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August 16

Common/Household "soft" magnets

What are some "soft" magnets that you can find in your household? —Preceding unsigned comment added by 71.52.198.240 (talk) 00:23, 16 August 2008 (UTC)

Soft vs. hard means the mobility of the magnetic domains inside the material. Iron is a soft magnet, since its domains will align with whatever magnetic field it is in. Mac Davis (talk) 03:15, 16 August 2008 (UTC)

I am told tht ordinary nails are made of iron (soft ferromagnetic material). If you magnetize a nail, it easily loses its magnetism when the current in the coil surrounding it is turned off, or it you tap it sharply. Most common ferrous household objects are steel (hard ferromagnetic material), such as appliance casings, hinges, shelf standards, beams and angles, and even sewing needles. In transformers, the cores are soft ferromagnetic matrial. Relays would have soft ferromagnetic cores as well, as would doorbell or solenoid cores, so that they lose their magnetism when the current is turned off. The nature of a ferromagnetic material could be analyzed by measuring the hysteresis curve and seeing how much magnetism is retained. Edison (talk) 05:00, 17 August 2008 (UTC)

Don't forget speakers; the electromagnet inside has to change its properties very quickly for the device to produce any usable sound. --Bowlhover (talk) 09:50, 17 August 2008 (UTC)

In modern speakers I have seen a permanent magnet and a coil of copper wire attacked to the voice coil. Where is the soft ferromagnetic material?Edison (talk) 14:56, 17 August 2008 (UTC)

Vitamins

Is Vitamin short for Vital Minerals? Or is it something else. --69.152.197.134 (talk) 04:06, 16 August 2008 (UTC)

It's derived from the Latin "vita", meaning "life", and from the Latin "amine". Scientists at the time believed vitamins were amines; when this was shown to be false, the "e" was dropped. See [1]. --Bowlhover (talk) 04:37, 16 August 2008 (UTC)

Or of course you could read Vitamin. --Heron (talk) 09:37, 16 August 2008 (UTC)

food science

Why does liver of meat especially of chicken turns green while cooking? What's the name of the pigment in shrimps responsible for changing into red while cooked? —Preceding unsigned comment added by Gnanesh123 (talk • contribs) 06:11, 16 August 2008 (UTC)

To the second question, astaxanthin. See [2] and [3]. --Bowlhover (talk) 07:18, 16 August 2008 (UTC)

IUPAC Nomenclature of carbohydrates

What do the letters R and S stand for in the IUPAC nomenclature of carbohydrates?(e.g., fructose is (2R,3S,4R,5R)-2,5-Bis(hydroxymethyl)oxolane-2,3,4-triol)Leif edling (talk) 08:09, 16 August 2008 (UTC)

I don't really have a clue, but my guess would be the chirality. -- Aeluwas (talk) 09:39, 16 August 2008 (UTC)

Yes, it describes the chirality. In organic compounds such as carbohydrates, wherever there is a carbon atom that has four different groups attached to it, there are two possible arrangements of those four groups. Each is a mirror image of the other, and one is designated as R and one as S. -- Ed (Edgar181) 16:12, 16 August 2008 (UTC)

As for what the R and S are abbreviations of, it is Rectus and Sinister, the latin words for right and left, respectively. (See Chirality (chemistry)#By configuration: R- and S-) -- 128.104.112.147 (talk) 21:19, 16 August 2008 (UTC)

Obtaining special dc power supply.

i need to obtain special dc power supply for a project. the power supply system will supply varying power with respect to time.

File:Dc power supply for project.jpg

the horizontal axis indicates time. how this type of power supply system can be constructed? where in the web i can obtain a circuit diagram for this power supply system? —Preceding unsigned comment added by Shamiul (talk • contribs) 09:28, 16 August 2008 (UTC)

You don't indicate any units on your graph, so we can only guess at the voltage (or other electrical parameter) and frequency that you want. One well-known circuit that produces this waveform is the flyback circuit for a CRT. There are many circuits [4] for this on the web. [Safety information: the flyback circuit can generate tens of thousands of volts, so don't use it unless you know what you're doing. For smaller voltages, use the humble 555 timer IC. Its data sheet will tell you how to get a triangle wave out of it.] --Heron (talk) 09:35, 16 August 2008 (UTC)

theres programmable psus you could buy to do this. One I remember was a BOP (bipolar operation power supply) make by Kepko. However if its only a fixed application you need, you could build your own by using a ramp generator followed by a power output stage. —Preceding unsigned comment added by 79.76.213.193 (talk) 12:04, 16 August 2008 (UTC)

It should be pointed out that what you are looking for is an AC supply - not DC. If you Google "Ramp generator" - you'll find a ton of circuits you can build to make the waveform you seek. You can also search on "Signal generator" to find a bunch of pre-packaged systems that generate these kinds of waveforms - although typically not at high voltages or current. But without knowing specifics of current, voltage and frequency that you need to supply - it's impossible to recommend a specific product.

Actually the Op was correct: hes looking for a direct current supply. It may not be steady, but its direct. —Preceding unsigned comment added by 79.76.158.77 (talk) 01:35, 17 August 2008 (UTC)

Some power supplies are "programmable" in the sense that you can feed them an analog voltage (at a very low power level) and the power supply will output a voltage that is proportional to the "programming" voltage. If the frequency of your waveform is low (less than, perhaps ten Hertz), such a power supply could be used with any number of simple waveform-generation circuits to produce your desired effect. (See function generator.) Alternatively, consider an emitter follower connected between an ordinary constant voltage power supply and your load, with the 'follower being driven by a waveform generator. (Also, if your power requirements are small enough, you might find that a function generator itself can output the power directly.) You can also buy "bipolar power amplifiers" that can help here, but they tend to be pricey.

Atlant (talk) 18:02, 19 August 2008 (UTC)

Evaporating black hole

What stays back when a black hole evaporates? The formulas on Hawking radiation don't seem to apply to very lightweight black holes. The evaporation is done by absorption of one partner of pairs of virtual particles but does this still work when they become really small, say, smaller than the wavelength of a photon corresponding to the mass (energy) of the black hole itself? And how often do virtual particles pop up from vacuum? 93.132.173.139 (talk) 12:39, 16 August 2008 (UTC)

I'm not entirely sure I understand what you are asking, but I think when a black hole evaporates, it just evaporates into electro-magnetic radiation, so nothing is left behind when it evaporates completely. ScienceApe (talk) 20:59, 16 August 2008 (UTC)

Well, it's difficult to ask the question precisely. What I have in mind is this: as long as the black hole is big enough to catch virtual particles, the formulas about Hawking radiation apply. But when the BH is really small I imagine it becomes more and more difficult to catch virtual particles. That opens a lot of questions about virtual particles and photons in general: what size is a photon, can it be absorbed by a structure that is far smaller than its wavelength? At which probability? If this is not possible at all, something stable will remain. If this is possible, the rate of evaporation will not be the same as for larger BHs. And what if the BH swallows a virtual photon with twice it's energy? Would we have a black hole with negative mass? 93.132.189.68 (talk) 08:06, 17 August 2008 (UTC)

It may not be a good idea to think of the evaporation in terms of virtual particles. Hawking does mention the virtual particle picture in his 1975 paper, but he says it's "heuristic only and should not be taken too literally". His actual derivation of the effect ("The real justification of the thermal emission") uses nonperturbative field theory, which doesn't have the virtual particle concept. You can describe any particle decay in terms of virtual particles if you want to. For example, instead of saying that a muon decays into an electron and two neutrinos, you can say that an electron-positron pair appears out of the vacuum, the positron interacts with the muon producing two neutrinos, and the electron escapes without interacting and "becomes real". Since the electron carries energy and overall energy is conserved, the positron must have carried negative energy to the muon. So there are "enough" virtual particles in the vacuum to cause the decay of even a pointlike particle like the muon, and presumably there would be enough for a tiny black hole as well. But nobody really knows for sure, since the combination of classical general relativity and quantum field theory that Hawking used breaks down for very small black holes, and we still don't have anything better. People believe that black holes decay completely simply because every quantum object decays unless there's a conservation law preventing it, and no known conservation law would prevent the decay of black holes. (Hawking also mentions this argument in his paper.) I've heard it suggested that black holes don't decay completely but leave a "Planck scale remnant" behind, but this seems to be based not on the plausibility of a "Planck scale remnant" but on the fact that it would solve the black hole information loss problem. -- BenRG (talk) 12:06, 17 August 2008 (UTC)

Thank you for the link, that looks the right stuff but not the kind to be understood in a minute. 93.132.189.68 (talk) 16:27, 17 August 2008 (UTC)

Calcium and Acne

I notice that after taking calcium supplements, my acne gets reduced and I have heard similar claims from other people. What role does calcium play in reducing inflammation or is there a different mechanism? Or is this just a placebo effect? 96.242.14.160 (talk) 13:28, 16 August 2008 (UTC)

It could only be placebo if you believed in advance that calcium would do that. Is that true? If not - no placebo. SteveBaker (talk) 13:56, 16 August 2008 (UTC)]

It's not necessarily a placebo, but it's certainly an unreliable conclusion if you believed calcium would reduce your acne before you started the therapy. That's why most trials for drugs are blind. However, there is some evidence that calcium aids the body's endocrine systems and is occasionally used as a treatment for acne. See PMID 17576242. —Cyclonenim^T@lk? 14:24, 16 August 2008 (UTC)

Glucose question

under what conditions does a patient needs to be in drip of glucose. —Preceding unsigned comment added by 121.245.25.17 (talk) 14:54, 16 August 2008 (UTC)

I fixed your question to a new section. Acute hypoglycemia can usually be reversed with an oral dose of 10-20g of carbohydrates. However, when EMTs find someone in a state of severe hypoglycemia, they usually administer an IV drip. —Cyclonenim^T@lk? 15:01, 16 August 2008 (UTC)

Hospitalized patients are often kept on IV fluids containing dextrose (glucose) in order to maintain both hydration and caloric intake for short periods of time. This might happen in a number of situations: 1) they are ordered not to take anything by mouth in advance of a procedure, 2) they are undergoing or recovering from an operation, or 3) they are unable to take in fluids or nutrition due to their medical condition. Total parenteral nutrition (TPN) is a mixture of dextrose, amino acids, and lipids that is used to maintain a more complete nutritional status over longer periods of time. An individual with an inborn error of metabolism who is hospitalized for an acute episode of metabolic decompensation may be maintained on a higher concentration of IV glucose to prevent a catabolic state, as this can worsen the metabolic condition. Medical geneticist (talk) 17:01, 17 August 2008 (UTC)

Adding to the above, if someone has taken an overdose of a long-acting hypoglycemic agent (either oral or insulin), though they can be revived temporarily with oral glucose, a drip will be required to keep their glucose levels normal until the agent wears off. - Nunh-huh 01:31, 18 August 2008 (UTC)

(edit conflict w/Cyclonenim) Most of the time when people use the term EMT they mean EMT-B. In many jurisdictions, EMTs are not permitted to perform invasive procedures such as IV therapy until they upgrade their training to EMT-I (AKA CRT) or EMT-P (AKA Paramedic). EMTs can only give oral glucose, usually as a paste (which tastes gross) but often use other available sources such as juice or cake frosting. --Shaggorama (talk) 02:58, 20 August 2008 (UTC)

type I diabetics are sometimes first diagnosed in a state of ketoacidosis, and in such cases they are administered insulin and glucose drips simultaneously until the ketoacidosis clears and their blood-sugars stabalize. (See Diabetic ketoacidosis#Treatment) Oded (talk) 03:35, 20 August 2008 (UTC)

Electronically squaring a signal

This is for a home project/ personal interest. Can a unipolar signal be squared (ie multiplied by itself) by first taking the natural logarithm (using a log amp), multiplying by 2 then taking the anti log using an exponentiating circuit? And what would happen if the multiplier were not 2: could you get odd powers of the input signal. Would the odd powers introduce other frequency components in the output? —Preceding unsigned comment added by 79.76.158.77 (talk) 16:30, 16 August 2008 (UTC)

Yes, yes, and yes. Some analog computers work using logarithms. In fact Zetex released an IC a few years ago [5] that used the same principle. Raising a signal to a power is a nonlinear operation, so you would necessarily generate harmonics. --Heron (talk)

OK thats good thanks- but, would it be easier and/or better to use an analog multiplier for pure squaring rather than a log amp: I mean would the multiplier be faster than a log amp? (I have a time domain signal-- pulses)--79.76.158.77 (talk) 00:42, 17 August 2008 (UTC)

Just a niggle, but isn't a unipolar signal just 0 or 1? If it really were a unipolar signal all you have to do to square it is pass it on unchanged. Dmcq (talk) 21:52, 16 August 2008 (UTC)

No, that's a binary signal. Unipolar means it stays on one side of 0 V. --Heron (talk) 22:05, 16 August 2008 (UTC)

Is everything technically light?

I'm aware that our eyes can only see only a paltry slice of the electomagnetic spectrum. What's interesting to me, however, is how special equipment can take just about any wave and "interpret" it for our vision (such as untraviolet, thermal radiation,etc.).I vaguely remember reading somewhere this can even be done for sound! Apparantly some animals and insects can "see" wavelengths that we can only measure. So is all existence merely wavelengths of light? What profound lesson (if any) can be drawn from this?--Hey, I'm Just Curious (talk) 17:10, 16 August 2008 (UTC)

No, light is distinct from matter. Our eyes can only see the visible light spectrum. However, the other wavelengths of the electromagnetic spectrum are still there, it's just our eyes can't detect them. Infrared and ultraviolet light can be picked up by sensitive equipment which converts it into forms of visible light. It is not because everything around us is made from light. You may wish to see Thermographic camera for infrared and Ultraviolet photography. —Cyclonenim^T@lk? 17:34, 16 August 2008 (UTC)

In some broader sense the OP is right. Very much of what happens in everyday life is either due to gravitation or to electromagnetic forces. If you put your coffee mug on your table an it doesn't sink in, that is because the electromagnetic force exchanges photons (light) between the two. 93.132.173.139 (talk) 18:09, 16 August 2008 (UTC)

I don't think that was the OP's point, though. I think they were suggesting that matter and light are the same thing which isn't true. Unless we start going into wave-particle duality. —Cyclonenim^T@lk? 18:31, 16 August 2008 (UTC)

Light is a very specific form of electromagnetism. The force holding atoms apart is the same fundamental force, but I don't think it's really accurate to called it "light". --Tango (talk) 18:43, 16 August 2008 (UTC)

Light is different from other stuff. For instance, you can't capture light (or sound) inside a bottle and unless you have a multi-billion dollar lab, you can't make two particles of light collide and bounce off each other. This is a trick that most other stuff, like doors and foreheads, easily manage.

Electromagnetic waves of "Light" with different wavelengths than visible light can also behave so different from ordinary light that you may not have recognized it as light, could you see it. While visible light travel mostly in a straight line, radiowaves can curve around mountains like waves around a dock in a harbour and it would be more difficult to see anything but the biggest features of the landscape around you. So while quantum mechanics allow us to say that everything is both waves and particles, we can't say that everything is "light".EverGreg (talk) 15:33, 17 August 2008 (UTC)

Everything may appear to be light through an infrared camera, but this is just because most things emit infrared radiation, commonly known as heat. Only an object at Absolute Zero would not theoretically emit infrared radiation, and a normal infrared camera would display this object as completely black. RevenDS (talk) 21:17, 21 August 2008 (UTC)

"Most chemicals are white."

"Most chemicals are white."

This statement appeared earlier on this page in a discussion titled The End of the World. Is it true that most chemicals are white? If so, why?

Thanks, Wanderer57 (talk) 17:57, 16 August 2008 (UTC)

Because they rarely have a chromophore? --Ayacop (talk) 18:28, 16 August 2008 (UTC)

Continuing on, yes, unless there's a chromophore, it's actually colorless. Even my college chemistry students have a difficult time distinguishing between "color" and "transparency" (a white thing is colorless+opaque, not a "color" itself). Suspensions (as opposed to solutions) are non-transparent (tindall effect) and collections of small particles (as opposed to solid crystals) are often cloudy or opaque. Both of these are due to seeing reflections and refractions from the surfaces and edges of the pieces, not due to the chemicals of the materials themselves. So the common form of a colorless thing may often be white. Look carefully & closely at a single grain of salt, then look at a shaker-full of them.

The link above should be to Tyndall effect. Dostioffski (talk) 21:07, 16 August 2008 (UTC)

Thank you. I would like to restate my question taking into account the above discussion re transparency. Is it true that in crystalline form most chemicals are colorless? Is it true that in powder form most chemicals appear white?

I am puzzled as to why the topic was changed from the "chemistry of color" to the innate significance of colors. Quite a jump. Wanderer57 (talk) 18:40, 17 August 2008 (UTC)

Above comment copied without modification from below after I added a subject heading Nil Einne (talk) 19:04, 17 August 2008 (UTC)

The colour white

Added subject heading to what is basically a different question to avoid either question being lostNil Einne (talk) 19:00, 17 August 2008 (UTC)

What is there about the color white that gives it a special status in the minds of human beings? Some societies give some other colors a special significance, but this is consciously (or politically) assigned. rather than being innate in the human mind as white seems to be. White is a mixture of other colors in certain proportions. If the proportions of the mixture are changed, a color other than white will be obtained that will not have an innate status. Andme2 (talk) 02:19, 17 August 2008 (UTC)

If anything, I would argue that black may have innate significance, since humans tend to have an innate fear of darkness (due to the significant disadvantage it puts us at because we rely so much on our visual system). The contrast to this state is "whiteness", the impression of any combination of colors of light that equally stimulates all three types of color-sensitive visual receptors. That is more of a light/dark issue than one of specific colour, though it may explain the special status our cultures tend to give black/white over other colours. Dostioffski (talk) 04:56, 17 August 2008 (UTC)

Does white really have a special/inate status in the minds of humans? What evidence do you have for that? It definitely doesn't universal cultural significance. For example, white denotes purity and innocence in Western cultures but is associated with death and mourning in East Asian cultures. In South Asian cultures, it's associated with both... Red given the high contrast, similarity to blood and other factors also tends to have special status in many cultures. Nil Einne (talk) 15:16, 17 August 2008 (UTC)

Thank you. I would like to restate my question taking into account the above discussion re transparency. Is it true that in crystalline form most chemicals are colorless? Is it true that in powder form most chemicals appear white?

I am puzzled as to why the topic was changed from the "chemistry of color" to the innate significance of colors. Quite a jump. Wanderer57 (talk) 18:40, 17 August 2008 (UTC)

Don't ask me, ask andme. However since they are indeed seperate questions to avoid either question being lost, I've added a subject header to AndMe's question Nil Einne (talk) 19:00, 17 August 2008 (UTC)

Agreed. I should have started a new subject. Yes, white is given a different significance in different societies. But my point is that white has an innate, though indeterminate, significance. Red, also, seems to have an innate significance but less intensely so than white, and red also can be assigned an arbitrary association. Furthermore, the shade of red is less important. It can be pure red (one wavelength in the light spectrum), or a shade of red (that includes other colors). There is only one white color - it is always a certain mixture of colors in certain proportions. Off-white does not have an innate significance. In fact, off-white may be regarded with contempt as dirty white - something masquerading as the color that has real innate significance. I don't think anyone would refer to a shade of red as dirty red - it would probably be called dark red. Andme2 (talk) 07:53, 19 August 2008 (UTC)

Do you have any real evidence that all cultures,( particularly those who have little contact with the 'outside' world) hold white to be significant? And that all cultures consider offwhite as "dirty white"? Also, I would say there isn't actually just one white. For example, adding blue makes white seem whiter. But most people would still agree that not so blue white is white. There is a difference in white balance preference with monitors etc depending on culture (also with lighting). And pink is probably not red in most cultures. And I don't personally believe, nor have I seen any evidence that the significance of red is more or less inate then white. Nil Einne (talk) 11:24, 19 August 2008 (UTC)

The OP may be interested in Basic Color Terms: Their Universality and Evolution by Berlin and Kay. Our article is pretty short but gets the basic point across. If their theory is correct, black and white are both innately and universally significant, with red coming next, but somewhat lower. Matt Deres (talk) 20:19, 20 August 2008 (UTC)

I was glad to see in the last post that organized research has found that white, black, and to a lesser degree red, have an innate (though basically indeterminate) significance to humans. This corroborates my own thinking on the matter. I was not aware that research had been conducted on this, with scientific papers and at least one book.

Having that corroboration, I will go one step further – a step which I thought would be too "off-the-cuff" to be posted. (An anonymous poster has chided me because I did not refer to scientific papers. In fact, he told me to "shut up".)

First, we must bear in mind the distinction between spectrally-pure (sine-wave) color and complex color (which combines sine waves of differing wavelengths, thus forming a non-sine wave). Also, living things vary in their physiological and psychological response to colors. Some humans are partially or completely color blind. Women are said to prefer red and orange; men prefer green and blue. Cats, apparently, have sacrificed some color awareness to an ability to see in faint light. Some insects see ultra-violet (I wonder what it looks like to them).

From a human standpoint, I think white has only one shade that can truly be called "white". One poster has said that blue-white has an even greater psychological significance than white. Another poster said different societies have slightly different concepts of what "white" looked like. I don't know what to say to these thoughtful remarks, so I will proceed with the idea that there is only one white to the human conscious, not blue-white and with no other shades.

Red, though secondary in innate significance, has a great many shades. The shades may be given names other than "red" (such as "maroon" or "crimson"). But to the human mind all shades of red have an innate significance not shared by colors that are basically not red.

Lipstick (almost always red) comes in an amazing range of shades. A large drugstore will show a very large variety of lipstick shades in chart form in their cosmetics department. Sometimes I walk out of my way in a drugstore just to see the great display of red shades. (I am a normal male.)

As I mentioned, some shades of red have been given a name (such as "maroon" or "crimson"). These shade-words (for all colors) could be added to the 11 basic color-words in the English language that have been posited by Berlin and Kay. According to their theory, I think, the total number of color and shade words would indicate the degree of civilized advance of a society.

Many shades of red have not been given a name (except for fanciful, promotional names given by cosmetics manufacturers!)

(To be continued in my next post in this thread.) Andme2 (talk) 08:17, 22 August 2008 (UTC)

Here is my far-out, off-the-cuff, idea. Perhaps each shade of red has a unique message. It is not a message that can be put in words. It is also basically indeterminate, at least within a broad range.

Each musical composition, too, has a unique message. The message of music is expressed only vaguely (if at all) by the title or the words (if it has words). The music itself is the real message. Music has been referred to as a universal language. It is understood by people who have a knowledge only of different languages. (Though some societies do not understand the music of other societies.) Music is entirely separate from the socially-assigned meaning of sound-groupings (words) made by the human mouth and vocal cords.

As with music, so it is, perhaps, with shades of red. Each shade of red has a unique message. It is separate from any arbitrarily-assigned political or social significance. Shades of red are also permanent in their meaning. (Red had an indeterminate meaning long before it was associated with communism.) Each shade of red conveys a single meaning, unlike music in which notes are presented in sequence to convey a complex message. Presenting colors in sequence would not do anything, though juxtaposing different colors in a great painting has a significance much greater than the individual colors. Abstract painting, if good, would be one way to do it. (To be continued in my next post in this thread.) Andme2 (talk) 08:17, 22 August 2008 (UTC)

Perhaps colors other than white, red, and black also convey a message. (Black, strictly speaking, is the absence of color.) The message of these other colors is not as intense as white, black and red, but there is a message. When I am in a hardware store, I enjoy looking at the amazing range of colors in the paint color-sample charts. What are they all saying? It cannot be put into words, just as the meaning of music cannot be put into words. Andme2 (talk) 08:17, 22 August 2008 (UTC)

While you are obviously welcome to your opinion, you may want to read white, colour temperature [6], [7], [8] (look for Asia) &

Bluing (fabric) and consider how this correlates with your idea there is only one white. In particular, you may want to considr the difference between a blueish white and a yellow/red white and where precisely in the colour temperature scale is the 'true' white that you believe exists and also, how this fits in with the idea that colour temperature preference varies between cultures and whether all cultures will agree with your notion that whatever colour temperature you feel is the true white is indeed the true white or they may find another colour temperature more 'whiter' then you white (or several colour temperatures equally white). P.S. I came across [9] which appears to address your ideas more generally and does seem to support the idea, at least among the study groups, that white has special significance (although not the only colour with special signifance). P.S. Incidentally, those colour temperature preference ranges are for lighting, I'm pretty sure monitor colour temperature ranges are rather different although again I believe there is varience between person and this is at least part culturally bound. Nil Einne (talk) 12:28, 23 August 2008 (UTC)

Electromagnet, Again

What would be the force of newtons with two electromagnets; each magnet having 1.2 volts, with an iron core, and 70 wraps of copper wire? —Preceding unsigned comment added by Hovercraft Experts (talk • contribs) 21:23, 16 August 2008 (UTC)

Again, use the equations on the electromagnet page. You'll learn a lot more about this project if you are able to do things yourself rather than asking someone else to use a calculator for you. BTW, you still didn't provide enough information. DMacks (talk) 21:43, 16 August 2008 (UTC)

Much better information, but you need to know the amperes which flow through the 70 wraps, which depends on the resistance. This could be calculated from the gauge and length of wire. Then the physical geometry of the iron cores (bar or horseshoe shape? What cross section area?) and the distance between will have a huge effect. The information given would not allow a calculation even within an order of magnitude. Note that if 1.2 volts is the voltage with no current flowing, the actual voltage in operation will probably drop significantly with only 70 turns of wire, which is close to a short circuit. Edison (talk) 04:52, 17 August 2008 (UTC)

Also note that the direction the currents flow in determine the sign of the force. If the current flows clockwise as viewed from one end, that end is the south pole, and vice versa for a current travelling counterclockwise. Like poles attract, so electromagnets with electrons flowing in the same direction attract; they repel otherwise.

Another crucial factor is the number of turns of wire per unit length of iron core. Wrapping 70 5-cm-thick cables around a core is obviously going to produce a weaker magnetic field than using 70 0.1-mm-thick wires if the current doesn't change.

To calculate current, use Ohm's law, the resistivity of copper (approx. 17 nanohm-metres), and the electrical resistance formula. --Bowlhover (talk) 10:50, 17 August 2008 (UTC)

If the conductors are 5 cm thick or 1 mm thick, and carried the same amperes through the same number of turns, and were along the same lengh and cross section of core, I would expect the same magnetic field strength. The 1 mm conductors could be spaced apart, or the 5 cm conductors could be applied in multiple layers. Edison (talk) 14:54, 17 August 2008 (UTC)

That's true--it's the number of turns per unit length of core that affects the current. --Bowlhover (talk) 19:21, 17 August 2008 (UTC)

August 17

sarcodactylus OR sarcodactylis?

