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)

4
votes
2answers
73 views

How Smooth is an Event Horizon?

For a non-rotating Black Hole, the event horizon can be described by the Schwarzschild metric as a sphere. Assuming external observer, away from the Black Hole and also assuming that there is no ...
5
votes
1answer
115 views

What happen if Hawking radiation is not found?

I'm not a physicist but reading from a few popsci books. I read that Hawking has equations that explain Hawking Radiation precisely, right? Is this equation a proof that the radiation is real or we ...
3
votes
2answers
208 views

Sending someone into the future using the sun?

Ok so for starters, i am not nearly as smart as the least-smart person on here, but I have been reading a lot about all sorts of stuff. We now know that the ability to "send" someone to the future is ...
3
votes
1answer
29 views

Does the Bekenstein entropy bound present problems for unitarity in cosmological models that invovle a collapse or bounce?

If we expect the Bekenstein bound, or something like it, to hold in a collapsing universe, will that not eventually force us to accept some loss of information, or is there something I'm missing?
-3
votes
2answers
57 views

Big Crunch, then new Big Bang = exact same Universe? [on hold]

If after a Big Crunch, the new singularity explodes in a Big Bang, would we get the same Universe all over again? Since black holes retain all the information they've stored, would we get an exact ...
-3
votes
0answers
24 views

What does gravitational lensing equal in time diolation to gravity? [on hold]

What is the equivalent in gravity to time dilation around a black hole?
1
vote
0answers
95 views

Are strings in string theory actually little black holes? [on hold]

I sometimes read that strings in string theory are actually little black holes, or can be interpreted that way. Is this true? How is that consistent with that the particle that a string represents ...
4
votes
2answers
120 views

What is the black hole information paradox really? [on hold]

Preliminaries What is the black hole information paradox really? Is it a sophisticated way to ponder and debate the existence of an operator on the boundary that can tease out the interior of a ...
0
votes
0answers
25 views

Wave equation for odd spacetimes and source terms

It seems to be quite common practice, when solving the wave equation in spacetimes with odd topologies or horizons, to decompose the solution into a sum of the various origins (or destinations) of the ...
2
votes
0answers
33 views

How can matter be squeezed extremely dense [duplicate]

I'm fairly new to physics and currently reading A Brief History of Time by Stephen Hawking. Hawking is talking about black holes being the size of a few miles containing the mass of multiple times the ...
4
votes
2answers
123 views

What does string theory predict for the singularity inside a black hole?

The usual explanation for what's going on inside a black hole goes something like "General Relativity predicts a singularity with infinite curvature, but when matters gets so tightly compressed we ...
-6
votes
1answer
52 views

Black hole gravity at bottom (deepest end) [closed]

If black holes are hole in space and time then at bottom or below bottom there is no more gravity of black hole. As any hole has an end (Deepest end). Is this true ? ******** EDIT *********** http:/...
10
votes
3answers
2k views

Will a giant ball of protons form a black hole?

Suppose you have enough energy and resources to put together (in a momentarily static configuration in which they are all at rest at the same time) as many protons as you want to form a "proton star". ...
-1
votes
1answer
36 views

Can light change the shape of a black hole?

If light orbits a black hole without ever getting closer or further from it, and the black hole pulls on the light from all directions, does the black hole extend at its equator? Would more light make ...
1
vote
0answers
28 views

How to find tetrads corresponding to a given metric?

In General Relativity, sometimes doing calculations with a set of orthonormal tetrads $h^{a}_{\mu}$ where $h^{a}_{\mu}$$h^{b}_{\nu}$$\eta_{ab}=g_{\mu\nu}$ for some metric $g_{\mu\nu}$ is easier ...
1
vote
1answer
48 views

Why do stars having mass less than 3 X mass of sun do not collapse into a black hole

Why do stars having mass less than 3 X mass of sun do not collapse into black holes?? Is there a way to find out the radius of a black hole if we know the mass of the parent star and vice versa??
-3
votes
0answers
45 views

How to work out a black hole radius if you know its density

How would you work out the size of the Schwarzschild radius knowing the energy density of a black hole?
17
votes
2answers
2k views

What is black hole spin?

