All Questions
23
questions
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79
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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
views
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
1
vote
1
answer
109
views
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
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
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
1
vote
1
answer
115
views
(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
3
votes
1
answer
807
views
Why are inner horizons Cauchy horizons?
I know that RN black hole has two horizons, one outer one and one inner one. The outer one is the event horizon.
As far as I know, a Cauchy horizon is the boundary of the domain of dependence of a ...
- 339
3
votes
1
answer
72
views
Black hole metric of reflected shell of incoming light
At this point in Leonard Susskind's eighth lecture on general relativity, he begins a discussion about finding the metric of a black hole formed by an incoming, spherically symmetric shell of light. ...
- 737
1
vote
1
answer
85
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How to describe ‘when’ a black hole actually is? [closed]
If I look at any point in space I can think of it as being in the future because it takes me time to travel there. I can go there and an observer can watch me go there.
When I look at a black hole I ...
- 740
2
votes
1
answer
176
views
Near a white hole, what do lightcones look like?
In the vicinity of a Schwarzchild black hole, spacetime looks like this (at least according to a quick google search). Here, the centre of the black hole is supposed to be at $x=0$ and the event ...
- 77
4
votes
1
answer
394
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Penrose diagram for two black holes
There are well-known Penrose diagrams for black holes:
And for collapsing star:
Diagram for collapsing star is obtained by joining two Penrose diagrams:
Is it possible to join diagrams for two ...
- 5,707
1
vote
2
answers
207
views
Another form of Black Hole Information Paradox?
Consider the Penrose Diagram of Collapsing Gravitational matter :
Any radial light ray (say P) originating from $\mathscr{I}^{-}$ is bound to end up in the Black Hole. The ...
- 648
6
votes
1
answer
473
views
Why can't light travel past the event horizon?
Since the event horizon is defined as the boundary within which the escape velocity is greater than the speed of light, and escape velocity is the speed required for that object to reach infinity away ...
0
votes
1
answer
211
views
Black holes, and faster than light communication
Lets say we have a black hole that has a diameter > 1 light year and we can somehow push and pull this black hole {though the push and pull itself can never exceed lightspeed of course}.Will the other ...
1
vote
0
answers
132
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What would the eternal black hole look like?
The white hole and black hole regions in a Kruskal diagram are said to be actually two different locations. Given the problems with white holes it might be a silly question but, hypothetically, what ...
- 780