All Questions
127 questions
-1
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50
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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 ...
4
votes
1
answer
182
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How are objects inside a black hole affected by the gravity of objects outside the black hole?
There are many Q&As about whether something inside a black hole can escape the event horizon if another massive object gets close enough to pull it out. I realize the answer (I think universally ...
- 482
1
vote
1
answer
77
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Definition of surface gravity via the non-affine geodesic equation
I have found a discrepancy in the way different sources define surface gravity (or derive) via the non-affine geodesic equation satisfied by the a Killing vector $\xi$ on a Killing Horizon (KH), up to ...
- 2,029
0
votes
2
answers
56
views
Observing an event horizon while approaching one
A thought crossed my mind that I realized was hard to conceptualize so I decided to simplify the question by putting it in terms of event horizons.
If I am an observer approaching black hole "A&...
- 101
5
votes
1
answer
541
views
Do gyroscopes still work inside event horizon?
Imagine a gyroscope falls into the event horizon of a supermassive black hole, what would happen to the gyroscope? will it remains in the same position before it falls into the event horizon?
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1
vote
1
answer
106
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Closest possible flyby to a nonrotating supermassive black hole
Imagine you think Laplace is the last word on black holes. That is, you are aware of the radius for which the escape velocity is $c$, but you think gravity is Newtonian. You set your spacecraft on a ...
- 3,680
0
votes
1
answer
110
views
How to derive the surface gravity in terms of redshift factor in static spacetime?
I am following Carroll's book "Spacetime and Geometry" p. 245-247. I have the following expression for the surface gravity of a Killing horizon:
$$\kappa^2=-\frac{1}{2}(\nabla_{\mu}K_{\nu})(\...
2
votes
2
answers
180
views
Can a body escape a black hole by being thrusted? [duplicate]
I am told many time that nothing can escape black-hole because black-holes escape velocity is more than speed of light. But we know object don't necessarily have to exceed speed of light to escape a ...
- 41
7
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3
answers
648
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Why does rotation make black holes smaller?
A non-rotating black hole has a Schwarzchild radius of $2GM/c^2$. A rotating black hole of the same mass has a smaller outer horizon radius, down to a limit of $GM/c^2$ at the fastest possible ...
- 185
0
votes
0
answers
27
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Black Holes and Gravity [duplicate]
We know that nothing including light can escape the gravitational pull of black hole. Now special relativity says that nothing travels faster than speed of light. Then how can effects of gravity due ...
1
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0
answers
57
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Surface gravity inside a star
In general relativity, there is a definition of the surface gravity that is associated with the Killing vectors, is there a similar definition for the interior of stars/compact objects?
I would expect ...
- 63
0
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1
answer
495
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How small do you need to compress mass to make a black hole?
I am wondering if there is an equation for how close you need to compress a mass to make a black hole. I have seen claims that if the earth is shrunk to the size of a golf ball, it would create a ...
- 101
4
votes
2
answers
662
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Gravity of black holes
I was looking At a couple different replies to some questions on here and I kept seeing people saying that larger mass black holes have a lower gravitaional strength on their surface than lower mass ...
- 41
0
votes
0
answers
23
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Is it possible for a black hole singularity to interact gravitationally with other celestial bodies (if we analyze it using the concept of gravitons)? [duplicate]
It is known to all that the travelling speed of gravitons (the propagation speed of gravitational field) is not instant. So for black holes, the gravitons (the gravitational field) generated by the ...
0
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0
answers
61
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If the reduction in Grav. PE in falling into a large Neutron Star approaches $mc^2$, shouldn't we conclude that a black hole is a large Neutron Star?
If we integrate the potential energy due to surface gravity from the surface to infinite radial distance (deep space), with a bit of basic calculus we should get
$\Delta E = mgr [J]$.
This depends ...
- 247
24
votes
7
answers
8k
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Why light can't escape a black hole but can escape a star with same mass? [duplicate]
I'm new to astronomy and was wondering why light can't escape from a black hole but can escape from a star with the same mass. In theory, the gravity of a star 100x the mass of the sun, and the ...
- 341
1
vote
0
answers
79
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Collapsing shell of null matter
In Naked Singularity (1973), Penrose claimed that the inequality
$$A\leq 16\pi G m_0$$
is equivalent to
$$4\pi\int r^2 \Omega \leq [\int r(1-\nabla^2\ln r)\Omega]^2$$
in the spherical symmetric case (...
- 109
0
votes
1
answer
91
views
Is there any evidence black holes can grow (gain mass) by accretion?