First of, congrats to the people at LIGO. In this article, the BBC notes that the latest LIGO results show that a new black hole was formed with a spin of $0.2$ (dimensionless number). What exactly ...
-1
votes
1answer
22 views

How does the mass of a black hole varies?

Is there any change of the mass of a black hole if time passes ? As it absorb any mass near it, will the mass of the black hole gets increased, or as it radiates energy, will the mass of it gets ...
-9
votes
1answer
156 views

Why does Ligo's second detection of gravitational waves and a black hole merger look absolutely nothing like the first? [closed]

Why does Ligo's reported second detection of gravitational waves and a black hole merger look absolutely nothing like the first detection announced in Februaray? Here is the data from the first LIGO ...
3
votes
1answer
36 views

AdS Black holes

How is the mass of the black hole defined in a asymptotcally AdS solution of the black hole? How can I find it? Beacause in asymptotcally flat solution I can read it from the $g_{tt}$ component of the ...
5
votes
2answers
127 views

Black hole simulation

NASA published a computer generated black hole image. In the image you can clearly see the event horizon and the light of background stars graduating from "smeared" to normal. However, between the ...
0
votes
0answers
17 views

Object traveling towards a black hole close to the speed of light [duplicate]

Fluff: I will start by saying my knowledge of physics lacking to say the least. I am asking this question purely out of curiosity. The question: If an object was traveling close to (or at (if it ...
53
votes
8answers
12k views

If the speed of light is constant, why can't it escape a black hole?

When speed is the path traveled in a given time and the path is constant, as it is for $c$, why can't light escape a black hole? It may take a long time to happen but shouldn't there be some light ...
2
votes
2answers
62 views

Effect of black hole mergers on life [closed]

Would the gravitational energy released in the explosion affect life in any way if it were on a planet close by?
-1
votes
1answer
74 views

Faster than light travel…? [duplicate]

So - Black holes possess a gravitational force so great it can pull anything in. Including light. So placing these side by side, If I were to shoot a laser pointer out of a cannon at a black hole, ...
2
votes
1answer
52 views

GR Verification for a Charged Black Hole

For a charged ($Q$) rotating ($L$) mass ($M$), the Kerr-Newman equations give the angular deflection of light. Has there been observational verification (I would prefer to use only the angular ...
1
vote
1answer
32 views

Does binary black holes evaporate slower because they compete for attracting virtual particles?

Is it true that if 2 black holes are near each other, such as binary black holes, will evaporate slower than when they are far apart, because they are competing for attracting virtual particles each ...
2
votes
2answers
54 views

Merging black holes makes them less dense, so

According to What is exactly the density of a black hole and how can it be calculated? (more specifically, John's answer here made me think: if you merge a whole load of chunks of an element heavier ...
1
vote
1answer
107 views

Does space itself fall into a black hole? [duplicate]

Long time ago I heard someone say that it is space itself that falls into a black hole. Yesterday I saw a little animation that suggested the same (although I´m not sure, because the person who put ...
-6
votes
1answer
95 views

Must the fine structure constant be an irrational number? [closed]

This question concerns a conjecture on the nature of $\alpha = e^2/4\pi\epsilon\hbar c$ in light of black hole holes. Let me consider the Reissnor-Nordstrom metric for a charged black hole $$ ds^2 = \...
5
votes
1answer
136 views

Is Hawking radiation really the same as Unruh radiation

I read that Hawking radiation is the same as Unruh radiation. However, there seems to be a paradox here. If you have an extreme black hole (say with maximum charge), then it has temperature 0 and ...
2
votes
2answers
103 views

What is the singularity of an actual collapsing black hole?

