Questions tagged [black-holes]
A black hole is a region of spacetime from which nothing can escape. More formally, the future light cone of any observer within the black hole is completely contained in the black hole, and the black hole region is not within the past light cone of any observer that goes to spatial infinity in an infinite amount of time.
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What can escape from a blackhole and via what path... if all paths constitute a full closed circle? [closed]
I am referring to the first explanation for black-holes that every layman hears. Straight lines are bent by gravity to the extent that they close in on themselves, thus nothing can leave/escape that ...
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Wald theorem 8.1.2 and the proceeding corollary
I am currently confused on the corollary of theorem 8.1.2 in Wald's book, specifically the paragraph separating the two. I've attached a screenshot below.
Why does Wald say that using theorem 8.1.2 ...
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Problem in deriving Killing equation
I am studying derivation of Killing equation by Wald (also reading some other literature) but having some problem in understanding the math.
Let $\chi ^a$ is killing vector on the horizon
$$\chi _{[a \...
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Gravitational time dilation – clock falling to event horizon
Given probe that falls into BH (simple non-moving mass M) staring from distance r=D, velocity v=0 and clock t=0, what will the clock show when entering the EH (or in general as function of the ...
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What is the difference between Innermost Bound Circular Orbit (IBCO), Innermost Bound Spherical Orbit (IBSO), and Sphere Radius?
⚠ My question is related to Schwartzchild and Kerr Blackholes.
In a few words:
Innermost Bound Circular Orbit (IBCO):
It is the constant radius at which this circular orbit occurs at 1.5 Schwarzchild ...
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Question on the poloidal and toroidal Magnetic field lines close to a astrophysical black hole
I'm trying to fully understand the force-free region near a Kerr black hole. A common picture is depicted as Figure 1, 1.
In this context, a generic magnetic field is:
$$\vec{B} = (\vec{B}_{P})+\vec{...
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What happens to kinetic energy that enters a black hole?
A little thought experiment:
We are in a space ship, safely orbiting black hole A.
We fetch 2 smaller, identical black holes, B and C and we position them on the opposite sides of black hole A.
We ...
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Equivalence of Hawking Temperature Between Black Hole and its KK Reduction
Consider the following $2+1$ dimensional metric describing a stationary black hole:
$$ds^2 = g_{tt}dt^2+g_{rr}dr^2+g_{\theta t}d\theta dt+g_{\theta\theta}d\theta^2$$
Where $t$ is temporal coordinate, $...
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I'm doing my project work on astrophysics. So what i want to find how much black hole grows over time
So problem in the code I have to find the velocity of gas that falling in the black hole at the point of event horizon,from this i can find the increasing $dm$ mass from Bondi equation over $dt$ time. ...
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What is the Carter constant?
The definition in Carter constant doesn't explain it clearly, I understand that
It is a conserved quantity for motion around black holes.
What does "quantity for motion" mean? And with ...
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How does the lifetime and temperature of a black hole scale with mass in universe with more then 3 spacial dimensions?
I've tried to find out how the lifetime and temperature of a black hole scale with mass in a universe with more then 3 spatial dimensions. I've spent a while trying to look up an answer to this ...
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Crossing an event horizon… at the end of time
I am not sure if my representation of entering into a black hole is correct, and so I would like to ask if I got anything wrong.
The black hole is getting closer. I know exactly when I will cross the ...
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Should density be considered in the role of the strength of a gravitational field?
Ok, here me out. Black holes are usually formed from the compression of the mas released during the death of a supermassive star, however they have the same mass as the star (or even less). Yet when ...
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Hawking Radiation: Why are advanced coordinates appropriate for past null infinity and retarded coordinates appropriate for future null infinity?
I've been reading Hawking's original paper https://doi.org/10.1007/BF02345020 https://projecteuclid.org/journals/communications-in-mathematical-physics/volume-43/issue-3/Particle-creation-by-black-...
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Is Schwarzschild spacetime in Eddington-Finklestein coordinates flat at Schwarzschild radius?
