A black hole is a volume from which photons, or any matter, can not escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

learn more… | top users | synonyms

1
vote
1answer
96 views

How small the earth should be for it to become a black hole

How small would the earth have to be squashed so that it would become a black hole?
0
votes
3answers
652 views

What would happen to the Moon if Earth is turned into a black hole?

Assume that all of sudden the Earth is turned into a black hole. And the moon revolves around the Earth (before turning into a black hole). What would happen to the Moon after earth changes to black ...
12
votes
3answers
1k views

Why is a black hole black?

In general relativity (ignoring Hawking radiation), why is a black hole black? Why nothing, not even light, can escape from inside a black hole? To make the question simpler, say, why is a ...
1
vote
0answers
28 views

Magnetic field-pulsed microwave transmission line

Here's the reference: The researchers showed that a magnetic field-pulsed microwave transmission line containing an array of superconducting quantum interference devices, or SQUIDs, not only ...
3
votes
4answers
471 views

Time inside a Black hole

If time stops inside a black hole, due to gravitational time dilation, how can it's life end after a very long time? If time doesn't pass inside a black hole, then an event to occur inside a black ...
-2
votes
1answer
219 views

The paradoxical nature of Hawking radiation [duplicate]

The definition of a classical black hole is when even electromagnetic radiation can not escape from it. Why then can Hawking radiation be emitted from semi-classical black holes? What is difference ...
6
votes
1answer
73 views

What is baryon loading in the context of gamma ray bursts (GRBs)?

I've read that with short hard gamma ray bursts (shGRBs) associated with the coalescence of NS-NS and NS-BH binaries are expected to be beamed along the axis of the orbital angular momentum (i.e. ...
1
vote
6answers
270 views

Is black hole bright at center?

As we know that light photon cannot escape the gravity of black hole so I was thinking that if that is the surface of the black hole would be bright as all the photons would be there only. Am I right ...
2
votes
1answer
1k views

Anti-matter black hole and time

I have recently read some hard science-fiction story based on an assumption that if time stops (from external observer's perspective) on the event horizon of black hole, then in an anti-matter black ...
6
votes
0answers
140 views

Do semiclassical GR and charge quantisation imply magnetic monopoles?

Assuming charge quantisation and semiclassical gravity, would the absence of magnetically charged black holes lead to a violation of locality, or some other inconsistency? If so, how? (I am not ...
12
votes
2answers
449 views

black hole no-hair theorems vs. entropy and surface area

I was revisiting some old popular science books a while ago and two statements struck me as incompatible. No-hair theorems: a black hole is fully-described by just a few numbers (mass, spin etc) ...
15
votes
3answers
1k views

No hair theorem for black holes and the baryon number

The no hair theorem says that a black hole can be characterized by a small number of parameters that are visible from distance - mass, angular momentum and electric charge. For me it is puzzling why ...
7
votes
4answers
2k views

If an anti-matter singularity and a normal matter singularity, of equal masses, collided would we (outside the event horizon) see an explosion? [duplicate]

If an anti-matter singularity and a normal matter singularity, of equal masses, collided would we (outside the event horizon) see an explosion?
12
votes
4answers
3k views

Why does a black hole have a finite mass?

I mean besides the obvious "it has to have finite mass or it would suck up the universe." A singularity is a dimensionless point in space with infinite density, if I'm not mistaken. If something is ...
0
votes
1answer
151 views

Can you enter a timelike hypersurface?

As I understand it, a timelike hypersurface is one that has only spacelike normal vectors. But does this not imply that a the geodesic of a particle crossing it must be spacelike at that point? But ...
4
votes
1answer
324 views

Diving into a charged (Reissner-Nordstrom) Black hole

Apparently there are two event horizons in this type of black hole, where the second one is known as the Cauchy horizon. According to Carroll, if you go into the first one, you will fall until you ...
2
votes
1answer
152 views

Reconstruction of the initial state from Hawking radiation?