I have found that some books give the Latin name for Buddha's hand (fingered) citron as Citrus medica var. sarcodactylus whilst others refer to it as Citrus medica var. sarcodactylis. Can any user please tell me which one is correct. Thank you.Simonschaim (talk) 07:01, 17 August 2008 (UTC)

They appear to be used interchangeably in the scientific literature, though a perusal of Google Scholar seems to suggest that Chinese scientists tend to use sarcodactylis while Western scientists tend to prefer sarcodactylus. Which is correct is likely a matter of opinion, though you could ask a Latin speaker at the Languages desk which is the more grammatically correct "fleshy finger" suffix. Dostioffski (talk) 07:24, 17 August 2008 (UTC)

Thank you. Simonschaim (talk) 07:34, 18 August 2008 (UTC)

Solar mining

I had a hard week and now my mind is wandering off a bit, so excuse me if this question is a tiny bit on the fictional side of science. Imagine that, by methods far beyond or current technology, we could manipulate solar flares to selectively eject some elements and retain others, and on a large scale, too. Would there be enough heavy elements in our sun to build two or three earth-like (in size and composition) planets? Would, on the other side, the depletion of heavy elements alter our sun in a significant way? 93.132.189.68 (talk) 09:33, 17 August 2008 (UTC)

According to Earth#Chemical composition, the Earth is about a third oxygen, a third iron, and a third everything else. According to Sun#Chemical composition, the sun contains about 3000 Earth-masses of oxygen and 200 of iron. We don't have information on the rarer elements (you could try looking up the cited sources), but at a guess, given the superabundance of those two, there's probably enough to get by. I don't know if the loss of a tiny fraction of its heavy elements will have much effect on the sun. Algebraist 11:36, 17 August 2008 (UTC)

Well, great, thank you! I'll just start right away! Don't worry if you see some minor flickering of the sun, it's just me .... well, now, wait, how many d'ya say? 200? Um, I've just cleaned out my attic but I doubt there's room for all of them ... . OK, to be serious again, what would be the densest known stable constellation for earth-sized planets inside the habitable zone? People have thought up things like Dyson spheres, there should be something about stability, too. I think a binary system taking only two planets would be fine, but putting another at the L₄ or L₅ would not be stable enough (I mean, another moon would be a great sight but I'm afraid my insurance policy doesn't cover a planet hitting the earth). Putting a gas giant inside the habitable zone and let a few dozen earth-like planets orbit it should work, but what to do with the others? 93.132.189.68 (talk) 15:50, 17 August 2008 (UTC)

I don't think 2 Earth's would be stable (one on each side of the Sun would be in equilibrium, but the slightest move away from that point would mess it all up and the other planets would cause such a move pretty soon), there are stable patterns for more than 2, though, I remember seeing some Java applets demonstrating them. I'll try and find them. As for the effect on the Sun, see metallicity - I'm not sure there's anything of use there, but it might point you in the right direction. --Tango (talk) 17:07, 17 August 2008 (UTC)

I was thinking of two earths orbiting a common barycenter, each as the other ones moon, like pluto and charon. I need some time to check all the links given by now. 93.132.189.68 (talk) 19:34, 17 August 2008 (UTC)

It's Wikipedia, so we have an article on this. See Klemperer rosette. -Arch dude (talk) 18:16, 17 August 2008 (UTC)

Well, I see a hard-to-spell Klemperer rosette firstly rotates around its common barycenter, not around a star --- I was not planning to put the sun aside. It is not clear from the article if a Klemperer rosette is stable, even less if it would be stable if orbiting (all of the bodies) around a star. 93.132.189.68 (talk) 20:45, 17 August 2008 (UTC)

According to the original paper linked, the rosette (the one with alternating light and heavy planets) is stable whether or not you put a star in the middle. There are some tight restrictions relating the radius of the system to the mass-ratio of the planets. Algebraist 20:53, 17 August 2008 (UTC)

OK, you beat me. I haven't read the large article completely. As I don't know when I'll have the time to do that, I promise not to create those 200 worlds until I have done so. You think I'm a megalomaniac? I hadn't even a chance to tell you about my three major plans with our local multiverses! 93.132.189.68 (talk) 21:18, 17 August 2008 (UTC)

Local multiverses? I'm intrigued! Not only are there multiple multiverses (each, presumably, containing multiple universes), but that's only taking into account the local neighbourhood? Wow! ;) --Tango (talk) 21:32, 17 August 2008 (UTC)

Just make sure our sun is not alive like the one in the Doctor Who episode 42. -- KathrynLybarger (talk) 15:26, 17 August 2008 (UTC)

Bah, I'll just tell it the procedure is good for its health and that the costs are covered by the national health service, no extra payment needed. 93.132.189.68 (talk) 16:08, 17 August 2008 (UTC)

Lower-metallicity stars have lower temperatures (http://nedwww.ipac.caltech.edu/level5/Sept01/Pagel2/Pagel4_3.html). By association, they also have lower luminosities and appear closer to red then to blue. Removing metals from the Sun will change its position on the Hertzsprung-Russell diagram towards the lower right. Oops, I misread the webpage. --Bowlhover (talk) 11:28, 18 August 2008 (UTC)

Doesn't that link say just the opposite? Anyway, I suspect that there are strong selection effects at play here: the oldest stars are also the coolest and the least metal-rich. I doubt that removing (magically and only) the metal from Sol would change its temperature significantly. --Tardis (talk) 16:04, 18 August 2008 (UTC)

Anti-ballistic missile

How effective are these anti-ballistic missile defenses? Are they really capable of intercepting ICBMs with 100% success rate? Otolemur crassicaudatus (talk) 14:22, 17 August 2008 (UTC)

There have been successful tests when the target missile had no countermeasures such as decoy warheads and flew a known path at a known time. Edison (talk) 14:50, 17 August 2008 (UTC)

But those tests have been isolated one-off tests. The chosen US technology is mid-course interception where the ICBM is shot down during its ballistic phase out on the edge of space. The handful of tests that have been done are not statistically significant indications of 100% success, even in those very limited circumstances - and it took a lot of tries before they hit one. The short answer to your question is: Their effectiveness is likely to be minimal against sophisticated ICBM's that could use stealth technology, could launch up to a dozen MIRV's and dozens more decoys. MARV technology might also render interception impossible. They might work against much simpler devices. All of those ideas have been in use in real ICBM's for decades.

Descent-phase ABM's have been used with some success - the "Patriot" missile had some success in the first gulf war - not 100% but it's been through several rounds of improvements since than. Of course you have to position them in large numbers around potential targets - which makes protecting an entire country essentially impossible. But they are "battle tested" - and they exist in reasonable numbers today. For shooting down nuclear weapons, they are not a great solution because the bits of destroyed warhead some showering down into the target zone - kinda like a "dirty bomb". That's better than having a nuclear detonation...but it's not exactly what you'd like to happen!

IMHO, what is needed (and would work a heck of a lot better) is launch-phase interception. In this situation, you catch the ICBM soon after takeoff. It's moving more slowly - it's moving in atmosphere (so it can't change direction quickly) - it can't deploy cheap decoys and it's MIRV/MARV's won't have split off yet. Best of all, the nasty, poisonous pieces of plutonium come raining down on the heads of the people who launched the darned thing rather than "burning up" on reentry - which spreads the incredibly poisonous stuff throughout the worlds atmosphere. I believe that if a ship-mounted ABM technology had been divised rather than following Reygan's crazy "Star Wars" ideas - then in times of international tension, ABM-equipped vessels could be positioned in international waters off the coast of any potential agressor as a way of saying "Don't even think about it". But for countries such as Russia and China which could position their launch sites far from international waters - this too has it's problems.

SteveBaker (talk) 15:53, 17 August 2008 (UTC)

• The claim that "the 'Patriot' missile had some success in the first gulf war" is disputed. See Patriot missile#Success rate vs. accuracy. --Anonymous, 03:56 UTC, August 18, 2008.

The problem with launch-phase interception is that you have pretty much no warning at all. How do you launch an interception missile in time for it to reach the ICBM soon after takeoff? --Tango (talk) 17:34, 17 August 2008 (UTC)

I find it unlikely that vaporizing the amount of plutonium in even a primitive warhead (~15 lbs of Pu) in the upper atmosphere would appreciably damage the health of people. Chernobyl dumped far more poisonous isotopes in far greater quantities into the environment at much lower altitudes and even that had relatively restrained effects (probabilistic increases of cancers, birth defects, etc., but no radiation poisoning other than people at the site itself.) As for ship-mounted ABM, see Aegis Ballistic Missile Defense System. --98.217.8.46 (talk) 17:56, 17 August 2008 (UTC)

You could always go back to the old plan of setting off hundreds of nuclear bombs in a line starting 150 miles west of Ireland and going up north. The plan was to destroy the Ruskie bombs with the electromagnetic pulses and radiation as they went past. And the trade winds would take any fallout over Europe away from America, they're all socialists anyway which is another word for commie. Dmcq (talk) 16:16, 17 August 2008 (UTC)

Of course, that wouldn't help you against SLBMs with flight trajectories other than over-the-pole, anyway. --98.217.8.46 (talk) 17:56, 17 August 2008 (UTC)

The answer is almost certainly no. But the practical effectively is perhaps secondary to the strategic and psychological effectively of having a co-ordinated anti-ballistic missile defense system that might work. The very fact that one exists, however potentially ineffective, could be a sufficient to deter anyone from launching a missile that would require intervention. The threat of MAD as a deterrence strategy so effective during the of the Cold War was deemed ineffective against potentially irrational "rogue" actors with nuclear access, so another strategy was needed. The anti-ballistic missile defense system was the result of this. Dostioffski (talk) 17:49, 17 August 2008 (UTC)

I'll take a little issue with your "history". The current ABM system comes right out of the work done on SDI in the 1980s—it never really stopped(see National Missile Defense). As for whether deterrence is ineffective against "rogue states" (meaning, presumably, North Korea or Iran), there's no evidence yet that it isn't, or that these leaders are willing to sacrifice their own political stability (or lives) for their ideologies (yes, they sabre-rattle, but so do all sides here). Iran is not al-Qaeda, and neither is North Korea. al-Qaeda does not have access to ICBMs (and the likelihood of them getting access is nil—if they are going to go nuclear, they aren't going to use ICBMs). --98.217.8.46 (talk) 18:01, 17 August 2008 (UTC)

I didn't mean to suggest that deterrence is not ineffective against "rogue states" or that the idea of a AMB system was a new one. I was noting that is part of the expressed reasoning for why the strategies used in the Cold War were deemed, by certain administrations, no longer effective and thus why other pipeline strategies became a priority. What we are told by governments, and what there is evidence for, often bear little relation. By the way, I think this reasoning was originally made in relation to former Soviet States. Al-Qaeda and the US were friends back then. Dostioffski (talk) 18:11, 17 August 2008 (UTC)

If you tell me when you plan to break into my house, and through which door or window, and with what tools, I have a good chance of preventing it. Edison (talk) 05:20, 19 August 2008 (UTC)

Yes - but if you tell me exactly how you protected your house - with what alarms and detectors, I have a good chance of breaking in. SteveBaker (talk) 02:13, 20 August 2008 (UTC)

And while you two were telling each other these things as length, I'd be stealing your cars! Or something like that. --98.217.8.46 (talk) 01:10, 21 August 2008 (UTC)

Dockin in orbit

Help! I am in orbit in my lander module having just taken off from the planet's surface. The command module is 100m ahead of me at the same altitude. I need to dock, but if i burn the forward boosters, I will increase my altitude and therefore be out of line. If I fire the retros, I will lag further behind command module. How the hell can I get docked? —Preceding unsigned comment added by 79.76.158.77 (talk) 03:44, 17 August 2008 (UTC)

...This is the Science reference desk, not the Read-Your-Mind-To-Figure-Out-What-The-Heck-You're-Talking-About-Desk =) --mboverload@ 03:48, 17 August 2008 (UTC)

'S ok mb, the question was not addressed at you ;) —Preceding unsigned comment added by 79.76.158.77 (talk) 03:57, 17 August 2008 (UTC)

Engage the autopilot. Algebraist 04:09, 17 August 2008 (UTC)

Auto pilot on shore leave. —Preceding unsigned comment added by 79.76.158.77 (talk) 04:16, 17 August 2008 (UTC)

Actually, you have got that wrong. If you fire your retros you will lose energy and fall to a lower orbit certainly, but you will also accelerate as you fall. In the lower orbit you will be travelling at a faster angular velocity and will start to catch the command module. For circular orbits;

$\omega ^{2}={\frac {GM}{r^{3}}}$

You might want to fire your attitude thrusters also to trim the orbit back to circular. After catching the command module you can now fire the forward thrusters to both bring you to higher orbit and to match velocities with the command module again. This is not professional advice, no liability accepted - if you scrape your spaceship talk to your insurance company, not to me. SpinningSpark 19:38, 17 August 2008 (UTC)

Your Orbital Maneuvering System and Reaction control system should help you do that. If you don't have a OMS/RCS you have no business being in space. Dostioffski (talk) 19:40, 17 August 2008 (UTC)

Why don't you simply harpoon the main module? Or shoot a harpoon backwards, the mass of the harpoon and the length of the rope carefully calculated so that, given the barycenter stays put at the distance with the main module, your lander module will just touch the main module. But be quick to grab the main module for you will be drawn back when the harpoon recoils on its rope. 93.132.189.68 (talk) 19:49, 17 August 2008 (UTC)

So what you're saying is that if you fire your engine in a line directly towards the command module then your orbital speed will increase - and so will the radius of the orbit. That's true. But you're pretty close to your command module - a very gentle nudge in the that direction won't increase your orbital speed by enough to make a significant difference to the radius of your orbit - and a very small change in orbital velocity will allow you to catch up the command module pretty quickly - it's only 100 meters away.

But if I understand what's behind your question - so let's pretend the command module is 100 kilometers away or something. Who says you have to aim your rocket DIRECTLY at the command module? Just rotate your craft until you're aiming a little BELOW the command module and fire your motors. If you time it right, the thrust will push you towards the planet - whilst simultaneously increasing your speed. Assuming you get your math right, the two effects will cancel out just as you arrive at the command module.

If your rotation thrusters were broken or something - and your were strictly only able to fire in a direction that's a tangent to the orbit then you could fire your main engine - let the orbit get a little larger then cut the engine and drift until you're a little ahead and above the command module. Then firse your retros at just the exact right moment and you'll fall behind it and reduce the radius of your orbit and arrive at the perfect spot.

SteveBaker (talk) 23:05, 17 August 2008 (UTC)

We have no rotation thrusters: this is Russian craft -we cannot afford luxuries like you Western capitalists. —Preceding unsigned comment added by 79.76.202.169 (talk) 01:15, 18 August 2008 (UTC)

The other consideration is that it's usually beneficial to minimise fuel consumption. As long as you're not in a hurry, a tiny nudge retro to change your orbit to a lower one, waiting to catch up, and then another tiny nudge to match orbits again will probably use less fuel than any other technique - you can use an arbitrarily small amount of fuel by just allowing it to take an arbitrarily long time to catch up. I'm not really sure what you happen if you thrust forward and slightly down, your logic sounds good - I expect you could do it in such a way as to intercept your destination, but I don't know what would be required to match orbits again, you would be in a completely different orbit. --Tango (talk) 23:25, 17 August 2008 (UTC)

You might ask Neil Armstrong. He was good at this, back in the day. Edison (talk) 05:17, 19 August 2008 (UTC)

Medicine

Please, what is the active ingredient in the prescription drug, Synthroid.

– — … ‘ “ ’ ” ° ″ ′ ≈ ≠ ≤ ≥ ± − × ÷ ← → · §  —Preceding unsigned comment added by 81.252.82.4 (talk) 19:40, 17 August 2008 (UTC) 

Synthroid. Wisdom89 _{(T / ^C)} 19:41, 17 August 2008 (UTC)

Or try www.synthroid.com --Tango (talk) 21:03, 17 August 2008 (UTC)

To paraphrase for those interested: It's a thyroxine analogue. Fribbler (talk) 22:52, 17 August 2008 (UTC)

Light in a time warp

Can light travel faster than the speed of light inside a time warp ? 69.157.227.243 (talk) 21:22, 17 August 2008 (UTC)

If you tell us what a time warp is, we might be able to help you. Algebraist 21:25, 17 August 2008 (UTC)

It's just a jump to the left... DuncanHill (talk) 21:28, 17 August 2008 (UTC)

Good call Duncan. We actually have an article on the sci-fi concept of time warps. Like Algebraist implies, there's no definition clear enough to answer the OP's question. But if this were asked on the Humanities Desk, I'd respond that as many sci-fi authors seem to imagine a time warp (as a tunnel-y sort of wormhole), light would travel at its normal speed locally while inside the time warp, while from a point of view outside the time warp, light (and everything else) would seem to travel faster than the speed of light (that is, back in time... assuming we're talking about the back-in-time kind of time warp). --Allen (talk) 22:19, 17 August 2008 (UTC)

To say this as clearly as possible: There is no such thing as a "time warp". Hence this question is utterly unanswerable. SteveBaker (talk) 22:52, 17 August 2008 (UTC)

Probably not what the OP was thinking of, but a "spacetime wrap" is contained in every black hole's event horizon. Since Einstein's theory of general relativity states matter bends space and light travels in straight lines, the path taken by light indicates the distortions caused by gravity. Light cannot escape from within an event horizon, so it's logical that the space is bent inward and no paths lead to the outside world. For an observer inside the horizon, light doesn't travel faster than usual, but an outside observer would not see light from inside the horizon at all. --Bowlhover (talk) 08:49, 18 August 2008 (UTC)

Olympic dope caught doping

What benefit would Kim Jong Su have gotten from taking propranolol? Clarityfiend (talk) 22:25, 17 August 2008 (UTC)

Our article on the drug says: "Propranolol is often used by musicians and other performers to prevent stage fright." Perhaps he took it to settle his nerves - you don't want your hands shaking when you're aiming a gun. --Tango (talk) 22:29, 17 August 2008 (UTC)

You're pretty much spot on, Tango. Propranolol works by blocking the action of epinephrine (a.k.a. adrenalin). Kim Jong Su competed in the 50 metre pistol event at the 2008 Summer Olympics. This is a game where the effects of adrenalin may not be a good thing, since adrenalin can makes edgy physically and mentally. As you can imagine, in a game of accuracy, this isn't fantastic. So by taking a β-blocker, Kim Jong Su could avoid this effects and have an advantage against other competitors. —Cyclonenim^T@lk? 22:36, 17 August 2008 (UTC)

Additionally This NY Times article says "In competitive rifle shooting, beta blockers are often used to reduce body bounce (caused by the pumping of the abdominal aorta)". That article discusses its effects for prone rifle shooters, but you'd think that similar effects would be felt by upright pistol shots like Kim. -- Finlay McWalter | Talk 22:46, 17 August 2008 (UTC)

Maybe students should be given a dose of propranolol before taking an important exam (especially an oral exam). Then they would not have the excuse, which is sometimes valid, "I knew the answer but I froze up." Andme2 (talk) 23:02, 17 August 2008 (UTC)

I know plenty of students who have taken beta-blockers before exams or, even more commonly, important presentations. They say it works wonders. Plasticup ^T/C 23:12, 17 August 2008 (UTC)

It is frowned upon, though... --Tango (talk) 23:16, 17 August 2008 (UTC)

It's not limited to students. It's widely known – if not widely commented upon – that professional musicians often engage in this sort of off-label use of beta blockers. See for example this page, which describes a survey of a large of number of U.S. symphony orchestra performers. (Few reported using beta blockers all the time, but a large minority took them for auditions and difficult solo performances.) TenOfAllTrades(talk) 18:30, 19 August 2008 (UTC)

Beta blockers seem to limit the max heart rate and stop you getting too excited. Certainly stopped me! Also did tend to reduce nervousness for that reason. Only problem: if you need to get your heart rate up with physical exertion etc, you cant! —Preceding unsigned comment added by 79.76.202.169 (talk) 01:20, 18 August 2008 (UTC)

How is that experienced, subjectively? I'd assume a high heart rate is needed during physical exertion, so do you feel "weak", "easily tired" or something more dramatic? EverGreg (talk) 08:18, 18 August 2008 (UTC)

I used to take propranalol for chronic headaches, and I once tried to ride my bicycle a bit too hard, and I passed out in the middle of the road. I don't remember passing out or falling, just waking up to see the beautiful blue sky and being unsure if I was dreaming. Then people were peering down at me. --Sean 14:17, 18 August 2008 (UTC)

Sedative effect of alcohol

The Sedative effect of alcohol is not explained in the article. Can anyone explain it here?--79.76.202.169 (talk) 23:19, 17 August 2008 (UTC)

Have you read short-term effects of alcohol#Pharmacology? Algebraist 23:26, 17 August 2008 (UTC)

Octopus

If an octopus has eight tentacles, with which one does it make love?--79.76.202.169 (talk) 23:24, 17 August 2008 (UTC)

With the Hectocotylus. Algebraist 23:26, 17 August 2008 (UTC)

August 18

Confidence boosting

This question has been removed. Per the reference desk guidelines, the reference desk is not an appropriate place to request medical, legal or other professional advice, including any kind of medical diagnosis, prognosis, or treatment recommendations. For such advice, please see a qualified professional. If you don't believe this is such a request, please explain what you meant to ask, either here or on the Reference Desk's talk page.

This question has been removed. Per the reference desk guidelines, the reference desk is not an appropriate place to request medical, legal or other professional advice, including any kind of medical diagnosis or prognosis, or treatment recommendations. For such advice, please see a qualified professional. If you don't believe this is such a request, please explain what you meant to ask, either here or on the Reference Desk's talk page. --~~~~

- Eron^Talk 01:35, 18 August 2008 (UTC)

Physics

What is an example of natural effects of heat transfer? —Preceding unsigned comment added by Ianf50 (talk • contribs) 07:22, 18 August 2008 (UTC)

What is your definition of "natural"? Does the fact that humans feel hotter in higher temperatures and humidities count? How about a lake warming up during the course of the day? If you're looking for meteorological phenomenon, almost all such phenomena involve heat transfer or the lack thereof. Perhaps the articles convection, thermal radiation, and heat conduction would help. --Bowlhover (talk) 09:00, 18 August 2008 (UTC)

A natural consequence of heat transfer is that in an isolated system, temperature differences among different parts of the system tend to reduce over time. In other words, the system tends to move toward thermal equilibrium. --71.162.242.81 (talk) 14:40, 19 August 2008 (UTC)

And fulfill the second law of thermodynamics. --Bowlhover (talk) 06:59, 21 August 2008 (UTC)

Sperm cell

What is a sperm cell? Where is it found? BINITA GUPTA (talk) 09:20, 18 August 2008 (UTC)

See Sperm. Zain Ebrahim (talk) 09:23, 18 August 2008 (UTC)

Building an atomic bomb

If the US was able to built an atomic bomb in the 40s, shouldn't rogue states also be able by now? Of course, they don't have the brilliant minds of that time, however, they also don't have to start by scratch. Mr.K. (talk) 10:36, 18 August 2008 (UTC)

Yes. The recipe for building an a-bomb can be considered common knowledge today. The challenge is buying the raw materials and building the infrastructure needed to build the bomb. This has been a backdrop for sanctions and actions against Iraq, Iran and North Korea. Another option is of course to buy a ready-made bomb. For instance, the US has leased nukes to Turkey. (See List_of_states_with_nuclear_weapons#Nuclear_weapons_sharing)

The article Nuclear proliferation may have more details. EverGreg (talk) 10:59, 18 August 2008 (UTC)

In the 40's, it took a 60,000 acre building and 1/6th of the entire US electricity generation capability to refine enough Uranium for just a couple of modest-sized bombs. It is indeed the issue of finding the raw materials that is the main obstacle to every nation having one. Uranium enrichment plants are LARGE and use a lot of power - that makes them hard to hide. They also take a long time to produce significant amounts of product. As for the actual bomb construction, the technical issues are spelled out in broad terms all over the place but some rather subtle details have to be gotten right to avoid the 'fizzle' that produced such a low yield in the North Korean nuclear test. The basic principles of bomb making were understood at the outset of the Manhatten Project - most of the effort went into resolving those fine details. SteveBaker (talk) 11:29, 18 August 2008 (UTC)

Though it should be noted the low yield for the NORK bomb was probably due to them trying to be especially fancy with it. If they had chosen a conservative design, I'm sure they could have pulled it off. The differences between the first atomic bombs (e.g. Hiroshima and Nagasaki) and a bomb small enough to fit onto the head of a missile are legion. Making a simple bomb, a gigantic kludgy thing like The gadget, is the easier thing to do. --98.217.8.46 (talk) 11:38, 18 August 2008 (UTC)

(ec) The difficulty in building an atomic bomb is one part knowledge, one part experience, two parts political ability, and maybe two parts materials. (I just made up the parts.) For a simple atomic bomb, knowledge is the easiest part, and always has been since 1945. There was enough knowledge released only two days after Nagasaki (see Smyth Report) to have a pretty good blueprint for how to make an atomic bomb. Experience is something anyone can develop—given enough time and other resources.

Political ability has proven a major factor in whether nations have or have not acquired nuclear weapons. Sanctions, pressure from allies and enemies, treaties, export restrictions, etc., have proven the only real bulwark against endless proliferation. It has hardly been a perfect system but the number of proliferators, while rising over time, is still relatively small (and could be much worse).

Materials are the lynchpin of the whole operation (and such has been known for a long time—the was the thesis of the Acheson-Lilienthal Report). Without raw materials for bomb making, you'll never get one even if you have all the other ingredients. Enriched uranium is not as hard to produce today as it was in the 1940s, but it's pretty dang hard. Plutonium is the "easier" route but it's no walk in the park either.