Schwarzschild metric in Eddington-Finklestein coordinates reads
$$ds^2=-\left(1-\frac{2M}{r}\right)dv^2+2dvdr+r^2d\Omega^2$$
My professor claimed that at $r=2M$, this metric becomes Minkowski metric. ...
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Proof of Bekenstein bound
I am trying to find a student-friendly (but mathematically rigorous) proof of the Bekenstein bound. Unfortunately, all the proofs I have found are too complicated to understand (most of them refer to ...
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An object moving close to $c$ falling into a black hole
Assume there is an object moving at $0.99999999999........c$ (upto like a thousand 9s) towards the center of a non-rotating black hole in a straight line
Since it can't escape once it falls in where ...
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How to model a Black Hole inside a Black Hole?
If you put a Black Hole inside of a Black hole, then you get a spherically symmetric vacuum outside the inner Black Hole and the Schwarzschild metric is the only spherically symmetric vacuum in GR. ...
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Reissner-Nordström black holes and naked singularities
I often read that there is no known mechanism that prevents a charged black hole, if the charge is high enough compared to its irreducible mass, from forming a naked singularity. But I don't get how ...
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A supermassive white hole that emits much more matter [duplicate]
Just as a supermassive black hole consumes a lot of material, does a supermassive white hole also expel a lot of material? If yes then how? I am puzzled by the thought of where the matter would come ...
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Interpretation of geodesics in Kerr metric when approaching the event horizon
I'm learning about the Kerr metric and Kerr black holes from the book "General Relativity: An Introduction for Physicists, M.P.Hobson" and I'm interested in an interpretation for the ...
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Confusion about JT gravity from extremal black holes, and AdS v. dS
I am currently reading section 2 of [1], trying to understand the motivations behind Jackiw-Teitelbom (JT) gravity.
It is known that the near-horizon geometry of an extremal RN black hole is $AdS_2\...
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Event Horizon vs. "Interior" of a black hole from perspective of distant oberserver
From the perspective of a faraway observer watching an object fall into a black hole, they will see the object approach the black hole's horizon at a slower and slower rate until eventually "...
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How do we know that at the Schwarzschild radius the falling speed is $c$? [duplicate]
I see something strange: The kinetic energy required for a falling body to reach the Schwarzschild radius is $(\gamma-1)mc^{2}$, so with $\gamma$ tending to infinity at the Schwarzschild radius, the ...
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Thought experiment with black holes and escaping rockets [duplicate]
Something about stuff ever falling in black holes always bothers me, so I made a thaught experiment that I would like to be scrutinized.
Element 1: We have a supermassive non rotating black hole.
...
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Is there a common mechanism that gives rise to spatial and time dilation in both Einstein's STR and GTR?
In both General and Special Relativity a body experiences both time and spatial dilation. For example, as a particle approaches C it experiences dilations in time and space. For a particle ...
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Would Hawking blackbody radiation theory still apply if there is no singularity in a black hole?
If it was shown that there was structure where a singularity should be, such as quarks and electrons, inside a black hole, would it still be expected to emit blackbody radiation as Stephen Hawking ...
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How can I (mathematically) see that, as you fell into a black hole, you would not see infinite time passing? [duplicate]
I'm currently taking a GR course and, as we were studying the Schwarzchild solution, we discussed how someone entering a black hole (Alice) could pass through the event horizon in finite proper time, ...
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Strange coordinate transformation in this proof of Birkhoff's theorem?
In this paper a proof of Birkhoffs theorem is provided. However, there is one step that I am quite suspicious of, so I would like to know what you think about it. The relevant part of the paper for ...
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How do we detect a black hole?
If light cannot escape from a black hole, how do we detect its presence? I mean there is nothing that can be faster than light so if light can not escape from the black hole there should be nothing ...
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Can something escape from inside of a black hole's event horizon by putting a sufficiently large mass next to it?
As far as I know, a lot of the maths surrounding black holes basically assumes that the black hole is the only mass in the entire universe. But what if something has fallen into the event horizon of a ...
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Black hole collision with a probe
I understand that for an outside observer it will take an infinite amount of time for an infalling object to reach the black-hole event horizon.
Consider a black hole that orbits around the center of ...