When LIGO detected GW150914, we saw for the first time the merger of 2 black holes and the gravitational wave evidence fit with our models and understanding for such an event. (Horizon meeting horizon)...
- 27
1
vote
0
answers
50
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Counterintuitive effect between event horizons of a 'binary black hole system'
I recently posted a question on my 'mobile phone profile' where I cannot post images and videos so I am trying to use this profile to solve my incomprehension of a gravitational-tidal effect that ...
- 467
1
vote
1
answer
49
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If two black holes orbit around each other should their tidal forces cause a shrinking of the closer parts of their event horizons?
I recently asked a question about the influence of external gravitational fields on the stability of the geometry of a part or all the event horizon of a black hole. I understood the answer in a ...
- 2,275
1
vote
0
answers
43
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Influence of external gravitational fields on the volume of a part of the black hole event horizon
FIRST QUESTION: In the event that a smaller black hole orbiting a larger black hole in an elongated elliptical orbit reaches the so-called perihelion of its trajectory, can it temporarily change the ...
- 2,275
1
vote
1
answer
143
views
Gravity near event horizon: very strong/ infinite or approaching zero?
From the point of view of a far-away observer of a black hole, is the gravity on an infalling object near the event horizon very strong or very weak?
There are two competing effects: On the one hand, ...
- 3,151
17
votes
2
answers
6k
views
Space falling faster than light after it falls inside the event horizon of a black hole?
Typing my question directly so people know what I am asking, afterwards providing background and context.
Q: What does it mean when space is falling, faster than light?
(I am specifically wondering ...
- 398
27
votes
5
answers
9k
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What exactly makes a black hole STAY a black hole?
I don't understand how, as a black hole gets smaller and smaller from the excretion of Hawking radiation, it retains its ability to capture photons. I imagine there would be a point in its life cycle ...
- 273
3
votes
1
answer
351
views
Are rotating black holes producing a frame dragging effect inside the event horizon?
Are rotating black holes producing a frame dragging effect inside the event horizon? Is that effect moving space inside the event horizon at speeds far greater than the speed of light?
- 2,275
2
votes
0
answers
140
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What happens when a neutron star becomes a black hole?
Suppose you have a neutron star that's as close as can be imagined to the required mass to become a black hole, perhaps just one proton mass away from this limit, when it collides with a dust grain ...
- 121
2
votes
1
answer
96
views
How compact can a thin shell be without collapsing?
I've heard of https://en.wikipedia.org/wiki/Buchdahl%27s_theorem.
It states that as the ratio between a spheres radius a it's Schwarzschild radius approaches $9/8$. the pressure at the center tends to ...
- 3,001
-1
votes
1
answer
312
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How deep is a gravity well of a supermassive black hole? [closed]
Just for the sake of curiosity I am trying to understand how far we should stay away from the black hole to avoid influence of its gravity on us.
In the first iteration I know about the 2nd and the ...
- 101
1
vote
2
answers
203
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How big is the force just above the horizon of a black hole?
There are huge tidal forces at work around a black hole. But the larger the hole, the smaller the tidal effects near the horizon. So the differences between nearby local forces is small.
What about ...
6
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2
answers
806
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Can two relativistic black holes' event horizons overlap and separate again?
I have read this question:
What I have not seen is a purely classical argument for the non-separation of a black hole merger. One can obviously take the time reversed spacetime manifold of a merger ...
- 29.3k
1
vote
0
answers
112
views
Gravitational collapse seen by a distant observer
Consider the gravitational collapse of a spherically symmetric dust cloud, as seen by a distant observer.
The Oppenheimer-Snyder solution tells us that, at some point during the collapse, the event ...
- 91
3
votes
1
answer
104
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Is there a possibility of frame dragging inside the event horizon?
Is there a possibility of frame dragging inside the event horizon? If frame dragging is related with gravity and space and as the black hole fills a space and produces a gravitational effect that ...
- 1,835
1
vote
0
answers
118
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If a black hole's mass is derived from its gravitational field, then why would mass be restricted inside the EH, while the field extends outside? [duplicate]
I have read this question:
Let's consider the simplest kind of black hole, which is one with no rotation or electric charge. The quick answer to the question is "yes, the mass is entirely within ...
- 29.3k
12
votes
3
answers
4k
views
Shouldn't black holes exert the same gravitational force as an object of similar mass but lower density?
If the earth got shrunk into the size of a peanut, it would turn into a black hole, which would have a higher density but same mass. Since the center of mass of both bodies would be the same, the ...
