All Questions
224
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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
1
vote
0
answers
51
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Does gravity accelerate you towards the geodesic of light between you and the mass?
If there's a planet far away, you will accelerate straight towards it due to gravity. If you place a Schwarzschild black hole right in the middle between you and the planet (the distance between the ...
- 171
2
votes
1
answer
161
views
When you are in a gravitational field, do object far away get physically closer to you as you get closer to the mass?
An observer A is close to a black hole and an observer B one light year away. They are both remaining at constant radial distance from the black hole. A is at 2 Rs away from the center of the black ...
- 171
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
26
votes
10
answers
13k
views
How do black holes move if they are just regions in spacetime?
If black holes are just regions of spacetime, how can black holes even move? When matter moves through spacetime, it bends the spacetime around it, but if black holes are just regions of spacetime, ...
- 403
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1
answer
116
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Time required to reach Black Hole's Event Horizon from outsider perspective?
Let's imagine a pair of particles that is entangled. One (call it $P_1$) is released and then falls to a black hole from a distant $x_0$, (for example $x_0=5r_s$) and velocity $v_0(=1/2c)$, while the ...
1
vote
2
answers
134
views
Keplerian Frequency of Schwarzschild Black hole
The Keplerian frequency/ Orbital frequency is the inverse of orbital period and for Schwarzschild black hole it is given by $$\frac{1}{2\pi}\sqrt{\frac{M}{r^3}}.$$ its unit is Hertz. Now To express ...
- 21
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votes
1
answer
67
views
What’s the condition to form an astrophysical wormhole?
According to GR, what’s the mechanism for a star to form a wormhole? How is it different from collapsing to a black hole? What’s the energy scale required?
- 195
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
3
votes
0
answers
88
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Intuition for the interior Killing vector fields in Schwarzschild?
The Schwarzschild metric represents a stationary (and static), spherically-symmetric, spacetime. These characteristics are manifested by the four Killing vector fields: one for time translation and ...
- 1,290
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votes
1
answer
102
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Flat space between colliding black holes
When 2 black holes approach each other, they both bend space in an opposite direction. There must always be a flat space between 2 colliding black holes.
However, I heard that they actually merge, ...
- 187
0
votes
1
answer
125
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Event horizon in stationary spacetime
In the case of non-stationary spacetimes finding the event horizon is no easy task.
The stationary case should somehow be less involved or so it is in some well known cases, such as the Kerr spacetime....
- 1,989
1
vote
1
answer
63
views
What effect causes stars to be optically enlarged at the Einstein ring of a black hole?
Sadly i do not know what this effect is called, but it can be seen in some simulations;
Credit to Alessandro Roussel
The same effect can be seen on the Wiki page for black holes.
For a while now, me ...
- 308
1
vote
1
answer
85
views
Metric of Einstein static universe (ESU) black hole
The Einstein static universe (ESU) has metric
$$ g = - dt^2 + d\chi^2 + \sin^2 \chi d\Omega^2 $$
With
$$ t \in \mathbb{R}, \chi \in (0,\pi) .$$
Is there a metric that describes an eternal black hole ...
- 743
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