All Questions
Tagged with black-holes event-horizon
1,182
questions
2
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4
answers
182
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Necessity of Singularity in General Relativity
The Schwarzschild solution is the standard example used to describe a black hole, its important points being the event horizon and the central singularity. But this solution is derived by assuming an ...
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9
answers
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Why singularity in a black hole, and not just "very dense"?
Why does there have to be a singularity in a black hole, and not just a very dense lump of matter of finite size? If there's any such thing as granularity of space, couldn't the "singularity" be just ...
5
votes
2
answers
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Hawking Temperature of the BTZ Black Hole
The metric of the BTZ Black Hole is given by
$$
ds^2 = - N^2 dt^2 + N^{-2} dr^2 +r^2(d\phi + N^\phi dt)^2
$$
with
$$
N^2 = -M+ \frac{r^2}{l^2} + \frac{J^2}{4 r^2}, \ \ \ \ \ \ N^\phi = -\frac{J}{2r}...
0
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0
answers
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End points of event horizon
I am reading The Nature of Space and Time by S. W. Hawking. In the last paragraph on page 16 he said that:
event horizon may have past end points but don't have any future end points
I understand ...
- 207k
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votes
1
answer
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Can you calculate the radius of a hypothetical singular surface inside a black hole from observing changes to its linear momentum?
Say there is a ball of unknown radius surrounded by a bubble. The ball represents a hypothetical singular surface inside a black hole and the bubble represents the event horizon.
If you threw marbles ...
13
votes
3
answers
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Why does angular momentum shorten the Schwarzschild Radius of a black hole?
Angular momentum causes the event horizon of a black hole to recede. At maximum angular momentum, $J=GM^2/c$, the Schwarzschild radius is half of what it would be if the black hole wasn't spinning.
...
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1
answer
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How to derive the surface gravity in terms of redshift factor in static spacetime?
I am following Carroll's book "Spacetime and Geometry" p. 245-247. I have the following expression for the surface gravity of a Killing horizon:
$$\kappa^2=-\frac{1}{2}(\nabla_{\mu}K_{\nu})(\...
2
votes
0
answers
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Can part of space be causally disconnected from the rest of the universe by being surrounded by black holes? [duplicate]
Is it possible for black hole event horizons to overlap and form a spherical wall around an island of space (that's not inside a black hole) while still being causally disconnected from the rest of ...
- 207k
3
votes
1
answer
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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 ...
- 1
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 ...
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2
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408
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Is black hole event horizon a transcendent tachyon?
In static spherically symmetric spacetime with metric $$ds^2=e^{2\nu}c^2dt^2-e^{2\lambda} dr^2-r^2 (d\theta^2+\sin{\theta}~d\phi^2)\equiv \delta_1~c^2 d\tau^2 \tag{1}$$ the geodesic equation in ...
- 1,948
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1
answer
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Black Hole Formation -- How Can an Event Horizon be Observed to Grow? [duplicate]
This is a question about black hole formation. To be clear, I’m not suggesting that black holes don’t form. It’s that I’m having trouble with the accepted explanation so there’s a flaw in my logic ...
- 740
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6
answers
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Gravity on supermassive black hole's event-horizon
$M =$ black hole mass
Gravitation is about $r^{-2}$
Schwarzschild radius, $r_{\text{S}}$, is $\propto M$
So, more massive black holes have weaker gravitation at their event horizon.
Consider a black ...
- 740
3
votes
2
answers
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When do we talk about spaghettification or pancakification in black holes?
So I've been doing some research for a while now, and yesterday came across the video of PBS space time talking about what happens to quantum information in a black hole. In the thought experiment ...
- 12.4k
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1
answer
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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 ...
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