All Questions
79
questions
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End points of event horizon
I am reading The Nature of Space and Time by S. W. Hawking. In the last paragraph on page 16 he said that:
event horizon may have past end points but don't have any future end points
I understand ...
- 59
3
votes
1
answer
79
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How to Understand Negative Energy in the Ergoregion?
I am trying to understand the Penrose process and having trouble explaining negative energy in the ergoregion.
How I interpret it is:
Energy is the dot product between the four momentum of the object ...
- 63
4
votes
0
answers
60
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Can wormhole inside a black hole become an escape?
I did not major in Physics so not sure if this is a proper question; but according to some Google search there do exist papers discussing wormhole inside black hole like this, which I am not able to ...
- 141
21
votes
5
answers
3k
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Does an object approaching a black hole ever cross the combined event horizon of the black hole and itself?
Once you start studying black holes, one of the first things you'll probably hear is that from an outsider's perspective objects falling into the black hole take an infinite time to do so due to time ...
- 356
2
votes
2
answers
136
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What happens if $ a^2 > M^2 $ in Kerr metric?
(Boyer-Lindquist coordinates and $ c = G =1 $ taken)
As I know, line element in Kerr metric $ d s^2 = - \left( 1 - \frac{2Mr}{\rho^2} \right) d t^2 - \frac{4 M a r \sin^2 \theta}{\rho^2} d \phi d t + \...
- 25
1
vote
1
answer
111
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Carter-Robinson Theorem
There are uniqueness theorems that classify Black holes according to its mass, angular momentum and charge. One of the theorem is Carter-Robinson theorem which has many assumptions and then it says ...
- 59
1
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0
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83
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What is the meaning to the switch $dt^2\to-dt^2$ and $dr^2\to-dr^2$ in the Schwarzschild metric?
What is the meaning of the change $dt^2\to-dt^2$ and $dr^2\to-dr^2$ in the Schwarzschild metric, leading to:
$$g=-c^{2}d\tau^{2}=(1-\frac{2GM}{c^{2}r})c^{2}dt^{2}-(1-\frac{2GM}{c^{2}r})^{-1}dr^{2}+r^{...
- 476
2
votes
2
answers
161
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If I were to drop my phone into a black hole, would I be able to catch it?
Say, for the sake of argument, I am outside the event horizon of a black hole and accidentally drop my phone (or some other object) into the hole. If I were to enter the black hole, would I ever be ...
- 121
0
votes
2
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113
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Light-like vector in Schwarzschild metric
If I shoot a light ray radially, assuming the Schwarzschild metric, what vector would I plug in to the metric tensor to get a relationship between the $t$ and $r$ coordinates? That is what vector do I ...
- 43
2
votes
2
answers
168
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Can a body escape a black hole by being thrusted? [duplicate]
I am told many time that nothing can escape black-hole because black-holes escape velocity is more than speed of light. But we know object don't necessarily have to exceed speed of light to escape a ...
- 41
2
votes
3
answers
118
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What is the radius of a black hole? [duplicate]
I know the formula for the event horizon is $$R_s = 2GM/c^2.$$ At this distance the escape velocity equals the speed of light so nothing can escape from a black hole from this distance or less. But ...
- 371
4
votes
0
answers
84
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Conformal Diagram for Astrophysical Black Hole
I have a question about the conformal diagram of an ‘astrophysical’ black hole which forms in finite time (but with no evaporation).
Usually I see the conformal diagram presented as something similar ...
- 55
3
votes
2
answers
96
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Influence of Schwarzschild radius and event horizon of ordinary objects
As far as I know, every body (with mass) has a Schwarzschild radius and therefore an event horizon. Thus, take an orange with radius $R$ and mass $M$ for example, then the Schwarzschild radius is ...
- 41
2
votes
1
answer
98
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How much time does it take for the gravitons generated by a black hole singularity to travel before exerting gravity forces on other celestial bodies?
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 ...
1
vote
1
answer
117
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(1+1)d collapsing null-shell?
I am trying to understand the following Penrose diagram (from https://arxiv.org/abs/1507.03489)
According to the authors, it is depicting the formation of a (1+1)d black hole from a collapsing null ...
- 329