So knowledge is a part of it. Not the entire thing, though. It is easier for a nation to get a bomb today than it was in 1945? Definitely. Nuclear technology and knowledge has become far more widespread—a nuclear engineering textbook today can tell you how to run a reactor to generate as much plutonium as possible, and that's something that was known to only a few people back in the 1940s. But an atomic bomb is still an immense technical program. Not as immense as it once was, but still pretty immense. --98.217.8.46 (talk) 11:35, 18 August 2008 (UTC)

One last little comment: if you look at pages like Fat Man and Little Boy and Nuclear weapon design you'll see LOTS of information that looks absolutely vital to making a bomb. At first glance it looks like EVERYTHING is out there. In reality though there is a lot of stuff that is not out there, but you have to be pretty well-trained to see where the big gaps are. For example, there's really nothing in the public literature about how plutonium behaves when under several megabars of pressure—as it is the center of a bomb. This is not accidental—it's a classification category. If one were a bomb-producer, this is the sort of knowledge would be necessary to investigate, and getting it wrong could have consequences to the effectiveness of your effort. If you don't know what the secrets are, it can be hard to judge how many secrets there still are. (I don't know what the secrets are either, but I know of the existence of some of the secret areas.) --98.217.8.46 (talk) 11:44, 18 August 2008 (UTC)

I have to question the mention above of a 60,000 acre building in the 1940s. According to our article List of largest buildings in the world, the largest building now by area is 990,000 square metres (244 acres). Where was this gigantic building, and what became of it? Thanks. Wanderer57 (talk) 16:01, 18 August 2008 (UTC)

Perhaps that was a misquote of the statement in Manhattan Project that the "Oak Ridge facilities covered more than 60,000 acres". --Heron (talk) 18:00, 18 August 2008 (UTC)

Sorry - yes. 60,000 acres for the entire facility - but it was mostly buildings. I saw some contemporary photos a while back - they were enormous factories. SteveBaker (talk) 02:33, 19 August 2008 (UTC)

K-25 was the giant enrichment plant, in case anyone is curious. --Allen (talk) 04:09, 19 August 2008 (UTC)

And the time it was built, it is worth noting, it was the largest building in the world. So SteveBaker's overall point is correct. (Even if his statement about the actual acrage, or even that it was "mostly buildings", are not quite true. Most of the Oak Ridge site was wilderness. It was intentionally quite isolated. see map. But really, we're splitting hairs here... it was one of many unprecedentedly large facilities developed during the Manhattan Project, at immense expense. The spent the equivalent of $5 billion modern USD per bomb. That's a lot.) --98.217.8.46 (talk) 05:01, 19 August 2008 (UTC)

I am suspicious of the statement that 1/6 of U.S. electric generation was used to make the 2 atomic bombs, since at the time there was no transmission network capable of carrying that much electricity from the various generating stations to Oak Ridge, Paducah, Los Alamos, or wherever. Electricity was consumed much closer to the generating plant than this implies and there were no humongous generators (with outputs equal to the total generation of several average states) near the gaseous diffusion plants. Edison (talk) 05:12, 19 August 2008 (UTC)

I don't recall if it's 1/6th but it is something huge like that (this document says 1/7th). It's mostly going to Oak Ridge, not the other installations. It took a tremendous amount of electricity to operation Y-12 and K-25—ridiculous amounts. One of the main reasons they set it up at Oak Ridge is so they could be right next to the Tennessee Valley Authority in order to suck up all that power. Power requirements were a major issue on the Manhattan Project (and continued to be well into the Cold War—in 1956, the three gaseous diffusion plants together consumed 12% of US electricity; more energy than was produced by the Hoover Dam, the Grand Coulee Dam, and TVA combined). --98.217.8.46 (talk) 00:22, 20 August 2008 (UTC)

In very general terms, a uranium "gun type" bomb is very easy to build and I suspect most Wikipedians could build a functional bomb if they had access to the necessary amount of highly-enriched uranium. But we don't so the world is safe from Wikipedian-built uranium gun-type bombs.

On the other hand, it's probably a lot easier to get plutonium, especially if you and your comrades aren't all too concerned about your personal safety during the extraction process (from nuclear reactor spent fuel). (And it would help to have "insiders" who arrange the nuclear reactor to breed more plutoniium than it might otherwise produce.) But plutonium is only effective in an implosion-type bomb and to build one of those, you need to not only be as intelligent as your average Wikipedian but also a damned-fine machinist who can form precise bits of plutonium, high-explosives, neutron reflectors, and the like. A neutron-producing initiator is also pretty-much required for an implosion bomb, and so far, RadioShack doesn't carry those. So again, the world is mostly safe from Wikipedians bearing nuclear weapons.

Atlant (talk) 17:45, 19 August 2008 (UTC)

Plutonium is actually quite difficult to produce in bomb-ready form and bomb-ready quantities. It's "easier" than uranium enrichment in some ways but it's no walk in the park. It gets played down by people who imagine it just involves sticking uranium in some heavy water but if you look at the facilities they actually developed to deal with the plutonium problem (General Groves' Now It Can Be Told is quite good in this regard), you can see how difficult it actually was to produce the tiny amount of weapons-grade plutonium they did during the war. (And you have to be pretty concerned for your own safety when dealing with spent fuel; it's a short-term hazard as well as a long-term one). --98.217.8.46 (talk) 00:22, 20 August 2008 (UTC)

I love the story (I think it's in one of Richard Feynman's books) about the room at the Manhatten project that had a large shiney yellow sphere sitting on the floor, holding open the door. Feynman goes to pick it up so he can close the door and finds that it's heavy....insanely heavy...impossibly heavy! He asks about it and it turns out that it's a sphere of solid gold! They'd commissioned it from Fort Knox because they needed something of similar size and mass to the plutonium warhead for some test or other. When he asked why such a valuable thing should be used as a door stop, he was told that the actual plutonium sphere that it stood in for was tens of thousands of times more valuable - and to emphasise that, they decided that in comparison, the gold might just as well be used as a door-stop. SteveBaker (talk) 02:11, 20 August 2008 (UTC)

While we're on the subject of heavy stand-ins... have you ever held a substantial piece of uranium metal? It's amazingly heavy. Completely tricks out your mind, which assumes, you know, little piece of black metal will weight more or less as much as lead or steel or some other metal we have usual contacts with, but man, is it ever heavier than it looks. It's unfortunately very hard to get larger than shard-sized pieces of uranium metal in the US (and no, I'm not talking about enriched, before some wiseguy asks—what I'd do to have a solid uranium doorstop! --98.217.8.46 (talk) 01:00, 21 August 2008 (UTC)

You don't. IF it is at/exceeds "critical mass"/"critical size"., you won't be here to use it as a door stop. 205.240.146.233 (talk) 01:58, 23 August 2008 (UTC)

That's probably why 98... warned-off the wise-guys by saying "unenriched". How big is the critical mass of that? And if we're speaking about depleted uranium, I think the critical mass is (quasi-)infinitely large as we prove by loading ammo bays with hundreds or even thousands of pounds of the stuff.

Atlant (talk) 13:10, 23 August 2008 (UTC)

Christmas suicide

Is it true that much more people commit suicide in Christmas or is it an urban legend? Mr.K. (talk) 10:37, 18 August 2008 (UTC)

According to this TIME magazine article, it's the 11th of June. Fribbler (talk) 10:45, 18 August 2008 (UTC)

As with many questions of this nature, Snopes has the answer. -- Captain Disdain (talk) 10:46, 18 August 2008 (UTC)

The TIME magazine article referenced above is from the year 1932. I would venture to guess that those statistics are outdated and no longer very accurate. cheers, 10draftsdeep (talk) 14:06, 18 August 2008 (UTC)

It also probably doesn't help that they are from the middle of the Great Depression. Not so swell a time. --98.217.8.46 (talk) 05:06, 19 August 2008 (UTC)

I doubt much more people in Japan, China or India commit suicide in Christmas... N.B. Our article on suicide actually mentions the holiday issue. It's usually helpful to read articles before asking questions Nil Einne (talk) 10:34, 19 August 2008 (UTC)

Two wheels, spinning in different directions

Suppose that you put a bike-wheel on a stick and started spinning it. If you grasp the wheel by both hands, you'll feel the gyroscopic effect, making it harder to turn it around. What if you put another wheel on there, but spun it in the opposite direction at the same velocity? Would the gyroscopic effect double, or would it be cancelled out? My intuition tells me it would be doubled, since both wheels are independent of each other, and the effect of the each is to stabilize the stick, but I've been told that that is incorrect, that they in fact cancel each other out. Which is correct? 90.235.5.91 (talk) 11:46, 18 August 2008 (UTC)

A single gyroscope

Short answer: They cancel out.

Longer answer: In the picture (from Gyroscope) you have the wheel spinning about the red axis. If you twist the axle (say) clockwise around the green axis, then there is a resulting rotation about the blue axis. Now, take two of those, rotate one through 180 degrees so the two red axles are pointing in opposite directions - then the blue (output) axis of one wheel points up and the other points down and the two green (input) axes are lying on top of each other. So - when you twist the pair about the green axis, one wheel attempts to rotate about the upward pointing blue arrow and the other about the downward one. Since they both try to rotate (say) clockwise about those opposite arrows - the motion cancels out. SteveBaker (talk) 12:45, 18 August 2008 (UTC)

Note that the canceling of such an effect by things rotating in opposite direction has very practical use in some helicopters (see Coaxial rotors). --98.217.8.46 (talk) 13:00, 18 August 2008 (UTC)

I think that it's just the fact that it doesn't cause the helicopter to rotate counter to the direction of the rotor. I'd expect taking away the gyroscopic effect would make it harder to fly, not easier. — DanielLC 15:41, 18 August 2008 (UTC)

The reason for having counter-rotating coaxial rotors isn't because of gyroscopic effects - it's because you can avoid the need for a tail rotor. Stability in helicopters comes from the 'coneing' of the rotor blades (where they don't spin in a perfectly flat plane - but rather follow the surface of an inverted cone) - this produces an effect like dihedral in a fixed wing aircraft - which confers much stability. SteveBaker (talk) 02:27, 19 August 2008 (UTC)

I was under the impression that you use the tail rotor to avoid gyroscopic effects? Aren't those the source of the torque that they counteract? And isn't the advantage of two rotors (among others) that when they rotate at opposite directions they counter the torque? Am I confused on the torque source? --98.217.8.46 (talk) 04:53, 19 August 2008 (UTC)

No. Gyroscopic forces only come into play when the axle is twisted in a direction different from the one it's rotating in. If a helicopter is just hovering in still air, there are no twist forces on the main rotor disk - so there are no gyroscopic forces. However, the tail rotor is still working hard to stop the helicopter from spinning. The way to visualize what the tail rotor does is to imagine a toy helicopter with a motor spinning the main rotor. If you imagine toy with the rotor spinning and grab the rotor and hold it still - what would happen is that the body of the toy would start spinning in the opposite direction - right? Well, the air resistance (drag) on the rotor blades in a real helicopter has the same effect. As the drag tries to slow down the blades, there is a force on the fuselage trying to rotate it in the opposite direction. To avoid the helicopter spinning around faster and faster, you need some kind of a force keeping it straight. That's what the tail rotor does. In effect it's a variable pitch propellor that pushes on the tail to counter the tendancy of the fuselage to rotate in the opposite direction of the rotors. By altering the pitch on the tail rotor you can increase or decrease that force and thereby steer the helicopter...but that's just a handy side-effect of it's main function. This YouTube video and also this one show very graphically what happens when the tail rotor fails when the helicopter is just hovering...the helicopter starts to spin faster and faster.

There are gyroscopic forces at play in helicopter dynamics - when the helicopter rolls sideways or pitches forwards or backwards, there are gyroscopic forces because you're twisting the main mast at right angles to the rotation of the blades. When you pitch the helicopter forwards, it'll tend to roll to one side or the other - and when you roll, it'll tend to pitch forwards or backwards. But that force is comparable in size to the original roll or pitch movement - and it's easily countered with the cyclic pitch control. It helps that rotors are kept as light as possible - it's not a solid disk and it's very light compared to the mass of the helicopter itself.

SteveBaker (talk) 12:50, 19 August 2008 (UTC)

Mail sorting

How is address recognition done for postal mail? Many address labels are hand-written, often with appalling handwriting - and a misreading of, say, a house number, or a postal code could send a piece of mail to the wrong place. However, I hear of very few stories of misplaced mail - how do the postal services do it? Do they rely heavily on humans reading the address labels, or do they have spectacularly good OCR? In the latter case, how come "commercial" OCR is still far off being able to reliably read handwriting? — QuantumEleven 13:45, 18 August 2008 (UTC)

Is it really? I thought banks are using it too for reading transfer forms. That said, it's certainly easier to recognize postcodes and numbers (as long as you're sure of their position) than general text. As soon as the post reaches that village, there will be persons reading the final address. Also, I'd guess handwriting OCR simply doesn't sell to the general public. --Ayacop (talk) 14:27, 18 August 2008 (UTC)

Forms that are going to be read by OCR usually have a separate box for each character which makes the job much easier. Addressing an envelope is much more variable. --Tango (talk) 17:03, 18 August 2008 (UTC)

The Post-service's business is getting mail to you efficiently. They will have invested very heavily in advanced OCR programs/processes and will pay for the best quality (or rather the one that provides best service at lowest price) system they can. The price that such software can command for business-systems that require it to be correct in 99% (or whatever) of cases will be huge, compare that to small-office/home-use OCR and it is a case of the company will sell a lower-quality/less intensive version of their software for the home-market - because selling their best product to home-users would ruin their profit-margin for business-sales. The R&D costs of developing a new improved OCR program are probably huge and they need to recoup them some how, seeing as it is most important to things like Post Offices and paper-business intensive industries (e.g. banks with cheques/etc.) then they can command a huge fee for that, but the relatively small home/small-business market cannot, so they sell a watered down version for them. 194.221.133.226 (talk) 14:55, 18 August 2008 (UTC)

Address OCR is helped by having a relatively small set of possible words. This makes it easier to guess the text than for general-purpose OCR. For instance if some letters in "Boston, Ma" are garbled, like "Bo**n, Ma" the only possible choices may be "Boston, Ma" and "Bolton, Ma". Then a street name in the rest of the address may exclude Bolton. A general-purpose OCR may have to consider words like "Born", "Boron", "Boomin", "Bow pen" as well as unknown words, making accurate OCR much harder. EverGreg (talk) 15:12, 18 August 2008 (UTC)

Is your mail really that accurate? I regularly have pieced lost and delayed for 3 weeks or more. Evidently a person in the mail sorting system thinks that Bermuda is in North Carolina and Southampton, Bermuda is Southampton, England. Consequently a significant portion of our mail is sent to the ends of the earth before being forwarded to us. I say it must be a person making this error because a computer would have been fixed by now. That 3 week delay is then compounded by the outrageously slow sorting in our domestic system, which can add another 2-3 weeks without batting an eyelid. A month and a half to deliver a postcard is pretty typical. Plasticup ^T/C 15:55, 18 August 2008 (UTC)

The country is supposed to go on its own line in all capital letters, after the rest of the address. So, Southampton (postal code) BERMUDA rather than Southampton, Bermuda (postal code) Have people writing you been doing this? --Random832 (contribs) 19:38, 18 August 2008 (UTC)

The initial sort for domestic mail is based solely on using OCR on the zipcode and is extremely fast (something like 12 envelopes per second). Looking for numbers in the last position on an address is considerably easier than reading arbitrary text, and should allow mail to reach the appropriate regional hub (or local hub if you include the full 5+4 zip). Illegible codes are handled by humans (actually by transmitting scans to computer screens rather than physical sorting). Yes, some mistakes get made which will get picked up at the regional hubs (when they try to process the rest of the address), and redirecting that mail can add a few extra days to transit. Once at the hub, OCR continues on the full text, but the list of valid addresses is considerably smaller which helps. Dragons flight (talk) 19:33, 18 August 2008 (UTC)

One thing that helps is that there is natural redundancy in the address. If you OCR both the street name AND the city name AND the zip/postal-code then you can check a bunch of things:

• Does that street exist in that city?
• Does that city use that postal code?
• Does that street lie within that postal code?
• Is there a building with that number on that street?

Even when something is misread - it's rather unlikely that it will be misread CONSISTENTLY. So a misread of the postcode is easily detectable if it doesn't match the city. Even if you misread both the city AND the postcode AND you do so in such a way that the incorrect postcode does indeed belong to the incorrect city - then there is still a good chance that the street name won't fall within the right postcode or indeed that the city won't even contain a street of that name. Providing the system can reject errors, humans can always pull in the slack and give you a reliable system.

SteveBaker (talk) 02:22, 19 August 2008 (UTC)

I might also point out that it's highly likely that the vast majority of mail the post office handles these days has typed addresses, often with bar codes. I get maybe two hand-written letters a month. I get maybe half a dozen printed items a day. The latter are obviously much easier to sort. --98.217.8.46 (talk) 05:04, 19 August 2008 (UTC)

Strongest acid!

I know that Aqua Regia is the strongest of all acids. But among Nitric acid, Hydrochloric acid and Sulpruric acid, which one is the strongest? Anyone to answer is heartily thanked.117.201.97.1 (talk) 14:40, 18 August 2008 (UTC)

Aqua Regia is not the strongest of all acids. According to its article, fluoroantimonic acid is the strongest acid known (measured by the Hammett acidity function). Algebraist 14:43, 18 August 2008 (UTC)

According to Strong acid, Hydrochloric acid is the strongest of the ones you mention. Fribbler (talk) 14:46, 18 August 2008 (UTC)

One problem with questions like this is that in casual conversation, it's easy to argue "Acid X is stronger than Acid Y because X not Y can dissolve Something". But acidity is only one of several issues involved in dissolving something. The great counterexample is that we store concentrated "strong acids" (sulfuric, nitric, hydrochloric) in glass bottles and they are stable there. But hydrofluoric acid, which is much less "acidic" than thoes others dissolves glass very easily! DMacks (talk) 15:40, 18 August 2008 (UTC)

I have read hypochlorous acid is the strongest acid in water. Because, it gives the largest quantitly of H⁺ions. —Preceding unsigned comment added by 59.92.105.11 (talk) 15:52, 18 August 2008 (UTC)

Might be time to (re)read the hypochlorous acid article. DMacks (talk) 16:01, 18 August 2008 (UTC)

It's time to give a plug to the helium hydride ion, the strongest acid known. It can protonate any other substance. Graeme Bartlett (talk) 01:36, 19 August 2008 (UTC)

Related question, are there any acids that are strong enough to melt/dissolve things like they do in the movies? Like melting metal and things like that? ScienceApe (talk) 05:05, 19 August 2008 (UTC)

What would be the effect of such acids on the human body? Can acids really be used for torture like in EVE Online?Avnas Ishtaroth _{drop me a line} 05:42, 19 August 2008 (UTC)

Hydrochloric acid can create an effect somewhat like burning of the skin, and could certainly be used for torture. Some of the acids mentioned above would be unsuitable: hydrofluoric acid is probably too lethal, while fluroantimonic acid's tendency to react explosively with water would make it far too dangerous. Algebraist 08:03, 19 August 2008 (UTC)

Why would you bother to treat your detainees with costly acids when you can do it with water and get away with it? 93.132.180.118 (talk) 16:21, 19 August 2008 (UTC)

So, does anyone have any pictures or videos of fluoroantimonic acid reacting with familiar household objects? :) --Kurt Shaped Box (talk) 16:41, 19 August 2008 (UTC)

That would be tricky, since you'd have to desiccate the air somehow to avoid the acid decomposing. Of course, it would decompose into hydrofluoric acid, which would then do horrible things to your household objects, and possibly (since the article mentions the decomposition reaction is explosive), yourself. Algebraist 16:53, 19 August 2008 (UTC)

science / about the reproduction of animals

name:rabbit young ones:? group:? adaptation:? —Preceding unsigned comment added by 117.192.225.219 (talk) 15:33, 18 August 2008 (UTC)

homework? See Rabbit. - Eron^Talk 15:37, 18 August 2008 (UTC)

See also Young Ones, group, adaptation, reproduction, and how to pose a question.--Shantavira|^{feed me} 08:20, 19 August 2008 (UTC)

Habitable planet/moon systems

There are many examples in sci-fi of habitable (and populated) planets with habitable (and often populated) moons. Based on our current understandings, how likely is it that such planet/moon systems exist (or can exist)? —Preceding unsigned comment added by 216.154.18.248 (talk) 15:43, 18 August 2008 (UTC)

Yes. First one to spring to mind is Talax in Star Trek: Voyager (Neelix is from one of its moons), although I suppose it's possible the moon was terraformed. If you allow terraforming, then there are loads of examples, but there are plenty without that (the number of times the phrase "M-class moon", or even "M-class asteroid" are mentioned in Star Trek is enormous!). In real life, however, I would expect it to be very unlikely. For a start, the mass of a planet capable of supporting human-like life has to be within a fairly narrow range (too heavy and you crush the people, too light and it can't hold an atmosphere), so you're probably going to have a double planet, rather than a planet/moon system. I'm not sure how likely it is for a double planet to be habitable - there would presumably be very extreme tides, but that wouldn't necessarily be a problem (there are examples of life on Earth that depend on tides to live). What kind of day/night pattern they would have, I don't know, the extreme tides would probably result in them being tidally locked, which would probably result in a very long day (similar in length to a month on Earth). That would tend to cause large temperature differences between night and day (the atmosphere would reduce it, though, so it wouldn't be like night and day on Earth's moon, where the temperature changes are in the order of 100's of degrees). All this would probably result in very different life than we have on Earth, but it wouldn't rule out life of some form. --Tango (talk) 17:16, 18 August 2008 (UTC)

Tango, I'm not sure I follow you on day length in a tidally locked system. Wouldn't that depend on the angular momentum of the entire system? If the two equal bodies rotated about their mutual centre (barycentre?) every 24 hours (with the plane of rotation in the orbital plane), they would both have 24 hours days, wouldn't they? Franamax (talk) 22:36, 18 August 2008 (UTC)

Yes, but that would be a massive amount of angular momentum. I would expect the system to start with the same amount of angular momentum as other similar systems, and that will remain constant as the tidal forces adjust things towards a locked state. This is currently happening between the Earth and Moon (the Moon is already locked, the Earth will be in about 50 billion years [if it still exists by then, which is unlikely!]) and the predicted final period is about 47 days. (According to Orbit of the Moon.)--Tango (talk) 23:05, 18 August 2008 (UTC)

Assuming their co-orbit lies more or less in the plane of the ecliptic (a reasonable assumption) - then if they are tidally locked and of similar sizes then whenever the side of your planet that's perpetually closest to the other one is facing towards the sun (ie anytime around midday), there would be a total eclipse of the sun. So the two sides of those planets facing each other would get vastly less sunlight than the sides facing away from each other. That (and the lack of tides) would result in some pretty freaky weather patterns. SteveBaker (talk) 02:09, 19 August 2008 (UTC)

The inclination would have be be really small to get total eclipses every day/month with any decent separation between the planets. Partial eclipses would be pretty common, though. --Tango (talk) 17:31, 19 August 2008 (UTC)

Extreme weather patterns, within a reasonable limit, may actually be a benefit to life. More activity implies more possibilities for complex organic molecules to gather, concentrate, and react. Evolution should also proceed faster because there's a greater pressure to adapt. --Bowlhover (talk) 07:36, 24 August 2008 (UTC)

Jupiter's moon, Europa, is now considered one of the prime candidates for extraterrestrial life in the solar system, due to the likelihood of liquid water under an icy surface and evidence of other useful chemicals. Of course, such life, if it exists, is almost certainly going to resemble bacteria more than it does humans. Given the number of extrasolar planets discovered so far that are several times larger than Jupiter (which is probably due more to selection bias than the actual distribution of planet sizes), it is not unfeasible that there may be moons the size of, say, Mars, which could be inhabited. Confusing Manifestation(Say hi!) 22:58, 18 August 2008 (UTC)

Yes, but Jupiter isn't habitable, so that doesn't fit the OP's requirements. --Tango (talk) 23:05, 18 August 2008 (UTC)

Not habitable by life like ours, perhaps, but I see no problem with floating bags of hydrogen living high in the atmosphere. StuRat (talk) 01:55, 19 August 2008 (UTC)

The amount of radiation emitted from Jupiter's interior is pretty daunting for life - also, there are strong VERTICAL currents in the atmosphere that would push any delicate floating gasbag down to crush-depth and then up to high altitude "POP!" risks. It's not impossible - but it's a really tough call. SteveBaker (talk) 02:09, 19 August 2008 (UTC)

Well, how likely is it that there are habitable worlds out there AT ALL? We really don't know. How many of them are inhabited (as well as habitable) is even more of a wild-assed-guess. But if we suppose that there are plenty of inhabited worlds - then the question as to whether significant numbers of them have inhabited moons is a more interesting question. There are some things we can say at the outset:

• If the planet's orbit is in the 'habitable zone' of a suitable star - then it's moon is also in the habitable zone...that's good!
• If it is indeed a requirement for life that there be oceans with sizeable tides (this is a common theory - but it's not proven) - then the planet has to have a decent sized close-in moon in order to have life. Obviously, the moon will have tides too...big ones probably!

• We know of two planet/moon systems (Earth/Luna and Pluto/Charon) within our solar system where the planet and the moon are of reasonably comparable sizes - to the point where they could justifiably be called "Binary planets" (remember - Pluto used to be called a "planet" - but it's much smaller than our moon - so we could reasonably call Earth/Moon a "binary planet"). In that case, both would at least have enough gravity to (in principle) sustain an atmosphere and perhaps an ocean. That's good too because it suggests that such "binary planets" are likely to be very common.

• We believe that there is a strong chance that a large impact on one planet could send chunks flying off and arrive at another nearby planet. We also believe that primitive life (Bacteria, virusses) could survive a journey on such a rock. So if life developed on one planet of a binary pair, it could certainly be transferred over to the other moon/planet if conditions were reasonable upon it's arrival. This is also good for life developing on both objects more or less at the same time...and probably sharing much basic biology too. More good!

• BUT: It seems that large moons such as Charon are "captured" relatively long after the formation of the solar system - and our own moon seems to have come about from an exceedingly violent collision of some other minor planet with the Earth. In neither scenario is it likely that both planet and moon will have the same composition. Our own moon has hardly any water and no atmosphere. Charon also has a totally different atmosphere and surface to Pluto. Hence if one object supports life - there appears to be absolutely no guarantee that the other will also have the right chemical make-up. But we're going on a sample of only two cases - so it's possible we just got unlucky in our Solar system. There is no easy way to know.

Conclusion: If life is commonplace then I don't think it's at all unreasonable for life to develop on a pair of objects such as a small planet with a large moon - or a true binary planet. But then maybe life isn't commonplace - and/or maybe pairs of objects with similar composition are somehow very unlikely - in which case, it would be exceedingly unlikely that you'd find this kind of thing going on. We just don't know...but we soon will.

Science fiction writers like this scenario because it puts two civilisations sufficiently close together that they can reasonably interact in ways that make for a good plot. Possibly the most extreme example of this (and a really great SciFi book if you haven't read it) is "Rocheworld" (by Robert Forward) where a pair of planets get so close that their surfaces have distorted into teardrop shapes and nearly touch each other.