I hear that unitary evolution and information conservation must imply that information about information content that defines the initial state of matter used to create a black hole can be inferred ...
5
votes
2answers
1k views

Do neutrinos of any flavor get trapped in black holes?

This question has been bothering me a bit. I know that neutrinos have super small mass and they interact via the weak force. Since they have a non-zero mass, they should be affected by black holes and ...
4
votes
2answers
617 views

The Uncertainty Principle and Black Holes

What are the consequences of applying the uncertainty principle to black holes? Does the uncertainty principle need to be modified in the context of a black hole and if so what are the implications ...
2
votes
2answers
416 views

Future light cones inside black hole

In Caroll's Spacetime and Geometry, page 227, he says that from the Schwarzschild metric, you can see than from inside a black hole future events all lead to the singularity. He says you can see this ...
0
votes
0answers
94 views

Are quarks the limits? The end of the fundamental science. For collisions on more higher energies will lead to black holes after all?

Are quarks the limit and the Plank scale is believed to be the limit of distance when the very concept of space and length cease to exist (10^-19) Any attempt to research the existence of shorter ...
3
votes
0answers
87 views

What happens when one black hole eats another black hole? [duplicate]

Everyone seems to know about black holes. They have an absolutely massive mass, and if you go into them, time stops from an outside perspective. When a planet gets too close to a black hole, it is ...
0
votes
3answers
160 views

Is singularity at the exact centre of a black hole?

I've read that all paths inside the EH lead to singularity. ALL paths. Even the ones pointing away from it, right? Because there's NO pointing away from singularity, since ALL paths point to it. So ...
13
votes
3answers
951 views

Has the black hole information loss paradox been settled?

This question was triggered by a comment of Peter Shor's (he is a skeptic, it seems.) I thought that the holographic principle and AdS/CFT dealt with that, and was enough for Hawking to give John ...
2
votes
0answers
137 views

Falling into a black hole emitter vs observer

Let's say we are working with the Schwarzschild metric and we have an emitter of light falling into a Schwarzschild black hole. Suppose we define the quantity $$u=t- v$$ where $$dv/dr= ...
1
vote
1answer
139 views

Spacelike slicing of Schwarzschild geometry

I am having trouble understanding how to obtain a spacelike slicing of the Schwarchild black hole. I understand there is not a globally well defined timelike killing vector, so we can define t=cte ...
2
votes
2answers
660 views

Which way does a black hole spin?

As far as I understand, and from what I have been shown in renderings of black holes, they spin (like water going down a drain). My question is, firstly, does the matter being pulled into a black ...
6
votes
2answers
190 views

Relaxation time for deviations from spherical shape of a black hole's event horizon (and waves)

A different question about truly spherical objects in nature (Do spheres exist in nature?) made me think of a lecture I had been at where, as I recall, it was mentioned that the most perfectly ...
0
votes
1answer
154 views

Is it possible that a black hole sucks energy that is the origin of another universes big bang?

The universe expanded from nothing, right? And black holes may be a "gate" to parallel universes, is it possible that stars that is being sucked in by black holes in our universe may be the origin of ...
1
vote
2answers
215 views

about the 1D singularity of black hole

I saw some responses here saying that the singularity into the black hole is one dimension object so my question is : is it possible that the singularity is simply a merger of the 4 dimensions of the ...
4
votes
1answer
283 views

How small are the smallest black holes?

How little mass can a black hole contain and still be a "stable" black hole? What would the diameter be, in terms of the event horizon?
6
votes
3answers
440 views

Charging a black hole?

What would happen if we have a black hole and we start shooting at it a single electron at a time, and go on doing it forever? Would the electrons start to bounce off eventually?
1
vote
1answer
200 views

In the big crunch theory, when the big crunch singularity forms, can the resulting black hole decay through hawking's radiation?

I've been pondering about this and I couldn't really find the answer for this. The big crunch theory postulates that the universe will eventually stop expanding and reverse back in on its self into a ...
3
votes
1answer
362 views

Hawking Radiation: how does a particle ever cross the event horizon?

