All Questions
83
questions
4
votes
1
answer
169
views
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 ...
- 450
1
vote
1
answer
63
views
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 ...
- 1,989
0
votes
1
answer
99
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:
$$\kappa^2=-\frac{1}{2}(\nabla_{\mu}K_{\nu})(\...
2
votes
2
answers
168
views
Can a body escape a black hole by being thrusted? [duplicate]
I am told many time that nothing can escape black-hole because black-holes escape velocity is more than speed of light. But we know object don't necessarily have to exceed speed of light to escape a ...
- 41
7
votes
3
answers
557
views
Why does rotation make black holes smaller?
A non-rotating black hole has a Schwarzchild radius of $2GM/c^2$. A rotating black hole of the same mass has a smaller outer horizon radius, down to a limit of $GM/c^2$ at the fastest possible ...
- 185
0
votes
0
answers
27
views
Black Holes and Gravity [duplicate]
We know that nothing including light can escape the gravitational pull of black hole. Now special relativity says that nothing travels faster than speed of light. Then how can effects of gravity due ...
1
vote
0
answers
51
views
Surface gravity inside a star
In general relativity, there is a definition of the surface gravity that is associated with the Killing vectors, is there a similar definition for the interior of stars/compact objects?
I would expect ...
- 63
0
votes
1
answer
378
views
How small do you need to compress mass to make a black hole?
I am wondering if there is an equation for how close you need to compress a mass to make a black hole. I have seen claims that if the earth is shrunk to the size of a golf ball, it would create a ...
- 101
4
votes
2
answers
379
views
Gravity of black holes
I was looking At a couple different replies to some questions on here and I kept seeing people saying that larger mass black holes have a lower gravitaional strength on their surface than lower mass ...
- 41
1
vote
0
answers
72
views
Collapsing shell of null matter
In Naked Singularity (1973), Penrose claimed that the inequality
$$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
90
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
1
answer
136
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, ...
- 3,155
27
votes
5
answers
8k
views
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
2
votes
1
answer
95
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 ...
- 2,908
-1
votes
1
answer
289
views
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