SteveBaker (talk) 01:57, 19 August 2008 (UTC)

Hi. I once read a list of moons where life is would be the most likely, likely meaning probably less than 0.001% chance. I've seen Mars's underground and Jupiter's atmosphere mentioned as likely candidates, and Saturn's moon Enceladus and even Neptune's moon Triton were on that list as well, in our own solar system, but it's more likely for life to exist in other solar systems, although we haven't really observed a perfectly ideal exoplanet just yet. ~AH1^(TCU) 16:38, 19 August 2008 (UTC)

The fact that we haven't observed one doesn't mean there couldn't be large numbers of them - our techniques for observing exoplanets can't (or, at least couldn't until very recently) detect such small planets even if they were there. --Tango (talk) 17:31, 19 August 2008 (UTC)

light chains of immunoglobulin

why two types of light chains are required by the antibody —Preceding unsigned comment added by 59.92.105.11 (talk) 15:48, 18 August 2008 (UTC)

See Immunoglobulin#Light chain. --JWSchmidt (talk) 00:59, 19 August 2008 (UTC)

Aero engine lubrication

tryings to find out lubricant used on Merlin aero engines during WW2--79.76.158.69 (talk) 16:39, 18 August 2008 (UTC)

Our article Rolls-Royce Merlin says that early models used pure ethylene glycol, later models used a water/glycol mix. DuncanHill (talk) 16:52, 18 August 2008 (UTC)

Fraid not: Ethylene glycol coolant was circulated by a pump through this passage to carry o

Coolant is not lubricant--79.76.158.69 (talk) 16:58, 18 August 2008 (UTC)

Sorry, misread your question. DuncanHill (talk) 17:09, 18 August 2008 (UTC)

There are some still flying. 'Phone the BBMF? Philip Trueman (talk) 17:15, 18 August 2008 (UTC)

According to "History of Aircraft Lubricants" on Google Books - it was a mineral oil with some "secret additives". The exact nature of the additive is unknown - except that it contains "anti-scuffing agents". Speculation is that it was "tri-cresyl-phosphate"...there is also discussion about it being an especially high viscosity oil. This seems to be a really complicated matter - with the composition of the lubricant changing over the life of each individual engine - and over the years as supercharger pressures increased. That book seems to be the bible for this - if you can get access to a copy, you'll know all you ever wanted to know - and probably a lot more!

     History of Aircraft Lubricants
     By Society of Automotive Engineers, Society of Automotive Engineers
     Contributor Society of Automotive Engineers
     Published by SAE, 1997
     ISBN 0768000009, 9780768000009
     164 pages

SteveBaker (talk) 01:25, 19 August 2008 (UTC)

Red meat

What causes "red" meat to be red? wsc```` —Preceding unsigned comment added by 75.85.203.191 (talk) 17:21, 18 August 2008 (UTC)

Mostly myoglobin. -- Coneslayer (talk) 17:30, 18 August 2008 (UTC)

See also this previous discussion. Dostioffski (talk) 21:27, 18 August 2008 (UTC)

mama

The mother article says that for most infants, the first word sounds like 'ma' hence, most if not all languages have similar sounding words for mother. Is there a reason why most infants say 'mama' first? is it because 'ma' is easy for infants to pronounce?

If so, I wonder if infants that say much more difficult words first end up having a higher IQ or something! Coolotter88 (talk) 20:06, 18 August 2008 (UTC)

Interesting. While I know the stereotype is 'mama', I'm pretty sure I was reading not long ago that the most common first 'word', as it is the easiest sound, was 'papa' or 'dada'. Perhaps people 'count' 'mama' first? Hmmm, must search. 217.42.157.143 (talk) 20:58, 18 August 2008 (UTC) Ignore that. Read this article and be enlightened! The whole thing is good, but cut to section 6 if you want to get straight to your answer. Or read Mama and papa :) 217.42.157.143 (talk) 21:58, 18 August 2008 (UTC)

I believe infants usually start "speaking" by making repeated consonant-vowel combinations. Few of those would be recognised as words in most languages, so odds are good that "mama" will be one of the first "words" spoken, since there aren't many other options. How many other such words are there? "papa", "dada", "baba" (could be interpreted as "baby" by an optimistic parent) are all I can think of off the top of my head. Remember, it needs to be a word a parent could realistic expect their child to say, otherwise they'll accept it for the coincidence it really is. --Tango (talk) 21:13, 18 August 2008 (UTC)

FYI: My son learned "dada" as his first word - he didn't learn "mama" until he already had a vocabulary of perhaps 100 or more words (much to the annoyance of my wife!!) - but he used it to name all adult humans - so I think it was just a simple misunderstanding of the meaning of the word. SteveBaker (talk) 00:58, 19 August 2008 (UTC)

Antiperistasis

I was looking at this article and thinking it could do with some improvement. I could do with some help in finding keywords and directions to look in.

For one thing, it refers to wells being warmer in winter than summer. I suspect that this is down to groundwater being about the same temperature when the outside temperature is colder, and could also lead in to a mention and perhaps discussion of how humans perceive things largely through contrast. Hence lukewarm water feeling hot when you're acclimatised to cold and cold when you're acclimatised to hot. This seems relevant, but I can't think of the name of this phenomenon or similar things. Also, while I'm pretty sure this is what's going on with the well water, I'm not as sure as I'd like to be, nor do I have a source.

If anyone could offer links, sources, useful words, ideas, etc. that would be nice :) I'd ask for contributions to the article itself, but I'm sure you're all terribly busy... 217.42.157.143 (talk) 20:55, 18 August 2008 (UTC)

Well, that article was apparently written in 1728 AD [10], so it could probably use an update :) Reading the original, they seem to actually be saying that well water is not warmer in winter, among other blazing refutations of the whole idea.

I'm not sure that you can usefully introduce the idea of temperature contrast as a retroactive explanation for the extent to which ancient Greek philosophers took the idea. It seems like more of a yin-yang approach than one of perception using the senses. Franamax (talk) 22:21, 18 August 2008 (UTC)

The original idea is clearly wrong. But you can see why they may have thought that. If you are outside in the snow, freezing your ass off - 10 degree centigrade well water might well seem warm - but if its in the height of Greek summer, then you're at 25 degrees C and 20 degree well water seems cool. Then they saw well water "smoking" in the depths of winter (which is because cold air tends to be very dry - so evaporation of warmer water is easier than in hot weather where the humidity is higher). They put two and two together and made five. The Greeks were notorious for never doing actual experiments to back up their theories - so this should come as no surprise.

Certainly humans perceive most things as differences rather than absolute values - so this phenomenon would seem to them to be pretty commonplace. This perception by 'difference' rather than 'absolute' appears in many (if not all) of our senses - Color constancy is the visual version of it. But you can also "get used to the smell" of you pet dog or whatever in your home - and cease to notice it. Visitors to your house may be all too aware of it. It's true for the sense of touch too - when you put on an item of clothing over bare skin, you initially feel it - but after you've been wearing it a while, you stop noticing. The underlying basis being that it's useless to have your senses continually sending your conscious mind the same data over and over again - it's more efficient to only tell you when something changes.

SteveBaker (talk) 00:53, 19 August 2008 (UTC)

This book calls it sensory adaptation. It seems to have been a significant concept in the history of philosophy, when the idealists were battling it out with the realists. --Heron (talk) 18:41, 19 August 2008 (UTC)

Does human perception of time speed up with age?

Just wondering if an adult perceives time as going faster than a child, for example. If 3 hours seems like the longest, biggest night in the world to a child, but to an adult it's just a normal event. That kind of thing. Also is it true that in those terms half your life is over at age 20?--Quadrilateral Tertiary (talk) 23:00, 18 August 2008 (UTC)

I've always personally believed that time perception varies depending on how quickly you wish the time to pass, with quicker = slower. --Kurt Shaped Box (talk) 01:00, 19 August 2008 (UTC)

Time perception is pretty complicated. There are a lot of factors involved. Is age part of it? I wouldn't doubt it at all that children get impatient much quicker than adults (I recall when I was 6 or so thinking that an hour was an unbelievably long amount of time—that if I had to wait an hour for something I might as well just give up; now hours fly by, I hardly notice). But I doubt very much that it is a linear function, even one that changes much once you become "an adult" of some form. I don't think old people perceive time as flying by on a day to day level (even if the years have "flown by"). --98.217.8.46 (talk) 03:17, 19 August 2008 (UTC)

My grandmother, a generally trustworth observer, insisted that time passed faster as one aged, and that a summer seemed like a long time when you were 6, but went by in a twinkle when you were 76; that babies grew to adults very fast when you were old. I have found no basis for disagreement with her observation. As one ages, each year seems to slip away faster. This is difficult to test in a meaningful formal experiment. Edison (talk) 05:07, 19 August 2008 (UTC)

There are plenty of people on the Internet asking why time seems to pass faster with age, so I would assume adults do perceive time intervals as being shorter when looking back. The reason, on the other hand, is not so clear.

This new scientist article suggests "some elderly people feel that the days seem to drag, but that the years flash by" and continues that this may be because few events take place in an elderly person's life. Thus, elders believe a given period of time is long until they examine it retrospectively and note that not much happened. The article also says that other factors such as memory and IQ may affect time perception.

The same logic should extend to adults. Tasks that one has long been accustomed to seem like an inconsequential part of life, not memorable moments, and are hence not well-remembered. A 7-year-old remembers only a few previous winters and might find the first snowstorm of the year interesting, but a 40-year-old would likely not take much note.

The top answer given at Answerbag is also convincing: as a person ages, every unit of time that passes is a smaller portion of his life. When looking back, younger people find that a greater portion of their experiences are from that unit of time and conclude it must have lasted a long time. --Bowlhover (talk) 07:46, 19 August 2008 (UTC)

Well, my own experience is definitely the same as your grandmother's -- not that I'm that old, but the summers of my childhood and this summer have passed at a completely different rate, it seems, and things only seem to be speeding up. I've got a birthday coming up, and it seems like I just had one. At this rate, I've subjectively lived through the vast majority of my life alrady -- and that's a cheerful thought, isn't it? This is speculation, of course, but I think a lot of it has to do with the fact that when you encounter something new, it kind of stops you in your tracks and forces you to take account of it, you know? And when you're a kid, everything is new. When you get to be in your thirties, though, most of it is old hat; it's not really making much an impression. It's probably as much a function of memory as it is one of time perception. -- Captain Disdain (talk) 07:13, 19 August 2008 (UTC)

I don't think you can tell by your own experience. Is it that your perception of time has changed as you got older - or is it that your memory of your perception of time has changed as you got older? Our brains don't have enough storage capacity to remember every detail of everything - and we selectively forget unimportant detail and compress old memories into shorter and blurrier incidents. It's not unreasonable to assume that this deletion of old data would be accompanied by a mental note relating to how busy you were.

Suppose (to pick a trivial example) that your memory were to operate like a computer and maintained a count the number of times when you were busy doing something that interests you in every month of your life. As memory deletes details of precisely what you were doing, all you have left is this "busy counter". Kids play all the time - so your early memories would indicate that you were REALLY busy - although (of course) you wouldn't remember every tree you climbed, every rope you jumped, etc. Old people do less with their time - so more recent memories would have much smaller "busy counter" values. This might consciously translate to: "A lot of things happened each summer when I was a kid" - which could easily be perceived as "Back then, summers went on forever" - simply because more things happened.

I'm not saying that's the exact mechanism (we don't know) - but it's perfectly possible that AT THE TIME you perceived time identically when you were young and now - and it's only the MEMORY of that which has changed over the years as your old memories were compacted, sieved and slotted into place to make room for new ones.

SteveBaker (talk) 12:26, 19 August 2008 (UTC)

Well, sure. Though I should probably point out that the difference between the summers when I was in the first grade and when I was in the sixth grade felt pretty major even when I was in the sixth grade, and the summers when I was in the sixth grade seemed much longer than, say, this past summer. But of course I couldn't say whether that has more to do with my memory or my perception of time at the time these events took place. On a hunch, I'd say it's probably a combination of the two. It's an interesting phenomenon, to be sure. -- Captain Disdain (talk) 12:34, 19 August 2008 (UTC)

I think one way to test time perception would be to ask movie-goers of different ages: "how long is it since you saw (say) Star Wars - Revenge of the Sith?". (and also ask the same question for several other movies.)

In fact, Revenge of the Sith came out just over three years ago. My theory is that older people are more likely than younger ones to underestimate how long ago they saw a film. Perhaps, in the case of the Sith film, they might answer "it was a couple of years".

There are complications to the experiment. One is that some people will know some answers very precisely from memory landmarks. "I saw that on my 12th birthday so it was 3 years ago." This type of answer does not rely on a general sense of "how long ago it seemed".

I wonder if this experiment or a similar one has been done. Wanderer57 (talk) 16:31, 19 August 2008 (UTC)

August 19

light

If light travels through space(a vacuum) at a constant speed does this mean that in theory light can travel on forever 207.118.239.28 (talk) 00:39, 19 August 2008 (UTC)

Anything can continue traveling indefinitely in the absence of interfering forces or materials, per Newton's First Law of Motion, and light is no exception. However, space isn't a perfect vacuum, and thus probability dictates that light eventually hits something. — Lomn 01:41, 19 August 2008 (UTC)

If it doesn't hit anything that can absorb it, yes, it will keep on going for ever. The photons we detect as cosmic microwave background radiation have been travelling for over 13 billion years. Because of the expansion of the universe, light travelling for a long time is redshifted, so if it started off as visible light it won't be visible after a while (I'm not sure how long, billions of years probably, longer for blue light, shorter for red). Also, if the light is coming from a point and spreading out in all directions (like from a star, say, as opposed to a laser beam), it's intensity reduces with the square of distance (inverse square law). Neither of those things will ever stop it, though, it gets weaker and weaker and redder and redder, but it never entirely goes away. --Tango (talk) 01:42, 19 August 2008 (UTC)

Are other EM wavelengths going around in space in addition to the cosmic microwave spectrum? For example, is ultraviolet light scooting around out there waiting till it is old enough to be visible?

If wavelengths get gradually longer with time, is the cosmic microwave distribution shifting toward longer wavelengths, or has it stabilized for some reason? Thanks. Wanderer57 (talk) 03:46, 19 August 2008 (UTC)

Yes all those different wavelengths are radiating through space, see ultraviolet astronomy coming from hot stars, and X ray astronomy from pulsars. Also the cosmological redshift keeps working even today. What could stop it is the end of time. Graeme Bartlett (talk) 05:58, 19 August 2008 (UTC)

The end of time, or just the end of expansion. If the universe stopped expanding, the redshift would stop (and would turn into blueshift if it started contracting). Current theories do not expect that to happen, however, so redshift should continue indefinitely. --Tango (talk) 18:11, 19 August 2008 (UTC)

Here's a point about light intensity. The total intensity does not decrease with the square of the distance. Assuming nothing blocks the light, total intensity remains constant, regardless of distance. What decreases is the intensity per unit of area normal to the direction of radiation. A telescope, at any given distance, magnifies this area. That makes the image dimmer but allows smaller detail to be seen - provided the detail remains sufficiently bright to be perceived. (Hence the value of time-exposure photograpy in astronomy.) Andme2 (talk) 06:03, 19 August 2008 (UTC)

According to Intensity (physics), intensity is a measure of power per unit area, so it is correct to say the intensity is reducing, it's the total energy which is staying the same (I'm not sure even that is true once you take relativity into account). --Tango (talk) 18:11, 19 August 2008 (UTC)

Also, I should point out that telescopes don't just magnify. The aperture (width of the main lens, roughly speaking) is usually much greater than that of the human eye, so it also increases the total amount of light that reaches the eye. --Tango (talk) 18:14, 19 August 2008 (UTC)

I think the definition of intensity depends on your field. As Intensity (physics) says: "In photometry and radiometry, intensity has a different meaning: it is the luminous or radiant power per unit solid angle. This can cause confusion ..." Gandalf61 (talk) 18:23, 19 August 2008 (UTC)

Railguns, rail errosion

If it is possible to coat the rails or the projectile in conductive plasma, could this help mitigate rail erosion at all? ScienceApe (talk) 05:03, 19 August 2008 (UTC)

Plasma will increase erosion as the ions and radicals react with the rail. Graeme Bartlett (talk) 05:54, 19 August 2008 (UTC)

How about reducing friction by raising the projectile above the rails through "magnetic levitation"? Is this incompatible with the propulsion system?

Is friction a significant issue with railguns? Wanderer57 (talk) 19:51, 19 August 2008 (UTC)

The projectile needs to have current flowing through it, see railgun. The coilgun has no wear on the barrel, but who wants to ONLY fire ferromagnetic projectiles. With a Railgun, you can launch depleted uranium slugs, yay! Coolotter88 (talk) 20:25, 19 August 2008 (UTC)

Yea, the railgun can launch anything that is conductive. I also think it can launch projectiles are higher velocities than a coilgun. But the rail erosion is a problem that coilguns don't have. Friction is primarily the problem. Plasma is conductive so I was thinking that maybe plasma could help "lubricate" the rails so to speak. Could a salt solution work instead? ScienceApe (talk) 00:16, 20 August 2008 (UTC)

What about silver conductive lubricant? 96.242.14.160 (talk) 15:30, 20 August 2008 (UTC)

Laser

How to measure the distance using laser? —Preceding unsigned comment added by Jiachun Zheng (talk • contribs) 09:45, 19 August 2008 (UTC)

Are you talking about a Laser rangefinder? Zain Ebrahim (talk) 09:48, 19 August 2008 (UTC)

Thank you.

Yes. I think. Part of it is "Laser transducor". I interested about it is used in the Olympic Games, specially about it is used in the field game.

I want to know more morden Tech. of the Laser rangefinder.Thank you.

--Jiachun Zheng (talk) 11:58, 19 August 2008 (UTC)

Using interferometry combined with time-domain reflectometry, you can use the laser to determine lengths with accuracy to sub-wavelength levels. I am not aware of commercial applications of such an apparatus outside of the research community. Nimur (talk) 16:59, 19 August 2008 (UTC)

A laser rangefinder is a time-of-flight device, not an interferometer. It sends a laser pulse and counts the time until the pulse returns. the accuracy depend on the accuracy of the clock that measures the time: light travels 300 millimeters per nanosecond. -Arch dude (talk) 02:14, 20 August 2008 (UTC)

Thanks. All friends

I interested it specially about how to be used at the field game in the Olympic Games,

I want to know more morden Tech. of the Laser rangefinder.

--Jiachun Zheng (talk) 03:27, 21 August 2008 (UTC)

The most basic device contains a laser and some kind of sensitive light-sensor that only responds to light at roughly the frequency of the laser. There is also a very high speed clock - one that "ticks" several times in every nanosecond probably.

Something (a computer probably) fires off a brief burst of laser light in the direction of whatever it is that has to be measured and simultaneously starts the clock. The light heads off towards the target (at the speed of light) - bounces off of it and then comes all the way back again and some of the reflected light enters the light detector. As soon as the detector senses that the laser light has returned, it stops the clock. By measuring the amount of time taken for the light to go from the rangefinder to the object and back again - and knowing the speed of light - you can calculate the distance the laser pulse travelled. Divide that by two to find the actual distance.

The difficulty is that light moves very fast - and making a clock that ticks fast enough is difficult. A good rule of thumb is that light travels about 1 foot (or 30cm) in a nanosecond. Making a clock that ticks once per nanosecond is possible - but it would only be able to measure the distance the light travelled accurately to 1 foot - and so the error in measuring the distance would be about six inches. In some situations, that kind of precision probably plenty good enough - in others, it's not.

The "Interferometry" approach is more difficult - but vastly more accurate. Light is a wave - and if you mix two lightbeams, you get "interference". When the peaks and troughs of the two waves line up exactly, you get a wave that's twice as big. When the peak of one wave lines up with the trough of the next, they cancel out. Between those two extremes, you get light that's somewhere between zero and the sum of the two beam brightnesses. This effect is called "interference".

So, you fire off your laser to the target, catch the reflected light as it comes back and mix the outgoing and incoming light together with some fancy mirrors and stuff. Then you can see whether the two beams are lined up, or out of alignment, or somewhere in between. If the distance to the object is an exact multiple of the wavelengths of the laser light - then the two beams line up peak-to-peak and trough-to-trough. If the distance is a little less, they won't add up to quite as much - and so on down to where they cancel out and the distance is half a wavelength more than an exact multiple.

This means that providing you know the distance to the object to the nearest half wavelength (which you can do with a really fast clock and the first method I described) - then you can use the interference result to figure out whether this is an exact number of wavelengths or half a wavelength off - or anywhere in between. This complicated method gives accuracies down to a few millimeters or better...but the machines are a LOT more expensive.

The other trick you can do with some of these machines is to use them like a police laser speed gun which measures the amount of doppler shift between the incoming and outgoing laser light - which tells you the speed something is moving.

SteveBaker (talk) 01:17, 22 August 2008 (UTC)

Thanks for this answer. Thank you for your writing so carefully. I get so much about it. I was a retired Physics teacher. So I think I am able to understand all of your writing. Thank you once again.

Could you give me an answer about what Laser rangefinder is used at the field game in the Olympic Games, specially at the 2008 Beijing Olymic Games. I guess probably,it is more morden Tech. of the Laser rangefinder.

You are so kind that I don't know how to express my mind.

good luck

--Jiachun Zheng (talk) 08:29, 22 August 2008 (UTC)

natural c onception after menopause

Can a woman conceive naturally after menopause? Are there natural foods or treatments to reverse infertility, after many years past menopause? —Preceding unsigned comment added by 64.21.220.29 (talk) 16:49, 19 August 2008 (UTC)

I think, by definition, a woman cannot conceive after the menopause, however I suppose it's possible to be mistaken about the menopause having taken place (the cycle may become irregular, but not have actually stopped completely). Being mistaken like that for many years seems unlikely. --Tango (talk) 18:05, 19 August 2008 (UTC)

Is a little lead deadly?

I bought a flat iron and it has a tag with a warning that it contains lead and I should wash my hands after handling it. And it says "The state of California recognizes that lead causes cancer and birth defects" or something. Should I freak out? Is it REALLY necessary to wash my hands every time I straighten my hair? Will having the product near me send lead particles into the air that will harm me?

And since this thing will be in contact with my hair, will the lead damage my hair?

Thanks so much in advance. 68.15.144.36 (talk) 18:18, 19 August 2008 (UTC)HarkUponTheGale

Just don't lick it. see lead poisoning. I wouldn't imagine that lead dust would fly out of your iron (they wouldn't sell it) but the time it takes to wash your hands afterwards isn't that much so it's not that inconvenient to be on the safe side and wash your hand with soap and water. Coolotter88 (talk) 18:32, 19 August 2008 (UTC)

Regarding lead and hair, note that (at least until recently) the hair coloring product Grecian Formula contained lead. In fact, the active ingredient was lead, which colored the hair by forming black particles of lead sulfide. -- 128.104.112.147 (talk) 19:07, 19 August 2008 (UTC)

My best guess is that the lead is in the solder for the electrical connections inside the iron. If so, the hazard really does not occur until it is disposed of and tossed in a landfill to leach into the soil. --—— Gadget850 (Ed) ^talk - 19:27, 19 August 2008 (UTC)

I really doubt you have anything to worry about. I remember buying a chair once that had the same warning (though it didn't tell me to constantly wash my hands). That message is required by the state of California for any product that contains lead somewhere, I guess. A product that could cause lead poisoning by its standard usage wouldn't get on the market, and the lead concentration is probably inside, like Gadget850 said.--El aprendelenguas (talk) 19:33, 19 August 2008 (UTC)

This appears to be a Proposition 65 warning [11][12] required when you will be ingesting more than 0.5 micrograms of lead per day. That's a very small amount, but the tag doesn't have to show the actual amount. The lead could be in the solder, in could also be an alloy in the iron body itself (for heat conduction). I wouldn't personally advise you to ignore any warning label. Your other option is to take the iron back to the store and look for one that doesn't have a warning tag. Also, as noted in one of the links, pregant women should minimize their exposure to lead from all sources. Franamax (talk) 21:02, 19 August 2008 (UTC)

Lead is nasty because it attaches to your brain and makes you stupid. The problem is much worse for small children than for adults for some reason...hence the warning for pregnant women. As an adult, I honestly wouldn't worry. So long as you aren't trying to eat the lead in the iron, it's not going to affect you. The biggest problem with lead poisoning is dust that you can inhale (from crumbling lead-based paint) or lead that's dissolved into something (such as happened in old buildings with lead water pipes). An actual solid lump of lead isn't going to hurt you measurably. SteveBaker (talk) 21:25, 19 August 2008 (UTC)

Well, not usually... - Eron^Talk 21:28, 19 August 2008 (UTC)

While we're on the subject of lead... I have two lead pigs that were given to me as an odd gift. They're not coated in anything—just heavy lead cylinders. At the moment they're just sitting on my radiation cover. I don't touch them. Is there any health risk? What about in the winter, when the radiation is turned on? I'm not asking for medical advice, I just want to know if there's some obvious danger I'm not thinking of. --98.217.8.46 (talk) 22:05, 19 August 2008 (UTC)

You mean "radiator" - not "radiation" - right? If you have radiation sources that turn on and off - the lead may be the only thing that's saving you!

But - no, just having a couple of lumps of lead in the room with you won't hurt you at all...doubly so if you never touch them. You might want to put them somewhere where kids can't reach them though. SteveBaker (talk) 22:12, 19 August 2008 (UTC)

I seem to remember a similar question being asked a few months back. I can't seem to find it in the archives now, but IIRC the consensus that time was that the owner of the cans should get them checked out (local university lab?) before even thinking of reusing them for any purpose, or even opening them up. --Kurt Shaped Box (talk) 00:24, 20 August 2008 (UTC)

Haha, yes, Freudian slip on radiatior. Anyway, thanks. I figured such was the case. No worries about kids (they are both out of the way, incredibly boring, and I have no kids around, ever). And yes, KSB, I was the original lead pig dude. I'm not reusing them. They just sit there. And are heavy. And are pretty boring. --98.217.8.46 (talk) 00:26, 20 August 2008 (UTC)

As I think I probably mentioned when you asked previously, whatever was previously stored in them may have left a dusty residue on the inside. These things are supposed to be decontaminated before disposal but, as is the way of the world and the people in it, that doesn't always happen. Really, the last thing you need is a faceful (followed by a lungful) of an alpha emitter compound if you accidentally jostle the pigs and the lid comes off one of them... --Kurt Shaped Box (talk) 02:07, 20 August 2008 (UTC)

Wall voltage through your body vs resistance

If the resistance between my hands is 3.9 million Ohms (measured with a multimeter), how come running 230 volts between them is so dangerous? Am I missing something huge, or are there simply points with so much lower resistance that it becomes dangerous?