The heuristic argument for Hawking Radiation is, that a virtual pair-production happens just at the event horizon. One particle goes into the black hole, while the other can be observed as radiation. ...
5
votes
2answers
140 views

following up dark matter accretion in supermassive black holes

A while ago, there was some conspicuous evidence that supermassive black holes didn't seem to be eating dark matter at the expected rate of 70%-30%, in fact, only 10% of the black hole mass increase ...
3
votes
3answers
1k views

Can a black hole collapse in itself?

As we know that the black hole is a lump of highly dense matter, and that's the reason for it's so strong gravitational force. Bat let us assume that it has sucked up a huge amount of mass and it's ...
0
votes
1answer
558 views

Could Dark Matter be a manifestation of tachyons tunnelling between black holes?

Among the many Dark Matter candidates, I wondered if there are any along the lines of the title. The inner horizon of a black hole, with its mass inflation, and/or the high spacetime curvature, seems ...
-1
votes
1answer
129 views

How about destroying a black hole?

Can we destroy a black hole? I know it is pretty powerful and hardcore stuff but there should be a way even theoretically as we know know nothing is perfect in this universe as it exist due to ...
2
votes
3answers
156 views

A Conflict with Black Holes

If a particle with rest mass falls from r = infinity to r = Rs of a black hole it is supposed to reach a velocity of c. But where does all that energy (infinite) come from to bring the rest mass to a ...
1
vote
1answer
126 views

An infalling object in a black hole looks “paused” for a far away observer, for how long?

As I understand, to an observer well outside a black hole, anything going towards it will appear to slow down, and eventually come to a halt, never even touching the event horizon. What happens if ...
2
votes
1answer
134 views

What happens at the moment black hole is created?

Suppose a body has just enough matter in it that just 1 more electron could turn it into a blackhole. When this happens, where does all that matter go?
4
votes
1answer
132 views

Rotation of Spacetime => Change in orbit/path

Along the idea of frame-dragging; Will the rotation of a black hole, which has some velocity v and angular momentum, influence its path in 3D space? I've seen the fact that depending on the ...
0
votes
4answers
181 views

Seeing The Light in a Black Hole

I assume if light can't escape a black hole, then light is in a black hole. Does the light 'shine' inside a black hole
8
votes
2answers
791 views

What happens to orbits at small radii in general relativity?

I know that (most) elliptic orbits precess due to the math of general relativity, like this: source: http://en.wikipedia.org/wiki/Two-body_problem_in_general_relativity I also know that something ...
10
votes
3answers
897 views

What is the capture cross-section of a black hole region for ultra-relativistic particles?

What is the capture cross-section of a black hole region for ultra-relativistic particles? I have read that it is $$\sigma ~=~ \frac{27}{4}\pi R^{2}_{s}$$ for a Schwarzschild BH in the geometric ...
6
votes
2answers
889 views

Why is BTZ black hole asymptotically $AdS_3$?

The metric for the BTZ black hole is $ds^2=-N^2dt^2+N^{-2}dr^2+r^2(N^\phi dt +d\phi)^2$ where $N^2=-M+\frac{r^2}{l^2}+\frac{J^2}{4r^2}$ and $N^\phi=-\frac{J}{2r^2}$. It is often said that BTZ black ...
3
votes
1answer
406 views

The Black Hole Problem

Consider a spherical body of uniform density $\rho$ and initial radius R. You can imagine this body containing another sphere of radius R/2 which touches the center and the periphery of the larger ...
1
vote
1answer
112 views

Black holes and entropy

1)I'm told that for Black Holes, when they radiate (Hawking radiation) particles and anti-particles) split at the event horizon, one going to infinity, the other into the BH. They then lose mass. How ...
4
votes
1answer
191 views

Gauss-Bonnet theorem in the Hawking/Ellis book

At the page 336 of Hawking, Ellis: The Large Scale Structure of Space-Time, the Gauss-Bonnet theorem is stated as $$\int_H \hat{R}\ d\hat{S} = 2\pi \chi(H) \qquad (1)$$ with $$\hat{R} = R_{abcd} ...