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Constructing Embedding Diagram of Reissner-Nordstrom Dihole [closed]
Is it possible to create embedding diagrams of $\phi=constant$ slices of axisymmetric two black home spacetimes, like the Reissner-Nordstrom dihole?
$d\Sigma^2=U(x,y)^2(dx^2+dy^2)$
I ask because the ...
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Is proper distance an invariant quantity?
I read on the Wikipedia page for proper length that proper distance is a quantity that is the same for all observers. I am not sure what it means exactly.
I learned from this answer that proper ...
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Is interpreting information conservation as time reversibility non-mainstream physics?
I understand the following as standard results in modern physics.
Black holes evaporate over time via Hawking radiation.
Small black holes evaporate over very short times.
There is an "...
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Why is the Einstein Field Equation relevant in the area theorem?
I am studying Area theorem and the first assumption is as follows :
If Einstein equation holds satisfy null energy condition...
I don't understand in general, what does it mean to satisfy Einstein ...
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What happens physically at the white hole - black hole transition in Kruskal-Szekeres coordinates?
Imagine you are an observer that starts on the timelike past $i^-$ of a maximally extended black hole spacetime and immediately shoots along a null ray hovering on the past horizon $\mathcal{H}^-$ of ...
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Rough estimate of the time until last photon radiated by object falling into black hole is seen by distant stationary observer
When an object falls into a black hole it takes an infinite amount of time according to a distant stationary observer until it reaches the horizon. From this point of view, such an observer will never ...
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Are there other ways we know of a massive star becoming a Black Hole other than the star going supernova? [duplicate]
Are Black Holes in the past, but also in the present, possibly formed by a process other than "very massive stars going supernova"?
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Would a star with the same mass as a Black Hole appear as a dark star?
In the case of the Black Hole (BH) the surface is a few Km from the center of mass whereas in the case of an active star with the same mass as the BH, the surface can be be many millions of Km distant ...
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When two black holes spinning in opposite directions approach one another is the Kerr metric destroyed as two regions of frame-dragged space meet?
The kerr metric describes the frame-dragged space just outside a spinning uncharged black hole. I have read in popular science articles that frame dragging is like a stick spinning in treacle causing ...
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Are black holes …white holes? [closed]
Time dilation is infinite at the event horizon, after all.
As you approach the event horizon, your frame of reference slows asymptotically to match that of the black hole. While the universe around ...
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Black hole radiating massive particles
Is it sensible to write the metric for a black hole that emits radiation that is composed of massive particles as
$$ds^2 = -\left(1-\frac{2M(t)}{r}\right)dt^2 + \frac{dr^2}{\left(1-\frac{2M(t)}{r}\...
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How can black hole mergers happen if we cannot see anything fall into a black hole? [duplicate]
If we cannot observe anything fall into a black hole as an external observer, how can we observe a black hole merger?
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How to prove Hawking area theorem using Raychaudhuri equation?
I recently started studying black holes and I am trying to understand the Hawking area theorem and its mathematical basis (at least at a simplified level). Some resources say that the Raychaudhuri ...
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Toroidal planets to toroidal black holes -- what changes?
It's believed that a sufficiently quickly rotating planet-sized mass could be stable in a toroidal planet formation (though vanishingly unlikely to form naturally). However, assuming no cosmological ...
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Violation of cosmic censorship by negative mass Schwarzschild geometry - weak or strong?
Can anyone tell me which censorship is violated by Schwarzschild geometry with negative mass? Is it weak or strong cosmic censorship? I think the weak cosmic censorship is not violated because it is ...
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What would the kinematic of a black hole falling on an object look like?
I wonder what would the kinematics of a black hole falling on an object be.
To make it realistic, I got told that the black hole needs to have a radius bigger than atoms, so that it can swallow them. ...
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Turning an apple into a blackhole: feasibility & effects on surroundings
I read that it is possible to turn an apple into a blackhole by compressing it to the size of 10^-28m, according to the Schwarzschild radius formula.
My first ...
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Event and cosmological horizons in black hole structures
Sometimes, we face two horizons in the dS black hole. How do we determine the difference between the event horizon and the cosmological event horizon? How is it calculated, and what is the difference? ...
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