By my (possibly faulted) calculations, the current passing between the hands when resistance is 3.9M ohms is 230/(3.9 * 10^6) = 5.9 * 10^-5 = .000059 A = 0.059 mA, where 10mA is quoted as being deadly (because of the heart). That's more than an order of magnitude more than 0.059 mA. -- Aeluwas (talk) 19:15, 19 August 2008 (UTC)

Did you see electric shock#Lethality of a shock? Coolotter88 (talk) 19:43, 19 August 2008 (UTC)

It's not so much a case of different points on your body as the condition of your body causing resistance to change. Is your skin dry? Is it wet? Is it wet enough? Grabbing hold of 230V is far from uniformly fatal, but it can be under the proper conditions. — Lomn 20:06, 19 August 2008 (UTC)

I'd take that measurement with a very large grain of salt. High resistances can be very difficult to measure accurately, and as others have already noted, the condition of the skin (especially the presence of moisture) can dramatically alter the conductivity of the body. Under moist conditions (electrodes or skin are damp), resistance between major extremities can drop at least a hundred-fold ([13], [14]) to one thousand ohms or less. Inadvertently piercing the skin (as, for example, with the sharp cut end of an electrical wire...) will also sharply reduce the resistance of the body by eliminating the resistance of skin at the point of contact. 230 volts across a thousand ohms resistance is 230 milliamps—more than sufficient to be very fatal. TenOfAllTrades(talk) 20:52, 19 August 2008 (UTC)

OK - PLEASE LISTEN VERY CAREFULLY: People have been killed by multimeters...doing more or less exactly what you did. Sure, the resistance is high - and even 240volts won't directly deliver enough current to kill you. But that's a VERY simplified view of what's going on. The crucial piece of information is that the majority of that huge resistance is in the first few millimeters of the skin. Once you get past the skin, the human body is mostly just salty water. Salty water (and therefore blood) conducts electricity very well and veins and arteries handily "wire" everything directly to your heart and brain. So if you were to grip the meter probe where you have a cut your finger (DO NOT TRY THIS!) and measure the resistance, the resistance will be MUCH lower and the current supplied by a humble 9 volt battery is quite enough to kill you.

I have gotten many 240v shocks several times over the years - and I'm still here to tell the tale - so indeed, 240 volts THROUGH THE SKIN is not usually lethal (it's far from pleasant though!). The key is that the current flow through the muscles of your arm causes them to contract super-quickly. So the current is only there very briefly. People who die from electric shocks tend to be doing something like gripping the wire so that their fingers contract around the wire and they can't let go because of the very muscle contraction that saves you in more ordinary situations. What happens then is that the energy from the current flow turns into heat which is concentrated where the resistance is highest...in the first millimeter of your skin. This heat causes the cell walls to break down - within a second or two you get burns and blistering (and you still can't let go - no matter how hard you try). If you haven't let go of the wire before that happens, blood gets in contact with the metal and then the resistance falls spectacularly...then you die.

Please treat electricity with respect - even a 9 volt battery can kill you.

SteveBaker (talk) 21:04, 19 August 2008 (UTC)

Hmm, interesting! I'll certainly think twice before doing that again. Still: does anyone have two multimeters around, to measure the voltage they send out when probing resistance? I doubt it's the full 9 volts; another site measured it to 0.3V. -- Aeluwas (talk) 21:27, 19 August 2008 (UTC)

Steve, I believe you on general principle, but do you have a reference for the assertion that multimeters have killed people in that way? It would be a valuable addition to multimeter#safety. --Sean 23:24, 19 August 2008 (UTC)

This is a fairly detailed discussion of an apparent death by multimeter, though that page lists it as "unconfirmed" and hence possibly erroneous. Dragons flight (talk) 23:45, 19 August 2008 (UTC)

Yep - that's where I first read about it too. There was some chat about it here on the WP:RD a year or two ago. It's really "death by a surprisingly little amount of electricity" - the multimeter just provides a plausible reason why someone should be connecting their hands to the + and - ends of a battery. SteveBaker (talk) 03:42, 20 August 2008 (UTC)

Radio wave strength

Is the strength of radio waves broadcast by humans into outer space inversely proportional to the square of the distance from Earth or inversely proportional to the cube of the distance from Earth (like magnets)? Coolotter88 (talk) 20:47, 19 August 2008 (UTC)

The square. Electromagnetic radiation follows an inverse square law. --Tango (talk) 20:58, 19 August 2008 (UTC)

So it doesn't matter that the radio waves are broadcast from dipoles? Coolotter88 (talk) 21:07, 19 August 2008 (UTC)

Yep it's the square. It's easy to remember why. When you set off a brief radio pulse, the waves spread out at the speed of light in a spherical wave-front - the three dimensional analog of "ripples in a pond". When you double the radius of a sphere, you quadruple its area. So however much energy was in the original pulse ends up smeared out over the surface of the sphere...since the area of the sphere increases of the square of the range - the energy on each square meter of the sphere follows an inverse square law. It doesn't matter what kind of antenna you use - even if you put out a fairly narrow beam of energy - you're broadcasting over a small segment of the surface of the sphere - which still doubles in area as the distance doubles. The only way to dodge the inverse square thing would be if you could broadcast like an absolutely perfect laser beam - which might never diverge at all. However, even the 'tightest' laser beams do diverge slowly - so the inverse square law still applies to them. SteveBaker (talk) 21:14, 19 August 2008 (UTC)

Yes. For those who's interested, it might be helpful to look at the pyramid and check that the area of the base quadruples in size when you double the height or the length of its sides, regardless of the angle at the apex. So the law holds even when the rays spread only a little. (Since the base we're interested in here is really curved, the rigorous treatment would be found in Spherical cap, but the law holds here too, naturally.)EverGreg (talk) 12:04, 20 August 2008 (UTC)

The magnetic field strength from one pole actually decreases as the square of the distance. The reason dipole field strength obeys the inverse cube law at large distances is that the ratio between the poles' separation and the distance decreases as the observer moves away.

For a radio transmitter, the changing current causes an electromagnetic wave that radiates out in all directions. It is not the electric charge comprising the current that is detected as radio waves; it's actually the changing magnetic and electric fields produced as a result. The wave's intensity does not drop as the cube of the distance because it reflects the energy transmitted in a certain direction, not the force exerted by two poles. --Bowlhover (talk) 06:42, 22 August 2008 (UTC)

Radio astronomy would usually be in terms of a cone rather than a pyramid; the receptor would be in the form of a parabolic reflector having a round periphery. But the same principle of distance and area would apply. Square-periphery radio horns have also been used, resulting in a pyramidical form of the received-energy path. In light astronomy the thought would also be in terms of a cone rather than a pyramid; the light receptor would be a round lens or a mirror. Andme2 (talk) 07:38, 22 August 2008 (UTC)

healing effect of color on the elderly

How does color in the environment help heal the elderly patient76.196.253.118 (talk) 22:25, 19 August 2008 (UTC)

Fascinating question. Has it been shown that color in their environment helps "heal" elderly patients?

Has any particular color been found to be especially effective?

For what sort of conditions is the healing process improved by color? Wanderer57 (talk) 23:16, 19 August 2008 (UTC)

See colour therapy (or color therapy to you).--Shantavira|^{feed me} 08:15, 20 August 2008 (UTC)

I always cheer up when I notice the color of red wine in my glass --- always looking for a sponsor of a long term research project. 93.132.147.52 (talk) 18:36, 20 August 2008 (UTC)

Black holes

I read through the black hole article and found it difficult to comprehend as I'm not so erudite with Science. So I have some questions: 1) Where does the matter go when it is pulled into the black hole? 2) Is there a limit to how much they can "absorb"? 3) If black holes are created via imploding stars, what is underneath/above/around the black hole? Is it just space? I can't imagine anything flat in the middle of space pulling objects towards it. 4) I read through the Hawking radiation article and didn't understand it either. Do black holes ever disappear (or "evaporate")? Does it depend on their size (i.e. Do "smaller" black holes disappear, rather than "big" ones?)? If anyone could answer me these questions in simple laymans terms I would really appreciate it. Thank you in advance :-) Utan Vax (talk) 23:54, 19 August 2008 (UTC)

1) Matter is merely added to the black hole's total mass. It gets bigger the more it "eats".

2) As far as I know, no there isn't. OJ 287 is the most massive black hole we know of with a mass of about 18 billion solar masses.

3) Around a black hole is something called an accretion disk which is basically a giant disk of extremely hot matter surrounding the black hole. There are also polar jet at the poles of the black hole which spew matter and radiation.

4) Yes, they will all evaporate into photons eventually. The bigger the black hole, the slower the rate of evaporation. ScienceApe (talk) 00:22, 20 August 2008 (UTC)

Thank you very much for your answers :-) Utan Vax (talk) 00:26, 20 August 2008 (UTC)

We have to be very careful when saying "It gets bigger the more it eats". The idea is that the forces between the fundamental particles that made up the atoms of the original star are no longer enough to withstand the crushing force of gravity. All of the material in the star collapses to an infinitesimal dot. An object with zero size - no matter what the black hole has eaten, it's still a zero-sized object. (Actually, if it's spinning it can be a small, flat disk - but it would have zero thickness). When we talk about the "size" of the black hole - we're talking about the size of the "event horizon". That's more of a mathematical concept than an actual tangiable object. It's the distance from the infinitesimal point at which nothing (not even light) can go fast enough to escape the gravity field of that infinitesimal point. So as it swallows more stuff (gas, stars, planets, moons, light, little green men), it gets heavier - that means it's gravity field grows stronger - so the distance at which light cannot escape increases. So the size of the event horizon grows - but the infinitesimal dot in the middle is still zero sized - it doesn't grow. But the event horizon itself is nothing special - if you could fall though it, you wouldn't notice anything special happening.

Having said all that - it's really much more messy than that. As things fall towards the black hole, they go faster and faster - until (at the instant they hit the event horizon) they are travelling at the speed of light. But because of relativity, time slows down - and from the point of view of someone outside the black hole, they never actually reach the event horizon - they just get dimmer and dimmer until they are completely dark and splattered flat onto the event horizon with their time stopped dead. This is messy to understand and explain!

SteveBaker (talk) 01:59, 20 August 2008 (UTC)

I should correct a few things. You would notice something at the event horizon. Notably, you would probably be dead because the radiation from accretion disk and polar jets would kill you. Also, depending on the size of the black hole, the tidal forces would kill you too. ScienceApe (talk) 17:55, 20 August 2008 (UTC)

I don't think infalling objects ever reach the speed of light. From an outside observer's reference frame, time slows down and the falling object becomes stationary at the event horizon. For the object crossing the event horizon, a finite gravitational force is applied for a finite amount of time, because the object eventually hits the singularity. It doesn't reach the speed of light because the energy added is finite. --Bowlhover (talk) 06:40, 20 August 2008 (UTC)

Thank you. Intriguing stuff. But there seems to me to be a contradiction here. Black holes "will all evaporate into photons eventually." But "light cannot escape". If the photons cannot escape, they remain part of the black hole don't they? How is the black hole diminished? Wanderer57 (talk) 02:19, 20 August 2008 (UTC)

The way Hawking radiation works, the particles which constitute the radiation actually come into being just outside the event horizon, so are never inside it. It's really weird stuff! --Tango (talk) 02:52, 20 August 2008 (UTC)

Yes, infalling objects don't reach the speed of light at the event horizon. Nothing special happens at the event horizon. But I disagree with the part beginning "From an outside observer's reference frame". The slowdown you're referring to here is a Doppler shift, as I mentioned in the thread below this one. It doesn't have anything to do with reference frames; in fact nothing has anything to do with reference frames. The point of reference frames is that they're all equivalent: you can pick any one to do your calculations in and the result comes out the same. To the extent that you're making physical predictions, such as what's seen by the outside ship, reference frames don't matter. I'm of the opinion that one cannot achieve general relativistic enlightenment until one liberates one's mind from the concept of reference frames. -- BenRG (talk) 14:37, 20 August 2008 (UTC)

I don't understand you... reference frames are extremely important. An external observer sees something very different to an observer that's falling in. It's not just a matter of Doppler shift, it's gravitational time dilation as well (the two are related, however). When we say that all reference frames are equivalent we mean that the laws of physics apply equally to all of them, that doesn't mean they are all the same - different frames observe events differently. --Tango (talk) 17:14, 20 August 2008 (UTC)

(after ec's)

(1) It goes to the gravitational singularity, an mathematical point that contains all of a black hole's mass.

(2) There's no limit; a more massive black hole absorbs more cosmic microwave radiation and evaporates less quickly than a less massive one. There is no feedback mechanism that ejects mass/energy more quickly for more massive holes.

(3) Black holes are singularities with surrounded by event horizons. When an object travels closer to a singularity than the event horizon, it cannot escape. Hence, the space inside an event horizon is closed; there is no path which leads outside.

(4) The cosmic microwave radiation is constantly feeding energy into black holes, and this energy adds mass (remember Einstein's famous equation, E=mc^2, where E is the energy, m is the corresponding mass, and c is the speed of light in vacuum). In order for a black hole to lose mass over time, it must be evaporating faster than the CMB is adding energy. Because the rate of evaporation increases with decreasing mass, only black holes below a certain mass can be gradually wasting away. This mass turns out to be 0.8% of Earth's--must lower than the smallest holes (~3 solar masses) formed by gravitational collapse. All black holes are currently gaining mass instead of losing it.

Note that some black holes may formed shortly after the Big Bang and these can be of any mass. High-energy collisions may also create holes with extremely low mass. However, there is no evidence for the existence of these objects.

Also note that Hawking radiation contains more than photons. Any particle is possible, although according to http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html photons and neutrinos are the main components. --Bowlhover (talk) 02:36, 20 August 2008 (UTC)

The cosmic microwave radiation is constantly feeding energy into black holes, and this energy adds mass OK but Because the rate of evaporation increases with decreasing mass, only black holes below a certain mass can be gradually wasting away. This doesn't follow since at some time in the future the CMB radiation could be lower than the evaporation rate. But we're talking about 10¹⁰⁰ years here. --Ayacop (talk) 08:17, 20 August 2008 (UTC)

Sure, but my intention was to say that all black holes are gaining mass at this moment. To nitpick, the mass limit will exist even far into the future; it just won't be 0.8% of Earth's mass.

Edit: Sorry, I see my post implied black holes cannot evaporate away. --Bowlhover (talk) 11:31, 20 August 2008 (UTC)

Assuming the Lambda-CDM model is right, the CMBR temperature will soon start falling by a factor of 2 every 11 billion years or so, so even a billion-solar-mass black hole will be warmer than the CMBR in less than a trillion years. It will take on the order-order of 10¹⁰⁰ years to evaporate after that, though. -- BenRG (talk) 14:37, 20 August 2008 (UTC)

August 20

Black hole evaporation and time dilation

OK - so I have a question relating to the previous one. So I'm falling towards the black hole in by spaceship - my "mothership" is sitting a safe distance away observing my impending doom. From the point of view of the mothership, time for me slows down until I reach the event horizon and time stops dead. From my point of view, time in the outside universe (including the mothership) must therefore speed up. The closer I get to the event horizon, the faster time zips along. I see stars run out of fuel and die, I see the end of the universe sometime before I hit the event horizon.

But don't I also see the black hole evaporating? So doesn't the event horizon shrink as I approach it? Is it in fact the case that I can survive my death plunge because the event horizon always evaporates faster than I can go towards it? By the time I get there, the black hole weighs so little and is so tiny that I can escape it?

I'm sure there is a flaw in this - and in all likelyhood, the radiation from the rapidly (to me) decaying universe would cook me and spaghettification would have shredded me already. But this is a thought experiment.

SteveBaker (talk) 01:59, 20 August 2008 (UTC)

When the dude from Rush flew into the black hole it took long enough that they made a whole 'nother album. And at least a dozen more after that, so it must take a long time :) Franamax (talk) 02:30, 20 August 2008 (UTC)

The black hole is deeper in the gravitational field than you, so time for it is going even slower, not faster. Time further away from the black hole from you goes faster, time nearer goes slower. --Tango (talk) 02:54, 20 August 2008 (UTC)

OK - I'll buy that...so how come they evaporate at all when seen from the outside? From the point of view of someone a nice safe distance away, time has stopped at the event horizon so how can all of that virtual particle stuff happen? SteveBaker (talk) 03:36, 20 August 2008 (UTC)

It should be that the particle pair-production occurs outside the event horizon and one gets sucked in while the other flies away, that being the allegory usually used for Hawking radiation. So it takes finite time for an asymptotic observer to see the particle that escapes, though it should escape verrry slowly since its "formation". On the other hand, is its formation time well-defined since it's originally a virtual pair? SamuelRiv (talk) 05:13, 20 August 2008 (UTC)

I wouldn't say that "From the point of view of the mothership, time for me slows down". The mothership sees the light that you emit before crossing the event horizon redshifted into the far future. Redshift applies not only to individual wavelengths of light but to the overall duration of events, so the mothership sees you moving very slowly toward the horizon and never reaching it. But that's just a Doppler shift—it has nothing to do with the light as emitted by you or time as experienced by you. There is no reciprocal speedup, and you don't see the whole future of the universe. Here's a Penrose diagram of a classical (non-evaporating) black hole:

       ______*
       \    /\
    inside /  \
         \/ outside
          \    /
           \  /
            \/

This doesn't show the "shape" of a black hole but it shows the causal structure: nothing can travel along a worldline that's closer to horizontal than the slashes / and \. The horizontal line is the singularity and the line dividing "inside" and "outside" is the event horizon. The vertex marked * is future infinity, which is where the outside spaceship ends up. So it's easy to see that the outside ship will never see anything from inside the horizon. But you won't see the whole future of the universe either; no matter where you hit the singularity, your past light cone will exclude some neighborhood of the point *.

Here's a Penrose diagram of an evaporating black hole, taken from Hawking's 1975 paper:

             *
             |\
             | \
         ____|  \
         |  /   /
    inside /   /
         |/ outside
         |   /
         |  /
         | /
         |/

This uses a polar coordinate system and the vertical lines are r=0; nothing special happens there. The horizontal line is again the singularity. From this diagram you can conclude that (a) the black hole won't evaporate before you fall in; (b) you won't see the whole future of the universe before you hit the singularity; (c) the ship outside will see you fall in at the same moment it sees the black hole evaporate (in contrast to the non-evaporating case, where it has to wait forever to see you fall in).

However, it's not known if this diagram is correct, and many people suspect it isn't. So in truth nobody knows. -- BenRG (talk) 13:32, 20 August 2008 (UTC)

Using sig figs in calculations with both addition and multiplication

How do you use sig figs in a calculation when using both addition and multiplication?

For example: Find the molar mass of CO2

In this case, we would do 12.01g + 2*16.00g to find the molar mass.

Do I first round to the correct sig figs for each of the values (12.01g and 2*16.00g) and then add, using the correct decimal places, or should I find the result of (12.01g + 2*16.00g) then round using 4 sig figs?

Although doing it either way produces the same result in this example, in other cases that I've tried, rounding at different points produces different results. What is the correct way, and/or what method is most commonly used? —Preceding unsigned comment added by 68.111.75.89 (talk • contribs) 13:58, 20 August 2008

It's generally recommended to keep things as exact as possible until the very last step, since round-off error has a tendency to accumulate (i.e. the more often you round, the more information you lose, and hence the more likely it is that your final answer will be way off the mark). Confusing Manifestation(Say hi!) 04:01, 20 August 2008 (UTC)

You shouldn't report the final answer to 4sf at all, since you can't be sure it's accurate. Consider if the real values were 12.014 and 16.004 (which round to the values you gave), then the correct answer would be 44.022, which to 4sf is 44.02, however using the figures you started with you would get 44.01. Every time you do a calculation using an approximate value you lose some precision, so you should always report the final answer to a lower precision that you started with (I'm not sure how much lower, I'm sure there's some rule for it, though). --Tango (talk) 17:24, 20 August 2008 (UTC)

Well - be a little careful. You can often start off with a large quantity of inaccurate data and compute an average that has greater precision than any of the individual numbers in the input data. Providing the errors in the inaccurate data are random of course. SteveBaker (talk) 18:48, 20 August 2008 (UTC)

That's pretty much the only exception to the rule (that I can think of, anyway), but yes, you're absolutely right. --Tango (talk) 23:37, 20 August 2008 (UTC)

Hmmm - what happens to precision when you do stuff like squaring a number or calculating a square root? Let's not think in terms of "significant digits" - let's think about the actual worst-case error:

If I have a square, whose side I'm going to explicitly state has a length of 4cm plus or minus 0.5cm...I'm saying that there is a plus or minus 12.5% error. Now I calculate the area of that square: I end up with 16cm² - with some kind of error. We know that the "true" size of my square was definitely somewhere between 3.5 and 4.5 cm so the the area can really only lie between 12.25 and 20.25 cm² - so I should really say that the result is halfway between those answers: 16.25 plus or minus 4.0cm². The error is now plus or minus almost 25%. So squaring this number almost doubled the error.

BUT: Suppose we reverse the operation. If you told me that the area of a square is 16.25cm² plus or minus 4.0 and asked me to calculate the length of the side, I could (if I was being sufficiently anal about it) say that the true area lies between 12.25 and 20.25 - so the result lies between the square roots of those two numbers - which is 3.5 and 4.5 respectively - so I can reliably say that the result is 4.0cm plus or minus 0.5cm. So taking the square root of a number with a 25% error bar produced an answer with a 12.5% error bar. Hence, taking the square root of a number improves the precision of the result! Taking the cube root improves it even more.

In general - there are many situations where a small error in the input to a calculation can produce ENORMOUS errors in the output...and reversing those calculations has the opposite effect.

So no - it's not just averaging. The idea of teaching people to use "significant digits" as if it were a religious matter is bogus - it's a very rough rule of thumb when you need a rough idea of how much precision to put into your answer. Those rules cannot be applied rigorously. We should teach people to use error limits instead. In a bygone era when crunching the numbers was horribly time-consuming, then an approximate way to estimate error was an acceptable way to go. But these days, we can do much better.

SteveBaker (talk) 14:57, 21 August 2008 (UTC)

Earth's rotation

If the Earth is rotating so fast, why don't we feel anything?

If we were to jump 100m above the ground, should the Earth which is constantly rotating move under our feet so that we land somewhere else? —Preceding unsigned comment added by 68.111.75.89 (talk) 04:05, 20 August 2008 (UTC)

The rotation isn't very fast -- it takes a whole day to complete one revolution, and that's why the sensation of turning is far too little to feel. If you were able to jump say 100 km off the ground so that it took you several minutes to fall back down, then the Earth's rotation would accumulate enough for you to notice it.

You think the Earth's rotation is fast because of the linear speed of its surface motion. This is indeed fairly large: at the Equator just over 1,000 km/h or about 650 mph, in the mid-latitudes about 700 km/h or 450 mph. But you are moving along with it at the same speed, so you don't notice anything. It's just like when you walk around on a ship, train, bus, or airplane: you share the vehicle's speed, and you don't stop sharing it when you jump off the floor.

--Anonymous, edited 05:25 UTC, August 20/08.

When you jump into the air, you start off moving sideways at the exact same speed as the earth is moving - the air around you is also moving at the same speed...so there is nothing to "slow you down" - so you and the earth keep moving together until you come back down again. The only slight deviation is if you were to move very fast to the north or south when the "Coriolis effect" would cause you to be deflected off to one side a tiny bit. But the force is very small - you really don't notice it until you've moved a long distance in a relatively short time. Hurricanes feel the rotation - but that's because they are big enough. That's what starts them off spinning - and it explains why they spin clockwise in the southern hemisphere and anticlockwise in the north.

One way to really see proof that the earth is rotating is to make yourself a Foucault pendulum. This is a big, heavy pendulum that can easily swing in any direction and which has enough energy stored in it to continue to swing freely for many hours. If you imagine setting the pendulum up at the North pole - then as the earth rotated - the pendulum would continue to swing in the same straight line. From the perspective of someone slowly rotating with the Earth, it would appear that the pendulum was slowly changing its swing - moving at about half the speed of the hour hand on a clock so it would complete an entire revolution in 24 hours. But the experiment works even if you aren't at the North pole (although it works better the further north or south of the equator you go. SteveBaker (talk) 06:24, 20 August 2008 (UTC)

"... so you and the earth keep moving together until you come back down again" - I figured as much (not that I'm not the OP). :)

This should be equal to jumping in a train (moving at a constant speed), right? (For those who haven't tried, you don't slam into the car's back wall!)-- Aeluwas (talk) 08:55, 20 August 2008 (UTC)

Not exactly equal, but good enough. A person who jumps up initially moves horizontally with the Earth, but the Earth curves away while the person continues to travel in a straight line (Newton's first law of motion). If Earth were perfectly round and humans were capable of jumping straight up, the person would indeed land in a different place --Bowlhover (talk) 11:25, 20 August 2008 (UTC)

Surely the person doesn't travel in a straight line as they continue to be affected by gravity. AlmostReadytoFly (talk) 13:27, 20 August 2008 (UTC)

They would tend to travel in a straight line, not with the Earth (in a circle). --Bowlhover (talk) 20:37, 20 August 2008 (UTC)

Some quantitative calculations:

Because Earth rotates faster at the equator, the maximum centrifugal force an object can experience is felt if the object is at the equator. The force is given by F=mv^2/r, where m is the object's mass, v is Earth's tangential speed of rotation, and r is Earth's radius. r=6378 100 m and v=463.8 m/s at the equator, so every kilogram of mass feels 0.034 N of force. A 75 kg human feels 2.5 N which is about the weight of a 0.26 kg mass. However, the only effect this force has at the equator is weight reduction, and 0.26 kg is not very impressive.