- 169
2
votes
2
answers
136
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A closed laboratory free falls and approaches the event horizon of a Black Hole. Are measurements of physical constants affected?
A closed laboratory is in perfect free fall and hence is an inertial frame. This lab is falling toward the event horizon of a Black Hole. The lab is sufficiently small and the event horizon is ...
- 143
1
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2
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163
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Will this method work to take a photo inside a black hole?
Taking a Photo Inside a black Hole
The following is a “thought Experiment” which could describe an actual methodology to photograph (and be able to see the photograph) of the area inside a Black Hole’...
- 11
3
votes
4
answers
926
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Do black holes attract more in GR than Newtonian gravity beyond their event horizon? [duplicate]
I was once shown that if a black hole with the mass of our Sun replaced our Sun in the solar system (I believe it was in a simulation program), the attraction it has on the planets will be the same ...
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3
votes
6
answers
277
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What would you see dropping a sufficiently strong chain with substantial length into a black hole?
Here's a visual representation of the scenario
Here you can see we have a black hole on the left. The event horizon is the edge of the black hole. You are far away from the event horizon, and a chain ...
- 357
1
vote
4
answers
12k
views
What is the gravity at the event horizon of a black hole?
I've been reading and couldn't find any answer to this question. What is the minimum required gravity to prevent light from escaping?
- 111
1
vote
0
answers
55
views
Can singularities be ever formed from an outsiders perspective? [duplicate]
As far as I know, as an object gets closer to an event horizon, gravitation time dilation makes it move slower from an outside perspective, so that it looks like it take an infinite amount of time for ...
- 111
0
votes
1
answer
137
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Particular behaviour of 'black hole' horizon in modified gravity
When working in a particular theory of modified gravity, one can see that a solution for a spherically symmetric and static puntual mass is given by
\begin{equation}
ds^2=-B(r)dt^2+A(r)dr^2+r^2d\theta^...
- 77
2
votes
0
answers
35
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How can black holes emit detectable magnetic fields and gravity if nothing can escape the event horizon? [duplicate]
How can black holes emit detectable magnetic fields and gravity if nothing can escape the event horizon?
- 439
1
vote
1
answer
358
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Gravitational potential relative a black hole
If I understand correctly, the escape velocity is $c$ at the event horizon of a black hole. This would seem to mean that an object falling into the black hole from an infinite distance would attain ...
- 582
6
votes
2
answers
3k
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If I jump into a black hole, will I see myself passing event horizon? [closed]
Given a black hole with mass of the Milky Way (1.6x10^42 kg), I built a frame that 1 m above the event horizon. I am jumping from this frame my legs down (since I am afraid to dive into a dangerous ...
- 171
1
vote
1
answer
249
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Computation of surface gravity with Kruskal coordinates
The surface gravity of a Schwarzschild black hole (BH) is
$$
\kappa = \frac{1}{2}f'(r)\Big|_{r=2m}
$$
where $f(r)=1-2m/r$ is the component $g^{rr}$ of the contravariant metric in Schwarzschild ...
- 2,237
2
votes
1
answer
94
views
Behavior of a large spherical void in an infinite universe of uniform density matter
I realise how unrealistic this scenario is, but nevertheless, it does not seem unphysical and it does raise interesting questions. Imagine a flat stationary infinite isotropic homogeneous universe ...
- 2,413
3
votes
2
answers
735
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Horizon of extremal black hole
In David Tong: Lectures on General Relativity in section 6.2.5 there's discussion of extremal black holes.
Metric takes the form:
$$
ds^2 = -\left(1 - \frac{GM}{r}\right)^2 dt^2 + \left(1 - \frac{GM}{...
- 5,737
0
votes
1
answer
449
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Surface gravity and affine parametrization
Consider a Killing vector $\chi^\mu$ with the Killing Horizon $\Sigma$. From Carroll's book (pg 245), along the Killing horizon, the Killing vector obeys the geodesic equation
$$\chi^\mu\,\nabla_\mu\,\...
- 994
2
votes
1
answer
342
views
Effective Geometry of Non-Extremal Near Horizon RN Black Holes
We know that we can obtain AdS$_2\times S^2$ by considering the near horizon limit of extremal RN black holes in $4d$ with various asymptotics, i.e., either Minkowski$_4$, or (A)dS$_4$. How about the ...
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1
answer
44
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Is black hole supposed to oscillate in/out of singularity as matter falls on it?
So, as soon as star radius becomes smaller that Schwarzschild radius it becomes a black hole. But as matter continues to fall on it attracted by the gravitational pull its radius sooner or later will ...