In most general relativity texts, the singularity is treated as a point removed from the manifold, to avoid having to deal with the infinite curvature of the Ricci scalar. But in the case of a more ...
2
votes
1answer
47 views

Charge without charge and non-traversable wormholes

My question concerns the theory proposed in this classic paper by Misner and Wheeler. In the paper, the authors propose the idea of "charge without charge"--namely, that positive and negative ...
1
vote
2answers
83 views

Low density black holes and singularity

According to wikipedia "... the average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be less than the ...
1
vote
0answers
32 views

BTZ Black holes

I computed the two point function for two scalar fields in BTZ black hole that is defined as a local $AdS_3$ space time with discrete identification, as defined in many papers. Referring to this paper,...
10
votes
2answers
902 views

Orbits around the Photon sphere of a black hole (Schwarzschild coordinates)

This is a follow-up question to the answer given at What is the exact gravitational force between two masses including relativistic effects?. Unfortunately the author hasn't been online for a few ...
1
vote
1answer
56 views

Could gravitational waves have a large entropy in quantum gravity?

The question is broad, and we still don't know what is quantum gravity. But let me be more specific. In the question (on this site, see below) on whether gravitational waves have entropy the answers ...
5
votes
2answers
223 views

Could planets survive their star becoming a black hole? [duplicate]

Would the supernova responsible for its formation destroy them. The question becomes one of how destructive the supernova event is to its own solar system - does it completely evaporate all planets up ...
2
votes
1answer
36 views

Are the quasinormal modes scalar quantities?

I am studying the so-called quasinormal modes (QNMs) in the context of the AdS/CFT correspondence and I got stuck. For instance, if I choose a weird patch of coordinates for the, say, AdS5-...
2
votes
3answers
113 views

Would a particle the size of a neutron, if it had enough mass, collapse into a blackhole?

For example, a neutron is a particle that occupies a certain volume. If you pack enough mass into that volume, it would collapse into a black hole (I assume there is not enough mass now). At least if ...
1
vote
1answer
89 views

Kerr black hole horizons and infinite redshift surfaces

In the Kerr black holes we have infinite-redshift surfaces (where a infalling body is still according to the asymptotic observer) and event horizons (the escape velocity becomes greater than the speed ...
1
vote
2answers
66 views

Two Black Holes held stationary by EM forces

If two black holes with large enough mass (so that the tidal forces are minimal and the intersection is large) that are held apart by like charges (saddle point stability). Imagine the black holes in ...
3
votes
1answer
59 views

Torsion in kerr black holes

In General Relativity, we generally assume that the derivative operator is torsion-free, i.e., second covariant derivatives commute on functions. However, in Kerr black holes, spacetime is dragged (...
0
votes
1answer
37 views

Do the energies of cosmic rays approach infinite at the event horizon of a black hole?

Let's assume an observer orbits close to a black hole, he is not alone, massive cosmic rays, like electrons and protons and other kind of space dust comes from the outer space and may hit him. Since ...
3
votes
1answer
63 views

Schwarzschild metric black hole

Schwarzschild metric solution presents two singularities. An apparent one at $r=2GM$ and a real one at $r=0$. It is known that everything freezes at the event horizon from an outside observer point of ...
3
votes
4answers
94 views

Can we “see” into a black hole using gravity?

I believe the "no hair" theorem means all black holes settle down into a state only determined by a few parameters, typically listed as mass, charge and angular momentum. But I don't think they can ...
0
votes
2answers
69 views

Can objects exit a black hole?

I read an article claiming that an object had been observed to be ejected from a black hole. This was the newest article I could find published on it but they still seem uncertain what the object was ...
0
votes
0answers
44 views

Will two entangled particles changing Synchronously while one is in a black hole and the other is not?

1.If you change the attribute of one entangled particle then the other one changes too and instantaneously. 2.The time will go slower in (or near to) a black hole compare to our normal place like on ...
4
votes
0answers
28 views

Collapse of two large black holes in AdS

In $4d$ flat space, two black holes of mass $M$ can collapse to form another one of (roughly) mass $2M$. This process is spontaneous, as reflected by the fact that the black hole entropy $S=M^2$ ...