Assuming I've done the math correctly, the maximum horizontal force the centrifugal force can supply is half the force at the equator. At 45 degrees of latitude where the horizontal force is the strongest, a 75 kg human feels a force of 1.25 N pushing towards the south. 1.25 N is small compared to the forces involved in everyday life, so it's not noticed. --Bowlhover (talk) 11:08, 20 August 2008 (UTC)

Two corrections: (i) centrifugal force is a fictitious force - the forces on a object at the equator do not have to balance because the object is not in equilibrium - it is travelling in a circle (well, in a much more complex path if you take into account the Earth's motion round the Sun etc., but we can approximate the motion locally as circular motion); (ii) objects at 45 degrees of latitude do not experience a "horizontal" force because horizontal and vertical are defined relative to local perceived direction of gravity. In other words, a force even as small as 1% of the weight of an object would cause a snooker ball on a smooth table to roll - but we would attribute this motion to the table being slightly tilted, not to a mysterious horizontal force. Gandalf61 (talk) 12:37, 20 August 2008 (UTC)

Oh...Now you've gone and done it! Now I have to rant about people who complain about use of the term "centrifugal force". (a) It's a very handy shorthand that makes a lot of calculations easier to do in a rotating frame of reference. Anyone who gives a damn is fully aware of it's "fictitious" nature - so we don't need to be corrected - Mmmm'k? Thanksmuch. (b) Nearly every force we use in physics is "fictitious" in some way or other. You rest a brick on a table and Newton tells us that the table produces an equal and opposite "force" that opposes gravity...but really, that's the nuclear forces acting between all of the molecules of the table and brick. Gravity is considered a "force" - but really it's only an aspect of the curvature of space-time. Friction, drag, lift (as in airplanes), coriolis, air pressure...are all "forces" that don't really exist. If we have to expand every explanation down to the "fundamental" level - we'll never get any work done! Why does everyone pick on poor old super-handy centrifugal force for making nit-picky academic points? Bah! I intend to continue to use the term whenever and where ever it's convenient - I very much doubt that anyone will be even slightly confused. SteveBaker (talk) 16:24, 20 August 2008 (UTC)

...and the real reason you don't feel that lateral force is because the earth isn't a sphere. It's distorted into an oblate spheroid precisely because of the effect of (yes, Gandalf61: I'm going to say it) centrifugal force. Hence "horizontal" as defined as "tangential to the mean sea level surface of the earth" is handily orthogonal to the net local gravity+centrifugal vector...so you don't feel any lateral force - no matter how small. If you did - then the water in the ocean would flow sideways and thereby automatically adjust. "Mean sea level" would (as if by magic) correct for it. Hence, no lateral force. SteveBaker (talk) 16:31, 20 August 2008 (UTC)

Well, I wouldn't use sea level to define horizontal and vertical - I would use a spirit level and/or a plumb line. But we reach the same conclusion - there is no "horizontal" force. Any chance you might stop ranting at me now ? Gandalf61 (talk) 16:41, 20 August 2008 (UTC)

The ocean is a VERY big spirit level - it'll do just fine! Why would you trust a tiny little amount of liquid sloshing in a tube when you can use an entire ocean? As for the rant...er, sorry - but sadly, I just started another rant (below) about your incorrect ideas about friction! It's nothing personal...honest! :-P SteveBaker (talk) 17:31, 20 August 2008 (UTC)

I can't believe I forgot "down" is not necessarily "towards Earth's centre". Anyways, to SteveBaker: your rants, particularly those concerning angle of attack and Bernoulli's principle, have become famous on the science Reference Desk. See http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Archives/Science/2008_August_4#Why_can.27t_bullets_fly_.3F, for example. --Bowlhover (talk) 22:07, 20 August 2008 (UTC)

The answer here is strikingly simple. The real question, buried in this specific example, is what approximations are valid in this scenario. If you jump 100 meters, the scale involved is sufficiently small that gravity can be considered a constant force, the earth can be considered flat, and so forth; and you have a purely ballistic trajectory. This is a very easy math problem and is often taught in algebra-level introductory physics. If you wanted to jump, say, 100 kilometers, the scales would be quite different and the approximations would be very inapplicable, and you would have to use orbital mechanics to determine the new trajectory. There are some "universal" details you can still count on - momentum will be conserved, inertia still exists, energy is still conserved - only, your equations will be a little messier. Nimur (talk) 17:01, 20 August 2008 (UTC)

How a redshift affects the appearance of the moon on the horizon

A recent disscussion a got involved in has me curious and I need help in sttling an argument and that is wether or not the earth's gases are more condensed at the equator when the moon rises in the evening causing it to look oblong or just merely bigger than say half an hour after the moon rises. I know that the moon is only one size but is there certain times of the year (eg. summer equinox to fall equinox) when the redshift seems to make the moon appear more oblong or oval shape than other times of the year? —Preceding unsigned comment added by 69.31.224.52 (talk) 04:33, 20 August 2008 (UTC)

There are really three parts to this answer:

• Red-shift doesn't apply to the moon to any measurable degree. Red-shift happens due to the doppler effect when an object is moving away from you at some significant fraction of the speed of light. For objects that are an ENORMOUS distance away, the expansion of the universe makes them seem to move away from us faster the further away they are - so they red-shift. But the moon stays pretty much the same distance from us all the time and it's not far enough away for the expansion of the universe to be measurable - so definitely no red-shift for the moon. The appearance of a red-shifted object are that it's colours become more reddish - it has nothing to do with the apparent size of the object.

• The moon sometimes looks distorted when it's close to the horizon because of heat trapped close to the ground - hot air diffracts light differently from cold air. That explains the oblong/oval appearance - and it's more obvious in the summer when the ground is hot and more obvious still at the equator where the ground is hottest of all.

• The apparent change in size of the moon is an optical illusion. That illusion happens just as much in winter as in summer - so it's not related to the layers of hot air thing. We have an article about that illusion here: Moon illusion (of course we do!)

I think that clears up everything.

SteveBaker (talk) 06:08, 20 August 2008 (UTC)

And just for fun, the moon does periodically appear larger and smaller, but that's an artifact of its elliptical orbit and it's not of a magnitude remarked on by the casual observer. — Lomn 13:03, 20 August 2008 (UTC)

"The appearance of a red-shifted object are that it's colours become more reddish..." Not quite. redshift makes the wavelengths all increase, so red becomes closer to infrared, green becomes closer to red, blue becomes closer to green, violet becomes closer to blue, and ultraviolet becomes closer to violet. — DanielLC 15:51, 20 August 2008 (UTC)

size of earth

What is the size of earth? —Preceding unsigned comment added by 68.111.75.89 (talk) 04:46, 20 August 2008 (UTC)

You may want to check out our article for Earth, which, among other things, includes this information. Specifically, you want to look at "physical characteristics" in the sidebar. -- Captain Disdain (talk) 05:35, 20 August 2008 (UTC)

There's an entire field devoted to measuring the Earth: see geodesy. --Bowlhover (talk) 06:05, 22 August 2008 (UTC)

Banked Corner

Why does a car going around a banked/sloped corner slip down if it is not going fast enough?

125.238.173.197 (talk) 07:37, 20 August 2008 (UTC)

The same reason anything slides down a slope: because of gravity. But if a car is going fast enough on a banked turn, then its effective centrifugal force forces it away from the centre of the curve, keeping it up the slope. AlmostReadytoFly (talk) 08:21, 20 August 2008 (UTC)

Does it? It would be a very poorly designed road (and tyres) if a car "slipped down" because it wasn't "going fast enough". Vehicles need to be able to stop safely on any road. Even in icy conditions the camber is unlikely to be so great that this would happen.--Shantavira|^{feed me} 12:55, 20 August 2008 (UTC)

Oh, real roads. For the sake of the problem, I assumed frictionless roads. ;-) AlmostReadytoFly (talk) 13:23, 20 August 2008 (UTC)

While it's always dangerous to assume things, here in New England, winter icing occasionally produces conditions that are virtually friction-free and cars do, indeed, sometimes slip off of roads (and driveways!) in surprising directions including down a superelevated turn.

Atlant (talk) 14:46, 20 August 2008 (UTC)

Depends on the road and the anticipated speed. On the U.S. interstate, curves rarely exceed a one degree curvature and are banked to allow vehicles to negotiate them safely at 70 mph. At that speed, you don't need a whole lot of banking, but there are those who end up testing (and exceeding) the limits. By contrast, Phoenix International Raceway has curves with 9 degree and 11 degree banking. Record qualifying times are around 135 mph. — OtherDave (talk) 13:27, 20 August 2008 (UTC)

Are you perhaps asking about banked race-tracks? The Talladega Superspeedway Nascar track has a 33 degree banked corner. So if you parked your car on a 33 degree slope - would it slide down? Well, if it was icey, it certainly would. I know my car wouldn't slip because I have a lateral accellerometer (for measuring cornering forces in Autocross driving) - and I can exert almost 1g laterally before the car will slip - and that's when the wheels are rolling...you get better grip when parked. A 33 degree slope doesn't come close to producing a 1g lateral force. But a heavier car with narrower and worn-out tyres and poorly designed suspension might maybe slide on a 33 degree slope...it's hard to get lateral g-force data so I don't see how we could know for sure. SteveBaker (talk) 16:03, 20 August 2008 (UTC)

Umm .. isn't the coefficient of friction between tyres and road surface the only important factor as regards sliding down the slope when stationary ? Surely weight of car and width of tyres are irrelevant here ? Physics 101 says car can sit on 33 degree lateral slope without sliding as long as coefficient of friction is at least tan 33 degrees, which is about 0.65 (of course if its centre of gravity is too high, it might tip before it slips). Gandalf61 (talk) 16:23, 20 August 2008 (UTC)

This has been discussed before and in far greater detail (as well as by people who understand it far better than I), but friction is an excellent example of where a 101-abstraction maps incredibly poorly to physical reality. — Lomn 16:50, 20 August 2008 (UTC)

Surface area doesn't matter? Really?

Yep - that "friction doesn't depend on surface area" thing is complete nonsense out here in the real world. Take a look at a Formula I race car...or a dragster...what do you notice about the tyres? Kinda wide aren't they! If narrow tyres would produce just as much friction, why don't they use super-skinny tyres to save weight? Huh...maybe there's a reason for that? I drive Autocross events where grip is everything. Wide tyres make a SPECTACULAR amount of difference to cornering grip. Feynman's lectures on physics (which you certainly should have read in Physics 101) has a delightful rant about the "laws" of friction.

From our article Friction:

"This approximation mathematically follows from the assumptions that surfaces are in atomically close contact only over a small fraction of their overall area, that this contact area is proportional to the normal force (until saturation, which takes place when all area is in atomic contact), and that frictional force is proportional to the applied normal force, independently of the contact area (you can see the experiments on friction from Leonardo Da Vinci). Such reasoning aside, however, the approximation is fundamentally an empirical construction. It is a rule of thumb describing the approximate outcome of an extremely complicated physical interaction. The strength of the approximation is its simplicity and versatility – though in general the relationship between normal force and frictional force is not exactly linear (and so the frictional force is not entirely independent of the contact area of the surfaces), the Coulomb approximation is an adequate representation of friction for the analysis of many physical systems."

SteveBaker (talk) 17:25, 20 August 2008 (UTC)

The design of F1 tyres is governed by many engineering factors. Skinny tyres would have to be inflated at higher pressures. Higher pressures require thicker tyre walls and increase tyre wear. Skinny tyres would also deform more under high downforces and when cornering, when they must communicate lateral forces to the axles and chassis. Wider tyres are less affected by unevenness and small patches of oil on the track. Bicycles and motorcycles manage very well on skinny tyres. I suggest that the idea that lateral friction increases with tyre width is not as obvious as you say. Gandalf61 (talk) 22:34, 20 August 2008 (UTC)

We had this EXACT same argument on this very page two years ago - and you were wrong then too, Read Feynman's Lectures on Physics - then tell me that someone with a nobel prize in physics didn't understand friction as well as you do. SteveBaker (talk) 13:54, 21 August 2008 (UTC)

Steve - as I said before on your talk page, the tone that you are using in responses like this is somewhat offensive and is inapproriate for the RDs. Perhaps you could tell us exactly what Feynman says on the subject or provide a relevant link ? I think that would be more instructive than taking cheap shots at me - although possibly less satisfying for yourself and less entertaining for other readers. Gandalf61 (talk) 14:28, 21 August 2008 (UTC)

The last time (of several times) I discussed this here was: Spherical wheels on cars - I refer you to that discussion. SteveBaker (talk) 17:54, 21 August 2008 (UTC)

(unindent) Okay, I have reviewed that thread. I see that Feynman says the standard model of friction is a "good empirical rule" that is correct in "certain practical or engineering circumstances". The quote from our friction article that you gave above says it is "an adequate representation of friction for the analysis of many physical systems". One further source - Hyperphysics, Georgia State University says

“ Part of the standard model of surface friction is the assumption that the frictional resistance force between two surfaces is independent of the area of contact. While exceptions exist, the assumption has enough validity to be useful in many circumstances. For example, it implies that wide tires will not in general give better traction than narrow tires, and will not change the normal braking distance for a car. Better traction can be obtained with wide tires, or tires with lower pressure, if the pressure changes the coefficient of friction, as on a surface of snow. ”

So all these sources agree that the standard model is an empirical law that has limitations, but is valid in many scenarios. I see no reliable sources that support your bald assertion that it is "complete nonsense out here in the real world". Gandalf61 (talk) 20:38, 21 August 2008 (UTC)

What use is it if it has all of these exceptions? We shouldn't be calling it a "law" - that's dangerously deceptive - at best it's a rule of thumb - but not really even that because it's stating that the contact area doesn't matter (which is a qualitative statement) when in fact it should only ever be a quantitative statement: "under a lot of circumstances the effect of the contact area is negligable". Teaching it the way they do in schools elevates a hackish approximation that only works over some limited set of conditions - to the status of a Law like the laws of motion. There doesn't seem to be any well-defined set of circumstances when it works and when it doesn't. At least when Newtons laws of motion break down, we can say "they are accurate at speeds that are a small percentage of the speed of light". There is no such clear guideline for the applicability of F=un - just some vague hand-waving that says that it sometimes works and sometimes doesn't! That's hardly useful science! I have no idea when it's going to work or when it's a case when it's wildly incorrect (as is CERTAINLY the case with car tyres). It even fails to take account of the wildly different numbers you get for static and dynamic friction (a critical factor in answering this very question in fact).

Empirical rules that produce only small errors - or which work well - but only over a carefully specified range of conditions - are useful. Rules that just randomly fail without warning are useless! This is one of those. I drive an autocross car - I have an accelerometer in there with me so I can measure the force exerted by the tyres on the road by turning off the ABS, getting the car up to (say) 30mph, standing on the brakes and watching the accelleration. I tune things like braking distances by doing careful experiments. There are a lot of variables here - but I can control for most of them - the exception being the tread pattern...but we can figure that into the area calculations. The wide tyres give me about 30 to 40% more deceleration than the skinny ones made from the same rubber compound - and they have a "contact patch" which is about twice what the skinny ones have. That says that for rubber on asphalt at the pressures you get with a car, the frictional force is almost proportional to the contact area...that's a lot different from "negligable".

F=un says nothing about whether the surfaces are moving or static - and that's crucial to how wheels work without sliding about all over the place. The reason cars have ABS braking these days is 100% because the "Law" of friction doesn't work. SteveBaker (talk) 00:53, 22 August 2008 (UTC)

Brass shell casings corrosion rates in soil

I am trying to find out what exactly would be the effect on a 9mm brass shell casing after sitting in Frelsburg clay soil for 15 years virtually undisturbed. The casings were supposedly ejected from an undetermined semi-automatic weapon and therfore would not have been buried too deep in the backyard of a house in Burton Texas.

What type of corrosion would one expect to see on this fired brass cartridge casing which is 70 % copper and 30 % zinc after sitting the the clay for 15 years?

Should one expect just/only some discoloration, or would the metal actually be compromised to the point of seeing actual physical damage like decomposition, or just plain disintegrated ? These casings were not more than 1 to 6 inches beneath the ground's surface and were found with a hand held metal detector.

thanks ! I am trying to determine if some evidence in my friends murder trial was planted by the Harris County Sheriff's Cold Case Squad.

No one bothered question the integrity of the so-called evidence. —Preceding unsigned comment added by Pamfrancis (talk • contribs) 08:33, 20 August 2008 (UTC)

While I am skeptical of the value of using Wikipedia to refute police evidence, various references suggest that there would be little corrosion in the environment you describe: [CuZn30, CuZn33, CuZn36 and CuZn37 tarnish slowly to a uniform dull bronze colour with no pitting or localised attack. Hope that helps — Lomn 12:59, 20 August 2008 (UTC)

Lomm - the webpage you referenced above is from the metal expert /metallurgist.that we have hired - David Hendrix. The issue is that the shell casings that were extracted from the yard 13 years after Steve lived there( They were retrived in 2001) look pretty good. Other shell casings that did not match the crime scene casings through ballistics, extracted from the same yard - were too corroded for the Sheriff's office balistic expert to even test. Only the ones that matched balistically were in good condition, everything else was of no value ( looked like hell - as they should after 13 years) So why would these magic matching shell casing be in a state that he could use to examine match and the others were not - same yard, same amount of time, same clay soil. Also the Sheriff's didn't bother to document said extraction with photos, a diagram, a video, any witness, nothing. NO law enforcement person would skip these vital steps, especially since the location and the legitimacy of this extraction screams the need of this kind of documentation to prove that they got them from where they said they did. Pamfrancis (talk) 22:21, 21 August 2008 (UTC)

Pamfrancis, what you really want to do is talk to your friend's lawyer and a forensics expert, not a bunch of people on the internet. There are undoubtedly a lot of factors that are going to affect this whole thing that you either aren't aware of or haven't thought to mention, and anything we say is going to be a really crappy basis for any decisions you make. If this was for a hobby, that'd be one thing, but if this is really a murder trial, you don't want to take chances. (That said, though, I'm pretty skeptical that cops working on a cold case investigation would plant evidence unless they had some really, really strong motivation to do so. I mean, sure, some cops are just plain bad, but even then they don't really do this kind of stuff just because. It might be one thing to close an active case and get that stat, but people working on 15-year-old cold cases aren't generally expected to crack them, since they're, well, cold cases.) -- Captain Disdain (talk) 13:17, 20 August 2008 (UTC)

I agree that you should seek an expert - but I've gotta say that it doesn't look good. Brass is pretty corrosion-resistant. Metal detector enthusiasts frequently find brass artifacts that have been buried for a hundred years or more and they pretty much look new. This for example. It's going to turn a dull brown - but you aren't looking at the sort of corrosion you'd expect from iron or something. I really doubt you'll prove anything this way...particularly if you're trying to prove something as controversial as "planted evidence". But find a lawyer and an expert...don't take my word for it. SteveBaker (talk) 17:11, 20 August 2008 (UTC)

I am going to answer both Captain Disdain and Steve B here - The fact that only the matching shell casings look good and were able to be tested by the sheriff's ballistic expert is what makes this so questionable- I have hired a lawyer , I have all sort of experts lined up and ready to go now that we have the courts permission to view/test the evidence shell casings introduced at his trial in 2004. In the beginning, they couldn't find them. That was the first 2 years, then they found them, Now the judge in the case has given us his OK. Remember this is Texas, if you are indicted, your guilty. The Sheriff's don't care, that's there job. Both the DAs and the law enforcement believe it is their job to get an indictment - it is the courts job to throw out the evidence, or the defense lawyers job to show reasonable doubt. Neither happened - and i won't even go there - Remember I have been workin on this since the day of his conviction 4.5 years ago. This evidence was collected by the newly formed Cold Case Squad. It is the only "evidence' they could get on Steve who was never even a suspect in the oriiginal investigation. thanks for the help! Information is information where ever I get it, even on the internet. :)Pamfrancis (talk) 22:16, 21 August 2008 (UTC)

Plus I have a metal detector on my porch that I am going to use to go to the same yard to see if I can find some more shell casings, that look like they should - OLD - Pamfrancis (talk) 22:21, 21 August 2008 (UTC)

Brightness of the Sun and other stars

If the Sun were compared to a light bulb, what is its total output in lumens? Is the lumen output of a star proportional to the 'luminosity' as on http://en.wikipedia.org/wiki/Main_sequence_stars , or is it different due to being weighted to a different spectral response curve? If it is different, what is the approximate lumen output of each class of main sequence star?

-User: Nightvid (unregistered) —Preceding unsigned comment added by 128.8.238.179 (talk) 13:54, 20 August 2008 (UTC)

I'm not sure about light bulbs but this source says that "The ratio of sunlight to candlelight is about 259,000:1". —[[::User:Cyclonenim|Cyclonenim]] ([[::User talk:Cyclonenim|talk]] · [[::Special:Contributions/Cyclonenim|contribs]]) 14:14, 20 August 2008 (UTC)

That's just meaningless! At what distance from the candle? At what distance from the sun? If you are 10 feet away from the sun - it's a heck of a lot more than 259,000 times brighter than being 10 feet from a candle. If you are a billion miles from the sun and only one centimeter from the candle - then the candle is considerably brighter. You're quoting this number accurate to better than 1% - but there is considerably more than 1% difference between the brightness of the brightest and dimmest candles. This is the kind of crappy "factoid" that gets the world in trouble! What the article actually SAYS is that this is at 3 feet from the candle and at the surface of the earth - so at 93 million miles from the sun. (There is still far too much precision in the answer though.) SteveBaker (talk) 15:36, 20 August 2008 (UTC)

Well, the issue is how to measure "brightness", and the answer is measure one of the following "more pure" quantities:

• total power output
• power received at fixed distance per unit-area

Furthermore, you can adjust the "total power" to restrict only to "power emitted in visible light spectrum", or to normalize the power-vs-frequency profile based on an approximation of human vision, or any other contrived scheme. We have empirical units such as watts, candela, lumen, etc... Proper usage of these units requires understanding of what you are actually measuring. Nimur (talk) 17:07, 20 August 2008 (UTC)

That's as perceived on earth, though, right? I think the OP wants to compare actual light output. To the OP: yes, luminosity of stars is measured in terms of the raw power output, while the definition of the lumen is weighted towards wavelengths the human eye perceives strongly, so there won't be a direct relationship. I don't know how big a difference this will make or the answer to your last question. Algebraist 14:22, 20 August 2008 (UTC)

In lumens: A candle (nominally 1 candela) outputs 4 x pi lumens. The sun outputs 3.75 x 10²⁸ lumens. So the sun produces about as many lumens as 3x10²⁷ candles...3,000,000,000,000,000,000,000,000,000 ! Because we're talking lumens (which are adjusted to allow for the nature of human vision) and candle flames vary a bit in color and intensity, this is a very inexact comparison. SteveBaker (talk) 15:36, 20 August 2008 (UTC)

Luminosity of stars is generally stated in terms of Absolute magnitude - which uses the sun as a reference. The trouble is that this term derives from a time in history where someone just kinda eyeballed a star and guesstimated how bright it was. Nowadays there is a mess of derived terms - "Absolute Visual Magnitude", "Absolute photographic magnitude", "Absolute bolometric magnitude" - and they're all different. But Absolute Visual Magnitude could be roughly compared to a scale of lumens because they both talk about total energy output in the range of the human visual system. Remember though that magnitude is a logarithmic scale. SteveBaker (talk) 15:43, 20 August 2008 (UTC)

So using the sun's 3.75 x 10²⁸ lumens as a reference, the lumen output of other stars can be calculated by converting the "Absolute Visual Magnitude" to lumens, although this isn't exact due to magnitudes based on "eyeballing", right? Then my last question is equivalent to (short of a conversion factor) "What is the Absolute Visual Magnitude of each class of main sequence star?".

-Nightvid —Preceding unsigned comment added by 128.8.238.179 (talk) 21:22, 20 August 2008 (UTC)

But it's not that simple. Stellar magnitudes are represented on a logarithmic scale and most of them are negative numbers. A magnitude -3 star is 2.512 times brighter than a magnitude -4 star which is 2.512 times brighter than a magnitude -5 star. So you CAN'T simply say the conversion factor from magnitude to lumens is the sun's brightness in lumens divided by the brightness in absolute magnitude. You need some kind of equation which is going to have logarithms and stuff in it. I'm too tired to figure it out right now - but I'm sure someone here will do it. SteveBaker (talk) 03:22, 21 August 2008 (UTC)

(to Nightvid) I think you misunderstood slightly. Steve was saying that the visual magnitude system was based on eyeballing for most of its history, which started during the time of Hellenistic Greece. Since the human eye is not very accurate, cameras were later used to measure brightness, but early films were much more sensitive to blue light than red light. Hence, blue stars appeared much brighter to cameras than to the human eye, while red stars seemed to be much dimmer. The photographic magnitude system was invented as a result. Bolometric magnitude was later used to express a star's total luminosity for all of the electromagnetic spectrum, not just the visible band.

The measurement of brightness in astronomy is called photometry; if you read the article, you'll find it's a very exact science using expensive CCDs and software. The "V" band corresponds to the visual band, so this is the band to search for on Google for data on precise visual magnitudes.

In the magnitude system, every five-magnitude increase represents a dimming by a factor of exactly 100. Hence, a magnitude-0 star is exactly 100 times as bright as a magnitude-5 star. Since this system is logarithmetic, each magnitude represents an increase or decrease in brightness by a factor of 2.512 (the fifth root of 100).

The spectral classification article has data on the luminosities of stars in each class. The Sun has a bolometric magnitude of 4.75, so you can easily calculate the absolute magnitudes represented by the data. An O-type, for example, outputs ~1 400 000 times as much power as the Sun and has a magnitude lower by log (base 2.511) 1 400 000. That's about mag -10.62. If you want figures for visual brightness, use this conversion table. The data in the spectral classification table is so approximate that this is not necessary, but it's important for specific stars where exact photometric data is available. --Bowlhover (talk) 08:33, 22 August 2008 (UTC)

Scientific/medical reasons for Heartbreak

What are the scientific/medical reasons for experiencing heartbreak? What kind of person or what condition does a person have that cannot experience heartbreak? --12.33.211.29 (talk) 18:03, 20 August 2008 (UTC)

We have an article: Broken heart (also see grief as heartbreak is considered to be a form of grief-reaction). As for those who cannot experience heartbreak, well, that could only happen if they didn't care in the first place i.e. sociopaths, and psychopaths but even these may feel selfishly-motivated grief. Fribbler (talk) 18:07, 20 August 2008 (UTC)

Also find Broken Heart Syndrome for the physical effect of negative emotional trauma. Julia Rossi (talk) 06:26, 21 August 2008 (UTC)

Tea

Tea is made from the leaves of Camellia sinensis. Can a similar brew be made from other species of Camellia? DuncanHill (talk) 18:29, 20 August 2008 (UTC)

You could brew/seep just about any plant matter in water to make a at least a mildly flavored drink. It's a question of whether it tastes good enough to drink. I know, for example, Slashfood (a cooking and food blog) discusses "backyard teas" a few times. These are teas made not from Camellia sinensis, but from plants you would find in your backyard. Rooibos is a popular tea alternative as well. I assume tea could be made from other species of Camellia, but I have not found any evidence of any commercial production of such teas. --Russoc4 (talk) 00:36, 21 August 2008 (UTC)

You might want to be a bit more careful than that though - lots of plants are poisonous or have very serious effects on your health. I'd want to check before making drinks from any old random plant! SteveBaker (talk) 03:14, 21 August 2008 (UTC)

Under tomato Wikipedia says, "tomato leaves and stems actually contain poisonous glycoalkaloids, but the fruit is safe". A rather common flowering shrub planted in gardens is the oleander. Here is what Wikipedia has to say about this plant. "Oleander is one of the most poisonous plants and contains numerous toxic compounds, many of which can be deadly to people, especially young children. The toxicity of Oleander is considered extremely high and it has been reported that in some cases only a small amount had lethal or near lethal effects (Goetz 1998)." Andme2 (talk) 03:27, 21 August 2008 (UTC)

Well, yes, obviously you can't use anything you find. I thought common sense would dictate that. That's also why I only provided the link for some safe samples. Apparently it's not enough to help answer a person's question. --Russoc4 (talk) 15:01, 21 August 2008 (UTC)

Another note, the article Herbal tea suggests that "An herbal tea, tisane, or ptisan is an herbal infusion made from anything other than the leaves of the tea bush (Camellia sinensis)... In some countries (but not in the United States) the use of the word tea is legally restricted to infusions of Camellia sinensis (the tea plant)." These are not cited though. --Russoc4 (talk) 15:14, 21 August 2008 (UTC)

I've been looking, but I have not found any discussion on caffeine in Camellia plants other than Camellia sinensis. --Russoc4 (talk) 15:26, 21 August 2008 (UTC)

A lot of fruit plant leaves can be used for tea, and lemon, orange, grapefruit leaves even contain a bit caffeine. Tasteful too are strawberry, raspberry and the currants. --Ayacop (talk) 16:26, 21 August 2008 (UTC)

I was really just asking about Camellia. An estate in Cornwall has started growing camellia sinensis, and producing tea commercially from it. I was wondering if any of the other camellias which flourish in Cornwall could be used to make a drink like tea (that is, actually tasting like tea, not like raspberries, or chamomile, or suchlike). DuncanHill (talk) 16:56, 21 August 2008 (UTC)

Again, it's theoretically possible, but there does not seem to be any commercial value to Camellia plants other than the one. --Russoc4 (talk) 19:42, 21 August 2008 (UTC)

Black holes again: interior

It was stated previously that the mass of a black hole would be concentrated within a single point of the interior. I doubt this (as a general case, at least) based on the following: think of the earth and the surrounding galaxy, think of the interstellar space and the neighboring galaxies. The space there has a density, widely varying, but none-zero and (?) above a certain ground level (the cosmic background radiation, at least). Now think of a sphere with radius R around the earth (or any other point you like). Think R grow bigger and bigger and consider the mass contained. As the Schwartzschild radius of any mass is growing linearly with mass, but at constant (minimal) density, the mass grows cubic with radius, at some radius the mass contained within will will be contained entirely inside its Schwarzschild radius --- it will be a black hole, if looked at from outside. To cast it into a question: are we inside a black hole, if looked at from far enough away? 93.132.147.52 (talk) 18:54, 20 August 2008 (UTC)

I think you're describing the cosmological horizon. While it's a form of an event horizon, it's not classically understood as a black hole. — Lomn 19:02, 20 August 2008 (UTC)

I don't think that's what he or she is referring to. Imagine an infinite, homogeneous medium. Pick a point in that medium, and imagine a sphere centered on that point, of radius r. As you increase r, the enclosed mass will eventually exceed c²r/2G. The OP seems to believe that, at this point, r will be an event horizon, because it is the Schwarzschild radius for the enclosed mass. But the enclosed mass will not (necessarily) collapse to a singularity, and it's the singularity that results in the event horizon. A singularity severely warps spacetime; a homogeneous medium does not. (The OP actually allowed for an inhomogeneous medium, like the real universe, but for moderate clumpiness that's not forming singularities, you can just imagine a homogeneous medium.) -- Coneslayer (talk) 19:56, 20 August 2008 (UTC)

It depends on the shape of the universe, but it's quite possible that the (observable) universe is a black hole. If so, it just hasn't finished collapsing yet. --Tardis (talk) 20:02, 20 August 2008 (UTC)

The Schwarzschild solution and hence the Schwarzschild radius, implicitly assume that expansion is neglible in the local neighborhood of the black hole. As we know the expansion of the universe is not neglible when applied to the universe as a whole, these solutions are not applicable whem considering the universe as whole. Nonetheless, it is still possible that the universe as a whole does constitute a type of black hole (though recent measurements make this increasingly unlikely). If so, the singularity for the universe would occur in the future via the Big Crunch. More generally, all the mass inside a black hole must reach a singularity in finite proper time (time as measured by a local observer). So it is possible for transient inhomogenuities in mass to exist within a black hole, but they would have to be eliminated eventually. Dragons flight (talk) 20:14, 20 August 2008 (UTC)

This stuff is regularly fantastic. It's impossible to tell when you people are kidding. Wanderer57 (talk) 20:17, 20 August 2008 (UTC)

I've run into editing conflicts now several times. I'm not sure if I should get annoyed that I have to retype so often or to feel happy that the topic has caught so much attention --- I'm inclined to the later. As I can tell for kidding, I'm pretty sure for the OP and quite sure for the answers that the subject is treated seriously. 93.132.147.52 (talk) 20:26, 20 August 2008 (UTC)

OFF TOPIC TIP: If you get hit by an edit conflict, hit the "BACK" button on your browser - it should take you back to the editing window. Use the mouse to select the text you typed and use Ctrl-C to copy it into your clipboard. Now you can navigate to the newly edited version of the page (I usually hit the "BACK" button again then hit "RELOAD" to refresh it). Then you can hit the [EDIT] widget again and use Ctrl-V to paste your text back in again. Then you can hit "Save page" again - and this time, so little time will have elapsed between hitting EDIT and SAVE that it's very unlikely anyone will get in ahead of you. SteveBaker (talk) 03:12, 21 August 2008 (UTC)

You've clearly never had to write a reply on Keeper76's talk page during a busy period. Crickey. —[[::User:Cyclonenim|Cyclonenim]] ([[::User talk:Cyclonenim|talk]] · [[::Special:Contributions/Cyclonenim|contribs]]) 09:07, 21 August 2008 (UTC)

I was not thinking on the effects of totally (that's to say, sufficiently for the effect) homogeneous medium outside the black hole. That splits my question into two: what if the density would drop just outside the schwarzshield radius, leaving uncounted galaxies, unwilling to collapse to a singularity right now, inside? And what if two black holes of equal mass would decide not to drop into each other but to orbit around the common barycenter? And more, if there where three BHs, would it be possible for one of them to bend spacetime as much that it could be expelled? 93.132.147.52 (talk) 20:49, 20 August 2008 (UTC)

Anything inside a blackhole must collapse into a single singularity within finite time. General relativity does not permit any permanently stable orbits in the interior of a black hole. Achieving a stable orbit inside the event horizon, relative to the singularity, is functionally equivalent to going faster than the speed of light, i.e. it's impossible. Dragons flight (talk) 21:16, 20 August 2008 (UTC)

Thats common talk, but just why? From inside, there doesn't seem to be a need for it. And finite time for what kind of observer? Inside or outside? By the way, I've never been convinced that faster then light is impossible. 93.132.147.52 (talk) 21:30, 20 August 2008 (UTC)

Finite "proper time", which means time as measured by the object itself (the one that's falling in). From the point of view of an external observer, it takes an infinite amount of time just to reach the event horizon. If you don't accept the speed of light as a fundamental limit, then you have to reject the whole of relativity which includes the whole existence of black holes (some other theory may have something similar, but it won't be the same as what we know as a black hole), so the question becomes moot. If you accept the basic principles of GR, then it the maths is quite clear that objects that cross the event horizon have to hit the singularity in finite proper time. I'm not sure how to explain it without the maths, hopefully someone else can. There is a special case which is easy to explain, though - the case of an inert falling object, one without any kind of engine. If it's not going to hit the singularity it needs to either go into orbit, or leave the black hole entirely, and orbital velocity and escape velocity can both be calculated using simple Newtonian mechanics and you can see from their formulae that they would be greater than the speed of light once you cross the event horizon (actually, orbital velocity is greater from a even greater distance). --Tango (talk) 23:52, 20 August 2008 (UTC)

I feel slightly misunderstood. For the speed of light, I see that it's impossible to accelerate as much as to (exactly) reach it or exceed it, but I can't see that it would be ruled out that something could be faster than light (space like) right from the start.

Your "Anything inside a blackhole must collapse into a single singularity within finite time" holds in the simplest possible case (Deriving_the_Schwarzschild_solution#Assumptions_and_notation) but I don't see or have references if this holds for more complex situations, too. Although, an object moving in this metric must be small enough not to disturb the metric 93.132.168.99 (talk) 08:14, 21 August 2008 (UTC)

User:93.132.168.99 - you aren't misunderstood. I perfectly understand what you are saying - but it's still wrong. Objects moving faster than light ("tachyons") are completely forbidden by the very same math that prevents a normal object from attaining light speed. It's not like the situation with photons which can only move at the speed of light because they started off moving at that speed and have zero rest-mass. When you calculate the mass or length or time-dilation factor for tachyons using the lorentz transform, you end up having to calculat the square root of (1-v²/c²). If v (the velocity of the object) is greater than c (the speed of light) then v²/c² is bigger than 1.0 - which makes one-minus that be a negative number. Stick a negative number into your pocket calculator and hit the square root button! It says "ERROR"! When you take the square root of a negative number you get a 'complex number'. That would leave mass/length/time for a tachyon to be a complex number. But it's a rule that no matter what, no "real world" result can ever be a complex number - they are merely a convenience for doing calculations - in absolutely ALL real world calculations, the complex number ends up dropping out of the calculations before you get the answer. That wouldn't happen for tachyons. Therefore it's very safe to assume that they don't/can't exist. You can also show that such objects would require an infinite amount of energy in order to slow DOWN to anything less than infinite velocity. Something that has infinite velocity can't be at any place in space where you might observe it because it's gone already. Tachyons are *SO* not possible! SteveBaker (talk) 13:47, 21 August 2008 (UTC)

Don't think I agree with you there. You might as well say that spacelike intervals don't exist because ${\sqrt {dt^{2}-dx^{2}-dy^{2}-dz^{2}}}$ is imaginary. I would say that tachyons have "spacelike mass", or negative mass-squared. It's a perfectly well defined concept. In fact it's a key part of the Higgs mechanism in the Standard Model. There's also no reason you couldn't detect a particle moving with infinite speed—it's only necessary that the particle's worldline intersect the detector's worldline, which can still happen if one of the worldlines happens to be sideways. The real problem with tachyons is that, classically at least, they wreak havoc with causality. -- BenRG (talk) 15:24, 21 August 2008 (UTC)

I don't think I believe that the mass of a black hole is concentrated at the singularity. The Schwarzschild geometry is a vacuum geometry. But the singularity of a Schwarzschild black hole is in the future (as in the Penrose diagram I drew above). So, if there's mass there, where was it at earlier times? As far as I can tell, it was in the gravitational field. Gravitational fields can carry mass-energy but regions with a strong gravitational field are still called vacuum. So I would say that the mass of the Schwarzschild black hole is everywhere, but in a form that's missed by some accounting systems.

I know almost nothing about more realistic black hole models, but in the collapsing and evaporating black hole from Hawking's paper (the other Penrose diagram above) the singularity is still spacelike and times after it forms are not defined. All of the mass of the collapsing star ends up at the singularity, but there's no later time when it's "still there". I've never seen a realistic model with a timelike singularity, but I suppose they exist. Even in that case, though, time and space are so mixed up in black holes that I don't think you can ever say that the mass has "already" reached the singularity. If you're outside a black hole then by definition you're not in the causal future of any point in the hole, so every point in the hole is either causally disconnected from you or in your causal future. So it's perfectly consistent to take the position that nothing inside the event horizon has happened yet; you can even say that the event horizon hasn't even formed yet.

In the cosmological scenarios it starts to matter a lot how you define "black hole".

• People often say that a black hole is a region of spacetime from which not even light can escape. In that case the whole universe is a black hole, since light can't escape from the universe. But also the causal future of any point in spacetime (a future light cone) is a black hole, so black holes are forming all around us and engulfing us all the time.
• A better definition is that a black hole is a region in which you'll inevitably hit a gravitational singularity in the future. That doesn't work in a Big Crunch universe, where everything eventually hits a gravitational singularity. In fact there's no way to unambiguously distinguish the Big Crunch singularity from a black hole singularity. Black holes are just parts of the Big Crunch that happened early (except that, as I said, they haven't really happened yet). If the universe exists forever, as it does in the ΛCDM model, then this definition implies that no large region of the cosmos constitutes a black hole because (by assumption) it won't end up as a singularity. This definition also doesn't work with exotic wormhole geometries where you can avoid the singularity.
• An even better definition, which avoids mentioning the unobservable interior of the hole, is that a black hole is a region from which you can't get to future infinity. Again that doesn't work in a Big Crunch universe. It gets interesting in a universe that becomes de Sitter in the future (as ΛCDM says ours does), since future infinity in a de Sitter universe is a 3D surface instead of a point, reflecting the fact that different parts of the universe become causally disconnected in the future. So you could, if you liked, declare a "black hole" to be a region from which you can't get to our future infinity, i.e. the point the Local Supercluster is headed for. In that case we're surrounded by a black hole with an event horizon that's about 15–16 billion light years away in every direction ($\int _{\text{now}}^{\infty }{\frac {a({\text{now}})}{a(t)}}\,c\,dt$). In the future that distance will increase to an asymptotic value of about 16–17 billion light years (${\frac {c}{{\sqrt {\Omega _{v}}}H_{o}}}$). It's an inside-out black hole, i.e. the interior of the hole is the part outside the sphere. Phenomenologically it's very much like an ordinary black hole. The effective surface gravity at the horizon goes to infinity; objects falling through redshift out of visibility; and I think it Hawking radiates, but don't quote me on that. I don't think it would normally be classified as a black hole, however.

-- BenRG (talk) 15:09, 21 August 2008 (UTC)

21 Hockey Pucks

I work at a local ice arena, and just before I left for home, some guy walks in and asks to buy 21 pucks. While I'm gathering the pucks I, of course, ask him what he needs 21 pucks for. He says he needs puck for "practice, or something like that" and 21 "seems like a good number". I've worked at the arena for years now, and play hockey myself, so I know this kid doesn't play, and he couldn't answer a few basic questions about the sport. Can anyone think of any possible use for 21 pucks? That's between 115.5 and 126 ounces of vulcanized rubber. --Willworkforicecream (talk) 21:57, 20 August 2008 (UTC)

Some act of violence comes to mind. Then again, 21 is an usual number. Perhaps he's inventing some idiotic game for the next time he and his buddies are drunk.--El aprendelenguas (talk) 22:05, 20 August 2008 (UTC)

A google search reveals a few crafts projects you might do with a hockey puck, but nothing that specifically asks for twenty-one of them. I've heard of them being used as feet for things, perhaps he has four tables that need to be an inch taller?

I can't think of any acts of violence that wouldn't be better and more cheaply accomplished by a simple rock or brick.

That's what I figured, at $2.00 a puck, rocks seem like the way to go. --Willworkforicecream (talk) 03:39, 21 August 2008 (UTC)

If you're sure he doesn't play hockey, I'll bet he's using them as raw material in some basic project, Perhaps as weights or rollers or something. APL (talk) 23:15, 20 August 2008 (UTC)

Incidentally, do you always grill customers on the basic facts of hockey before selling them pucks? APL (talk) 23:23, 20 August 2008 (UTC)

I do if they're buying such large quantities of pucks.--Willworkforicecream (talk) 03:39, 21 August 2008 (UTC)

Maybe he was hired to test out the customer service of your store, a test which unfortunately you appear to have failed? Nil Einne (talk) 11:51, 21 August 2008 (UTC)

So, what potential bad scenario do you generally forsee when you encounter a non-hockey-player trying to buy pucks in volume? That the guy is going to start whanging them at people, windows and/or passing cars? --Kurt Shaped Box (talk) 12:06, 21 August 2008 (UTC)

To honor a visiting dignitary or head of state? TenOfAllTrades(talk) 23:42, 20 August 2008 (UTC)

That would presumably be quite painful. --Bowlhover (talk) 08:58, 21 August 2008 (UTC)

There are perhaps a range of other uses for hockey pucks - but why would he lie about his reasons for needing them? Maybe there is a school who are starting hockey practice - he might maybe work for the school or be something to do with a parent's group and have been sent out to buy the pucks on behalf of the team - although he might personally have little or nothing to do with the team. That's why he'd be vague about the precise reason they're needed - and he might easily not know anything about the sport. SteveBaker (talk) 03:07, 21 August 2008 (UTC)

I doubt it is another school starting a team, seeing as there are three school teams in the valley, and I have played on all three teams.--Willworkforicecream (talk) 03:39, 21 August 2008 (UTC)

It's not a good idea to be suspicious because some people who are simply shy. I used to buy equipment for various reasons ranging from science to toad keeping, but I rarely had non-innocuous motives. Nevertheless, I tried my best to hide my actual intentions for fear of being perceived as "unusual". It was due to my introversion and nothing else. --Bowlhover (talk) 05:47, 21 August 2008 (UTC)

I'm going agree with Bowlhover 100%. Honest, but slightly unusual answers like "I'm building a habitat for a pet tortoise." just invites a conversation I'm just not interested in having with some random stranger. Especially not if I'm in a hurry.APL (talk) 13:21, 21 August 2008 (UTC)

For backyard (air?) pistol shooting targets, maybe? Perhaps he only had enough money with him to buy 21 pucks? --Kurt Shaped Box (talk) 11:57, 21 August 2008 (UTC)

Inventor. --jpgordon^∇∆∇∆ 15:24, 21 August 2008 (UTC)

My brother used hockey pucks to put a body lift into his Jeep Wrangler. He's pretty cheap and didn't want to cough up for the commercial kits. Have seen this use on the internet in a number of places although I am too lazy to google it right now. 142.36.194.253 (talk) 22:17, 21 August 2008 (UTC)

That's plausible - but why 21 of them? You'd think he'd have gone with a round number - like maybe 20! But I could imagine there being 21 kids in a class of some kind taking hockey lessons. SteveBaker (talk) 00:20, 22 August 2008 (UTC)

I'm not sure that logic is right. If I needed some number of hockey pucks for a construction project then it would be the most natural thing in the world to pick up a spare in case I mucked one up, but it would seem weird to buy hockey pucks for a class and get exactly enough for the students. APL (talk) 05:08, 22 August 2008 (UTC)

Moving toilet water

I've been experiencing rain from Tropical Storm Fay all of yesterday and today, and noticed that the water in my toilet bowl keeps shaking prior to usage and flushing. I've just lifted the lid and the water slightly ebbs and flows, sometimes more noticeable than others. I've even flushed, waited 5 minutes with the lid open, and still the water is shaking. I haven't noticed this before the storm, so it makes me wonder if this is caused by low pressure or something? Or is my toilet just possessed? Thanks!--El aprendelenguas (talk) 22:00, 20 August 2008 (UTC)

How windy is it outdoors? A gusty and irregular wind can induce surprisingly large pressure differences in the vent pipe that goes up through the roof; I've observed one good blast can make the water go up, then down by half an inch or more. (For other reasons, once it goes down it doesn't come back up all the way, either.)

It's not inconceivable that the right kind of blowing can set up something like a standing wave in the vent that in turn causes vibration and rippling in the water. I can't prove that's the case here, but it's the best explanation I've been able to come up with for myself over the years :-) --Danh, 70.59.119.73 (talk) 23:44, 20 August 2008 (UTC)

I agree. I've known the toilet bowl dry out completely in high winds, presumably by some mechanism similar to what you describe. Once the water gets blown past the U-bend, it doesn't come back. --Tango (talk) 23:55, 20 August 2008 (UTC)

Are you sure you don't just have a leaky flap valve? When that big rubber thing at the bottom of the cistern leaks, you get a slow trickle of water running down the side of the bowl which is sometimes not at all noticeable - except that it causes ripples in the water at the bottom. Low pressure shouldn't make a difference - it could just be a coincidence that the flap valve developed it's leak at that moment. Not all toilets have flap-valves (the ones here in the US do - the ones in the UK use a syphon mechanism which develops different problems) - but anything that produces a constant tiny trickle of water would do that. SteveBaker (talk) 03:02, 21 August 2008 (UTC)

I can confirm that the water level in my toilet fell by about half and inch during a squall a few days ago. I was actually meaning to question it here... --Kurt Shaped Box (talk) 00:56, 22 August 2008 (UTC)

If you live in a high-rise building, it is normal for the building itself to shake or rock slightly in a high wind, which might agitate the water perceptibly. --Anonymous, 04:35 UTC, August 21, 2008.

Thanks for the answers. I think it is the wind and the air vent, like many of you suggested. Anyway, the storm has passed for the most part, and the water is still.--El aprendelenguas (talk) 19:57, 21 August 2008 (UTC)

August 21

Maglev

Are maglev trains generally built along the lines of the locomotive model or the multiple unit model? —Lowellian (reply) 02:44, 21 August 2008 (UTC)

The lift motor also generates thrust - since every car requires lift, every car produces thrust. So I believe they pretty much have to be multiple unit trains. But in another sense, the "motor" is the track - so in a weird way, none of the train is the power unit. SteveBaker (talk) 02:56, 21 August 2008 (UTC)

The locomotive article has a section on maglev (that heading entitled "Magnetic levitation") that gives some information about maglev but does not in the least relate that information to the locomotive or explain what any of it has to do with a locomotive. Perhaps that section should be removed? —Lowellian (reply) 18:45, 21 August 2008 (UTC)

Herbal infusion

Can I make a herbal infusion from the cannabis plant leaves.?

Cannabis (drug)#Drink. --JWSchmidt (talk) 03:08, 21 August 2008 (UTC)

Do you want to know a) if it is legal, b) if it is technically possible, c) if it is a good idea to do so, or d) other? Wanderer57 (talk) 03:25, 21 August 2008 (UTC)

Note the key point from the above article. Because THC is lipophilic, it's not a particularly effective way if you want to get stoned... That's why people tend to make cannabis cookies, cakes and food items containing a fair amount of fat. You could try perhaps some sort of cannabis Cendol or other dessert or drink with a liquid base containing a fair amount of fat if you really prefer it in drink form (never tried it myself but it seems plausible). Nil Einne (talk) 11:42, 21 August 2008 (UTC)

However, if you use one of those recipes for Tibetan butter tea that can be found on the net, your lipophilic troubles are eliminated. Butter tea needs getting used to, though. --Ayacop (talk) 16:07, 21 August 2008 (UTC)

Science! It like totally does stuff, you know dude? Oh man! SamuelRiv (talk) 00:51, 22 August 2008 (UTC)

Bhang Lassi. DuncanHill (talk) 08:56, 22 August 2008 (UTC)

eye damage from hand dryers

Hi, I read just now on a hand dryer in a public toilet, that if you blow the hot air in your face, there is a risk of eye damage, from something called "fusion." What is the story here? 134.115.68.21 (talk) 08:36, 21 August 2008 (UTC)

I don't know the specifics, but it seems blindingly (pun unintended) obvious that if your eyes get too hot, there's a strong risk of damage and given the low level of pain receptors in the eye, you may not even realise Nil Einne (talk) 11:46, 21 August 2008 (UTC)

Unless the dryer was made by the Fleischmann & Pons Corporation, I'd say they're referring to the danger of a plastic contact lens fusing to the eye because of the heat; ouch! --Sean 16:54, 21 August 2008 (UTC)

It might not even be the heat. Could it be that the drier would normally dry the liquid from the surface of your eye - but you'd blink and tear up a bit - and no harm is done. But one of those soft contact lenses that's permeable to water would dry out without you noticing and form a water-tight seal against the eye - with no way for liquid to get back inside and rehydrate it? That's a guess though. SteveBaker (talk) 17:42, 21 August 2008 (UTC)

Astronomy Question: Need a Repeating Celestial Event

I'm looking for an astronomical/celestial event for a story I'm writing. Thus far, I haven't been able to find anything that exactly fits what I'm looking for. My conditions are this: the event has to repeat every 200 to 300 years. It has to be visible from the same location on Earth. Also, I'd rather not go with a comet if I can help it. Can anyone give me some suggestions, or a pointer in the right direction? Thanks. --Brasswatchman (talk) 08:45, 21 August 2008 (UTC)

The Transit of Venus occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years. Of course the sun has to be up for one to see it, but as half of the transits currently occur in june, there are populated places with midnight sun that could have viewed it regardless. In any case, the article has a table of the timings. EverGreg (talk) 10:16, 21 August 2008 (UTC)

And if you did choose the transit of Venus (which I think is an excellent choice, personally), you'd have a rich set of historical anecdotes to draw on and even some interesting scientific controversies (the "black drop effect" has been a major issue regarding the transit of venus, and there is ample historical material out there about international attempts to resolve it in the 19th century, etc.). In 1874 it was referred to as the "astronomical event of the century". --98.217.8.46 (talk) 12:18, 21 August 2008 (UTC)

As with a solar eclipse, observing a transit of Venus is fraught with risk of serious eye damage. Whether this might affect your story, I don't know. Wanderer57 (talk) 12:24, 21 August 2008 (UTC)

The trouble with events that widely separated is that the previous occurrance would have been back in the 1700's when telescopes were still pretty primitive devices. So you're really needing a more or less "naked eye" event. Comets really are your best chance. If this is fictional - then a meteor shower might do what you need. Events like the Perseids happen annually as the Earth passes through a stream of debris that was originally a comet that broke up a very long time ago. That debris stretched out so far that pretty much the entire orbit of the original comet has debris within it. But if the comet had been one that repeated every 300 years and if it had only broken up relatively recently (on the scale of cosmic events) then there might well be a bunch of debris that only intersects the Earths orbit every 200 to 300 years. I don't know of any such events in reality...but fictionally, I don't see a problem with inventing one. Anyway - a meteor shower (especially from a relatively recently broken up comet) could be as spectacular as you need it to be. Anything from chunks of rock and ice hurtling out of the sky smashing buildings and splattering our Hero's one true love over half a county...down to a bunch of gentle shooting stars that commemorate that tragic event and bring our Hero's great, great, great, great grandson together with the (sadly splattered) heroines' great, great, great, great granddaughter together. Anyway - it would occur on the exact same night of the year - although not generally at the exact same time of the night.

Oh - and if you need the last occurance to have happened before 1753 - beware of calendar reform! Actually, this would make a great plot point! Suppose the original sighting of the event happened on September 1st 1752, with a prediction that it would happen again precisely 250 years later on Sept 1st 2002. Our modern day heroes might initially be unaware of the fact that 11 days were 'dropped' from the calendar in 1752 (so the day after September 2nd 1752 was September 14th 1752!!). They would gather with the bad guys holding guns to their heads (or whatever) at the appointed place on September 1st 2002 - and be horrified to find that the specified event doesn't happen as predicted. We're confused, they are confused, the bad guys go away - could the prediction really not be true? Then, just a week and a half later when hero and heroine are strolling in some romantic place believing that the prediction was a lot of nonsense - he proposes to her and at that exact moment the sky bursts forth with meteor trails (because it's September 12th) - which is actually 250 years later to the day exactly on schedule. A wandering Wikipedian strolls past and comments "Hey didn't you read Calendar reform?".

This stuff practically writes itself!

Good luck with your book! SteveBaker (talk) 13:21, 21 August 2008 (UTC)

And here I thought Steve was going to mention the change of New Year's Day, which in England took place about the same time, so that one year after March 24, 1750, it was March 24, 1752. --Anonymous, 21:39 UTC, August 21, 2008 (N.S.).

The trail of debris left by a comet will cause a meteor shower every year, but you can get more dramatic ones in certain years due to particular passes of the comet leaving clumps in specific places. It might be plausible to have a extra dramatic shower once every 250 years, or whatever. I think a transit of Venus is a better choice, though - it's almost tailor made for the OP's requirements. --Tango (talk) 17:55, 21 August 2008 (UTC)

Yes - certainly the Perseids have good years and bad years - suggesting a certain clumpiness. But it kinda surprises me that the debris spread so far through the original orbit of the comet. It seems that the debris was created about a thousand years ago and came from the 109P/Swift-Tuttle comet. For the purposes of a story, the comet could have broken up much more recently - leaving the debris in a relatively compact bunch that would orbit with roughly the same period as the comet - which could easily be 200 to 300 years. That would also produce an event with a fairly sharp onset and cutoff if most of the debris were still fairly close together. SteveBaker (talk) 00:17, 22 August 2008 (UTC)

Does it have to be set on Earth? How about a planet orbiting a flare star? (eg. a star like Groombridge 1618) Astronaut (talk) 18:30, 21 August 2008 (UTC)

"Green Bags"

There's an ad on TV for "Debbie Meyer's Green Bags", which promise to almost stop fruits and veggies from rotting. The claim is that they remove ethylene gas produced by food which would otherwise cause spoilage. They also say the bags can be reused up to 10 times. I'd like to know if these claims are legit:

1) Do all fruits and veggies produce ethylene gas ?

2) Does this gas promote spoilage ? If so, to what degree ? The commercial makes it look like there is no spoilage once this gas is removed.

3) Do the bags remove 100% of the gas, or only a tiny portion ?

4) Could they really be reused 10 times without losing their effectiveness ?

5) Are there other side effects, like stopping fruit from ripening ? I prefer yellow bananas over brown, but brown is still better than green.

6) I'd also be interested in seeing a cost/benefit analysis. That is, does the cost of the bags outweigh the financial benefit of avoiding lost produce ?

7) Is there a more efficient way of removing ethylene than disposable bags ? StuRat (talk) 12:37, 21 August 2008 (UTC)

Update: I found these reviews, which are mixed to negative: [15]. However, I'd still like to know more about the science that's going on with the bags, so still would like answers to the Q's above. StuRat (talk) 12:50, 21 August 2008 (UTC)

Ethylene as a plant hormone may be of interest, and this site has an interesting sensitivity chart. Moving into the questions:

1. Most, certainly. "All" is a difficult blanket statement.
2. Promote, absolutely. Sole factor, generally no. Per the sensitivity chart, reaction to ethylene varies.
3. Who knows?
4. It probably depends on how long you're keeping fruit in the bag on each of the 10 times. I'm betting on an over-optimistic marketing pitch, but that's just me.
5. Yes, rotting and ripening are both effects of ethylene.
6. Depends on you personally. How much produce are you losing without the bags? How much produce would you lose if you just bought produce on an on-demand (or nearly so) basis? It's unlikely you save anything if you're buying and storing produce responsibly to begin with.
7. Good ventilation is the easiest means of reducing ethylene quantities. Segregating ethylene producers from ethylene-sensitive produce is another useful approach. There are other methods, but they seem to be targeted at the industrial level. — Lomn 12:56, 21 August 2008 (UTC)

As for question #4, if they do in fact work, I wonder if telling you they can only be used for 10 times is just a ploy to get you to buy more. Just a thought; there may be a chemical in the lining of the bag that reduces ethylene that runs out after so many uses, and of course you don't want to reuse a bag for food too many times, but still, I wonder.--El aprendelenguas (talk) 20:10, 21 August 2008 (UTC)

The Large Hadron Collider and the Sun

Is the sole purpose of the large hadron collider to try to create black holes by accelerating hadrons to high speed and then crashing them into each other, or are there other experiments as well?

Is it likely that there are hadrons of comparable energy inside the sun? To put the question another way, is it likely that micro black holes are being created within the sun? If so, would this be a common event or a rare one? Thanks, Wanderer57 (talk) 16:04, 21 August 2008 (UTC)

Producing black holes is not a primary goal or purpose of the LHC at all. The only reason you hear about the LHC and black holes is that some cranks filed a lawsuit, because they thought it would cause the end of the world. See Large_Hadron_Collider#Research for its major goals. If you had to pick one goal, it would probably be the discovery of the Higgs boson. -- Coneslayer (talk) 16:13, 21 August 2008 (UTC)

It's interesting to note that we really do have machines which are reasonably likely to end the world at some point, but these folks didn't see fit to try to sue them out of existence. It's like fretting about spontaneous human combustion when there's a snake in the room. --Sean 16:46, 21 August 2008 (UTC)

Well, it sort of is apples and oranges from a legal point of view. --98.217.8.46 (talk) 19:52, 21 August 2008 (UTC)

The energy in each collision of individual nuclei in the LHC will be around 10^-4 joules. A rough calculation shows that this is equivalent to a temperature of around 10¹⁹ K. The temperature at the centre of the Sun is only around 10⁷ K. So LHC collisions will be much much more energetic that reactions within the Sun. Gandalf61 (talk) 16:19, 21 August 2008 (UTC)

For comparison, see Orders_of_magnitude_(temperature). --Ayacop (talk) 16:31, 21 August 2008 (UTC)

If your question was really "do such high energy particles/collisions occur naturally and have we seen any disastrous effects elsewhere?" the answer is yes for the particles and no for the disastrous effects, see Ultra-high-energy cosmic ray. Well if someone would be hit directly that would not be healthy, but neither the end of the world nor the end of our planet. 93.132.168.99 (talk) 16:54, 21 August 2008 (UTC)

Well, not quite. The idea that a cosmic ray could produce whatever weird/dangerous particles the LHC might produce is valid - but if it were that easy, we wouldn't need the LHC! The problem is that any particles appearing from a cosmic ray collision would continue to move at spectacular speed and probably go right through the earth and out the other side without anyone noticing. Even if you were hit by a mini-black hole coming about from a cosmic ray event, it would drill a hole through your body that would be considerably smaller than the diameter of an atom...you wouldn't notice. But if the LHC were to produce such an object, it could be stationary - or moving very slowly...so (the theory says) it could be a real problem. I think that's nonsense - but that was the thesis put forth in the law suite.

Anyway - the purpose of LHC is to "discover stuff" - and you don't always know what you're going to discover until after you've discovered it. It's hoped that there may be further evidence for dark matter - perhaps some of the predictions of string theory may be tested...and there is a lot of confidence that the Higgs Boson may be discovered. The Higgs is predicted to be a very heavy particle and because of E=mc² it takes more energy to make particles with more mass. Since the energy comes from slamming particles together, the energy comes from the kinetic energy of the particle that's doing the hitting. The giant circular tunnel accelerates particles to higher speeds than we've ever been able to do before.

SteveBaker (talk) 17:18, 21 August 2008 (UTC)

(ec with SteveBaker, somewhat redundant now) The purpose of the LHC is to see what there is to see in that energy regime. Theoretical particle physics has failed spectacularly at making predictions in the last 30 years, so nobody really knows what to expect. Almost everyone thinks it will see the Higgs boson, since the Standard Model limits its mass to a range that's easily accessible to the LHC. It's not really the main goal of the LHC to find the Higgs boson, it's just the one thing most physicists agree it will succeed in doing. Some people expect to also see black holes, some expect to see superpartners, some expect to see other exotic particles (about which I know nothing). Some people think the Higgs won't be found. Nobody wants the Higgs to be found; it would be much more interesting if it wasn't. The worst-case scenario for the LHC is finding a fundamental scalar Higgs particle and nothing else, giving theorists almost no clues about physics beyond the Standard Model. That could very well mean the end of particle physics.

I don't know much about conditions inside the Sun, but collisions at the energy the LHC will produce happen all the time. The only reason they need the LHC is to make them happen at a predictable place and time, so that they can stick a huge honkin' detector there and analyze the result. The proposed ILC would produce lower energy collisions than the LHC but under even better controlled conditions. -- BenRG (talk) 17:22, 21 August 2008 (UTC)

Apparently Natural Causes?

Could we please have medical input to the discussion here:

Talk:Jack the Ripper#Apparently Natural Causes!!!

Thanks, Wanderer57 (talk) 17:05, 21 August 2008 (UTC)

1. The standards of medicine in the 1880's were kinda poor so it's perfectly possible that nobody really checked for precise cause of death.
2. She died many months before the ripper murders became notorious - so there was no particular reason to pay careful attention to investigate and record cause of death - a lazy doctor could just have scribbled "Natural causes" in order to avoid a lot of hassle.
3. She died sometime AFTER she'd been discharged from hospital - over a month after she'd been stabbed - it seems unlikely that she was seriously injured at the time she died.
4. If she'd gotten some kind of general infection, it's perfectly possible that they would not have connected her death to injuries from the wounds - and perhaps a death from a serious infection would have been impossible to diagnose at the time.
5. It's unusual for someone who is only 38 years old to die of "natural causes" in this day and age - but even as late as 1900, life expectancies for urban poor were hovering around 45 years. So it's quite possible for the report to be 100% true.

I don't doubt that was the report of the time - but it's hard to know whether the report was accurate or not.

SteveBaker (talk) 17:34, 21 August 2008 (UTC)

Who she was and what she did for a living could also have prejudiced a doctor into not putting a lot of effort into her diagnoses. It's well known that cases of "undesirables" dying are often given far less attention by authorities, who are not looking to make more work for themselves if nobody cares or notices if they brush things under the rug. Just a speculation. --98.217.8.46 (talk) 19:42, 21 August 2008 (UTC)

Liquids going from perspiration to waste in humans

This may be a rather simple answer - at the point it reaches the kidneys, perhaps - but the womens' Olympic marathon brought to mind a query I'd had from a number of years ago. I'd consumed two bottles of water walking in 95 degree F heat for a couple hours,a nd hardly had to use the bathroom at all; it had all been sweated out.

My question is, at what point does water consumed go from being utilized as sweat to being discarded as waste? My guess from both articles would be that anytime before it reaches eht kidneys, it's game for being sweat, but afterward, it's automatically urine, but I'm not sure; it almost seems like if you've really gotta go, and yet you're running a lot, it can still wind up used as sweat even if it's in the bladder. Is the answer true for other liquids, too? Or, only with respect to the amount of water in them? (Weird, the thing to sign wasn't illuminated before. Now it is.) Somebody or his brother (talk) 19:03, 21 August 2008 (UTC)

The kidney is the irreversible/committing step where water goes to urine. Once it diffuses out of the blood and is not re-absorbed in the collecting ducts, there's no further chance for it to go anywhere else except ureter→bladder→urethra→outside. DMacks (talk) 19:09, 21 August 2008 (UTC)

Water also goes out through the digestive system but it can be recovered from the colon when needed. Franamax (talk) 22:37, 21 August 2008 (UTC)

what is important of material sciences?

Link title —Preceding unsigned comment added by 116.71.61.200 (talk) 18:56, 21 August 2008 (UTC)

Not really a coherent question here. But did you read the material science article? Think about how important it is that someone made the stuff your keyboard and monitor are made out of. DMacks (talk) 18:59, 21 August 2008 (UTC)

Studying it gives me something to do outside of Wikipedia. the wub "?!" 23:17, 21 August 2008 (UTC)

Telephone call to 911

Let's say that a person is incapacitated, cannot speak, and has absolutely no idea where they are (as a result of an accident or a crime or for whatever reason). The only thing the person can manage to do is to dial "911" on a telephone. Scenario A: If that person dials 911 from a landline phone, my understanding is that the 911 dispatchers can tell immediately what the location of the landline phone is. And, if the incapacitated person does not speak (after having made the 911 call), they will still dispatch help to the correct address. Is all of that correct? Scenario B: Now, what exactly happens if the 911 call was made from a cell phone? Does anyone know how exactly this works? I assume -- at some point -- and through technology -- they can somehow locate the cell phone and the person. Now, will this take a few minutes, hours, days, weeks, what? In other words ... in an emergency, how long would that incapacitated person expect to wait before help arrived, when summoning 911 from a cell phone? Also, are there procedures (similar to the landline phone) where they dispatch help no matter what, even if the incapacitated person on the cell phone does not / cannot speak? Thanks. (Joseph A. Spadaro (talk) 19:49, 21 August 2008 (UTC))

You should probably start with Enhanced 911. -- Coneslayer (talk) 19:52, 21 August 2008 (UTC)

The FCC's page on wireless 911 may also be of interest. — Lomn 19:53, 21 August 2008 (UTC)

I am fairly sure that 1. is correct at least in some locations. I have friends who dialed 911 and then hung up after a second or two and the cops did indeed show up their door. (It's a long story. It involved LSD. It worked out fine in the end. Just in case you're wondering.) As for a cell phone, hypothetically you can locate which cell tower the call came from (which puts you within a mile or so), and maybe even triangulate it based on information from multiple towers, I suppose, but emergency dispatchers do not currently have that capability as far as I know. If you don't tell them where you are, they don't know. Eventually I am sure this will change. --98.217.8.46 (talk) 20:05, 21 August 2008 (UTC)

As long as the phone is still turned on, you can track it more accurately. I'm not sure of the details, and I'm not sure if it's routinely used, but it can be done. --Tango (talk) 20:14, 21 August 2008 (UTC)

One thing the dispatchers get is the phone number, so that they can all back to see if the call was an accident, or a prank, or genuine. There is also a special number for tty machines if the user has such a facility. Graeme Bartlett (talk) 21:35, 21 August 2008 (UTC)

In many jurisdictions, if you don't talk to the 911 operator and simply hang up, the first thing they do is try to call you back at the same number. Also, if you have the misfortunate of living in my jurisdiction, you can call 911 and listen to a ridciulous "All operators are busy, please stay on the line" message for 10 minutes. This happened to me when trying to report a car crash. I can imagine that such delays have probably resulted in a least a few deaths from people that might otherwise have been saved. Dragons flight (talk) 01:00, 22 August 2008 (UTC)

The dispatch depends on the Emergency Medical Dispatcher protocols used by the authority having jurisdiction. In my county, this type of call goes out as an "unknown rescue;" fire, EMS and police respond together. Also see 9-1-1. --Shaggorama (talk) 03:28, 22 August 2008 (UTC)

Thanks. So, if the incapacitated person is on a cell phone and can't speak or offer any location clues ... how long would they have to wait for help to arrive? Minutes, hours, days, weeks? Thanks. (Joseph A. Spadaro (talk) 11:07, 22 August 2008 (UTC))

Solutions and volume changes

In a typical solution of a crystal solid in a liquid e.g. salt in water, is there a change in the volume of the solvent as the crystal dissolves? - is any change the same as or proportional to the volume of solute? —Preceding unsigned comment added by Webb202 (talk • contribs) 20:34, 21 August 2008 (UTC)

I'm pretty sure there is a change in volume, but I can't confirm if that change is equal to the amount of solute added. Coolotter88 (talk) 20:47, 21 August 2008 (UTC)

This is certainly interesting. Would the break down of the salt crystals to ions cause the salt to occupy less volume? I think the answer is yes but it would be very insignificant change.--155.144.40.31 (talk) 00:58, 22 August 2008 (UTC)

Some materials have a very large volume difference when dissolving ("volume of solute A + volume of solvent B >> volume of solution of A in B"). It has to do with the dissolved particles (ions or solvated molecules) fitting into the existing spaces in the solvent rather than completely displacing solvent molecules. Doesn't even matter if the solute is a solid or not (alcohol+water is an easy experiment to do at home). DMacks (talk) 02:42, 22 August 2008 (UTC)

Optical zoom

Does optical zoom refer to distances or area? If I have a *4 zoom and take a picture of a square "zoomed out" and it is 100*100 pixels, when "zoomed in" by *4 will it be 400*400 or 200*200? -- SGBailey (talk) 21:19, 21 August 2008 (UTC)

It's linear, so 400*400. It's the ratio of the focal lengths at the longest and shortest zoom positions. -- Coneslayer (talk) 22:40, 21 August 2008 (UTC)

It's linear if you're focused at infinity, but (if I've worked it out correctly) the width of the visible area of the focal plane in general is dw(1/f − 1/d), where d is the distance from the lens to the focal plane, w is the width of the detector (CCD), and f is the focal length. That would imply that the magnification is somewhat more than 4× if you're focused in close. However experimentation with my point-and-shoot digicam has failed to back this up, so maybe I made a mistake. -- BenRG (talk) 00:20, 22 August 2008 (UTC)

Ta -- sgb

For Chemistry Experts

What would result from the following?; If Methane (Ch4) is present in water (H2O) and Hydrogen Peroxide (H2O2) is introduced.Rukiddin (talk) 23:20, 21 August 2008 (UTC)

A solution of two gases in water. Need some activation energy to make something fun happen. In terms of organic mechanisms at least, you'd need to get the radicals going (H2O2 is often a difficult-to-start radical initiator unless there's something very reactive available to do something) and methyl radical is one of the least stable (hardest to form) ones:( DMacks (talk) 23:27, 21 August 2008 (UTC)

August 22

When someone says that they can see the intelligence in an animal's eyes...

...what exactly is the phenomenon that they're describing? I found myself pondering this whilst a Grey Heron and I were silently observing each other at close quarters earlier today. --Kurt Shaped Box (talk) 00:50, 22 August 2008 (UTC)

Anthropomorphism might fit. The more something appears to be volitional (an animal, a storm, the sea), the likelier humans are to believe the thing observes and reacts as humans do. I am not expert on herons, grey or otherwise, but since they feed off things like fish and frogs, the lack of motion and steady gaze likely come from hunting instinct. There's also not a great deal of brain in a creature weighing 2 kilograms. — OtherDave (talk) 01:00, 22 August 2008 (UTC)

I'm not an expert on herons myself (in fact, this is the first one I've ever seen at a distance closer than 100ft), so I have no idea where they rank in terms of intelligence and I accept that I may just be anthropomorphizing, but during seconds that my eyes met those of the heron, I got a distinct feeling that the bird was 'weighing me up' and 'observing', as opposed to just looking at me. Man, it's really hard to describe. I guess it's a bit like when a pet owner says that they can tell when their dog/cat/bird/whatever is curious or preoccupied by something by the way it looks at them... --Kurt Shaped Box (talk) 01:22, 22 August 2008 (UTC)

Birds can be surprisingly intelligent. I have similar experiences when making eye contact with our Cockatiel. It could just be a psychological thing that we're thinking way too hard about. --Russoc4 (talk) 01:52, 22 August 2008 (UTC)

On the other hand, magpies were recently added to the very short list of animals that are self-aware, an honor they share with great apes, Asian elephants, and the bottlenose dolphins. [16]. Dragons flight (talk) 07:45, 22 August 2008 (UTC)

There's a huge amount that's just how much sclera one can see. There's been a lot written about that of late—that humans judge eyes with a lot of white visible to be closer to their own intelligence, and the question of why humans evolved to have so much visible (unlike most animals). Additionally I think the ability of the animal to focus on one discrete object or make eye contact helps. Things that are looking all over the place look wild. When my dog makes eye contact with me and holds it, it's hard to argue that there's some kind of connection. --98.217.8.46 (talk) 02:38, 22 August 2008 (UTC)

I think the reason why an animal that looks into your eyes seems intelligent is as follows: This implies that they know you can see them with your eyes, which requires a certain degree of intelligence to work out. StuRat (talk) 02:59, 22 August 2008 (UTC)

There are a few references at Empathy#Empathy_and_animals which would imply that there is a possible difference between ability to identify with others of the same species, and other species. --Ayacop (talk) 08:06, 22 August 2008 (UTC)

Drunk Driving deaths by country

I'm looking for a refernce source that will list drunk driving and or alcohol-related deaths by country. So far it looks like I could probably find those statistics if I searched country by country, but I was hoping some website existed where it's already been aggregated. --Shaggorama (talk) 02:26, 22 August 2008 (UTC)

Unfortunately I can't answer your question, but I'm prepared to offer some comments and advice which will make it more difficult for you to trust any results you find. You'll need to be very careful with the definitions that you use; I suspect it will be difficult for you to find a genuine apples-to-apples comparison. Do you define drunk-driving deaths as those where an intoxication-related criminal charge was laid? Do you count the case as an 'alcohol-related death' if it involved a sober driver and a drunk pedestrian? Do you look at blood alcohol content—remember that the legal limit varies from country to country and may also depend on type of license. (A look through our Driving under the influence shows limits ranging from zero to 0.08%.)

Some anti-drunk-driving organizations are also a bit dishonest with their statistics, reporting fatalities as 'alcohol-related' as long as at least one individual involved in a collision had any measurable blood alcohol content, regardless of the circumstances of the accident or who was actually at fault. TenOfAllTrades(talk) 02:54, 22 August 2008 (UTC)

I'm with both of you. I'm not inclined to really trust the numbers I find, but I'm not in a position to produce them myself so for my purposes I'll take what I can get. One of the reasons I'm looking for one single resource is I'm hoping it would largely unify the definitions used in producing the stats. But I'm probably being naive, seeing as I am looking for statistics that, as TOAT pointed out, are often produced by biased propagandizing organizations. --Shaggorama (talk) 03:18, 22 August 2008 (UTC)

not a question, but... worth it (LHC)

"The Large Hadron Collider is set to fire up on September 10. Not sure why, but don't want to slog through tedious explanations of the Higgs boson and the Standard Model? Have a look at this informative rap narrative, delivered by persons in lab coats and hard hats."

http://blogs.spectrum.ieee.org/tech_talk/2008/08/switzerlands_nerdcore_scene.html

You'll laugh your head off. --Halcatalyst (talk) 02:34, 22 August 2008 (UTC)

I think they left out the part about how it'll blow up the world, right? ;-) --98.217.8.46 (talk) 02:52, 22 August 2008 (UTC)

The good thing about blowing up the world is that you can't end up looking bad because of it. If they'd rapped about the end of the world and it didn't happen, we'd all laugh at them. This way, whatever happens, they can't be proven wrong. --Tango (talk) 02:54, 22 August 2008 (UTC)

This is the way the world ends

This is the way the world ends

This is the way the world ends

Not with a bang but a whimper.

The Hollow Men --Halcatalyst (talk) 03:50, 22 August 2008 (UTC)

Frostbite

If frostbite is caused by the body trying to maintain the core body temperature, is it possible to get frostbite if only one part of your body is exposed to extreme cold, for example if you stick your hand in liquid nitrogen? —Preceding unsigned comment added by 218.5.84.167 (talk) 05:55, 22 August 2008 (UTC)

Did you read frostbite. It usually affects toes or extremities of the body. Graeme Bartlett (talk) 07:09, 22 August 2008 (UTC)

Extremities generally lose heat faster, but you can get frostbite anywhere if that part of your body gets cold enough. And yes, holding your hand in liquid nitrogen would do it. (Incidentally, it's not instaneous. Brief contact with LN2 will create a significant signal of pain and normally one would yank their hand back before it froze. The same way that touching hot things will generally cause you to recoil before any major damage is done.) There are cases where someone's leg stepped through a frozen lake and they quickly got frostbite on their leg while the rest of them was okay. That's probably most analogous to what you are asking. Dragons flight (talk) 07:38, 22 August 2008 (UTC)

Physics with Calculus queston, regarding differential mass elements

For chapter 9, my book describes the center of mass of a solid body on an x-axis as the integral from x1 to x2 of the differential mass elements all multiplied by 1/Mass

My question is it calls the differential mass elements, by the symbol dm which also occurs as the last symbol of the integral.

Are differential mass elements the mass of my object between 2 infinitesimal close points on the x-axis? In other words if I have a rod from x=0 to x=5, then divide this distance into infinite divisions, and the mass of each sub-distance totally comprises the mass of my object? Therefore summing all the differential mass elements will give you the "area under the curve".

The other way I can see it is that the term differential mass element means the change in mass between adjacent subdivisions of the 5 meter strip of the x-axis. I have helped improve the navigability for finding out information on differentials, but I'm not certain, by reading the wikipedia articles what a differential mass element is. It could either be f(x+dx) - f(x) or it could be dx*f(x+dx) - dx*f(x) is what I've narrowed it down to on my own. Thanks Sentriclecub (talk) 10:11, 22 August 2008 (UTC)

An engineer would say (mathematicians look away now) that the differential mass element dm is the mass of an infintesimally thin slice of the body between x and x+dx (a mathematician would talk about the "mass distribution function", but would get to the same result). So, for example, for a thin triangle with mass per unit area ρ between the x axis, the line y=x and the line x=a,

$dm=(\rho )dA=(\rho x)dx$

because the area of the thin slice is xdx. To find the distance of the centre of mass from the y axis you divide the first moment of mass about the y axis by the total mass of the body, so you have:

${\frac {1}{M}}\int (x)dm={\frac {2}{\rho a^{2}}}\int _{0}^{a}x(\rho x)dx={\frac {2}{\rho a^{2}}}\int _{0}^{a}(\rho x^{2})dx=\left({\frac {2}{\rho a^{2}}}\right)\left({\frac {\rho a^{3}}{3}}\right)={\frac {2a}{3}}$

Gandalf61 (talk) 10:56, 22 August 2008 (UTC)

Thank you alot, Gandalf, for your help. I understand a lot better using the language of infinitesimally thin slices than my book's language. (I learned calculus from a Stewart book, so its inconsistent with my physics book which is more pragmatic at how it throws around symbols). Thanks again. Sentriclecub (talk) 11:05, 22 August 2008 (UTC)

No problem. I have a mathematics background, but my son is a student engineer, so I am quite used to doing this type of "tranlsation" ! Gandalf61 (talk) 11:13, 22 August 2008 (UTC)

Tornados at night?

Do tornados occur at night as frequently as they do during day?Leif edling (talk) 10:49, 22 August 2008 (UTC)

No, but not for an entirely clear set of reasons. The biggest reason is that a warm front collides with a cold front, forming cells is the initiation of what can lead to a tornado. Pressure systems you could say are propelled by differences in air pressure between colliding "blocks" of air. The sunlight heats these blocks of air, and increases the likelihood of discrepancies between contiguous blocks of air. When the sun is out, these "moving blocks of air" are more likely to collide, and at higher kinetic energies, thus leading to increased likelihoods of the prerequisite conditions. As an analogy, pretend white and black mice are playing in a tank, moving at random. Now randomly sweep a lit cigarette around in the tank. This stirs up the mice and makes them even more likely to wind up one-atop-another. Sun feeds the weather system energy, and it makes things more volatile and chance climate events more probable. Sentriclecub (talk) 11:01, 22 August 2008 (UTC)

Apple

Why does an apple turn brown few minutes after when it cut? —Preceding unsigned comment added by Kelvin caesar (talk • contribs) 12:12, 22 August 2008 (UTC)

According to our article,

Sliced apples turn brown with exposure to air due to the conversion of natural phenolic substances into melanin upon exposure to oxygen. Different cultivars differ in their propensity to brown after slicing. Sliced fruit can be treated with acidulated water to prevent this effect.

Algebraist 12:20, 22 August 2008 (UTC)