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.
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How does gravity escape a black hole?
My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. It would seem to ...
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7answers
3k views
How can a black hole produce sound?
I was reading this article from NASA -- it's NASA -- and literally found myself perplexed. The article describes the discovery that black holes emit a "note" that has physical ramifications on the ...
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2answers
591 views
Analog Hawking radiation
I am confused by most discussions of analog
Hawking radiation in fluids (see, for example,
the recent experimental result of Weinfurtner et
al. Phys. Rev. Lett. 106, 021302 (2011), ...
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7answers
558 views
Do all black holes have a singularity?
If a large star goes supernova, but not enough mass collapses to form a black hole, it often forms a neutron star. My understanding is that this is the densest object that can exist because of the ...
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1answer
325 views
Overcharging a black hole
Hubeny's 1998 paper got a lot of people interested in determining whether cosmic censorship can be violated by dropping too much charge onto a black hole. It suggested that you might be able to get a ...
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6answers
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How fast a (relatively) small black hole will consume the Earth?
This question appeared quite a time ago and was inspired, of course, by all the fuss around "LHC will destroy the Earth".
Consider a small black hole, that is somehow got inside the Earth. Under ...
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3answers
840 views
From where (in space-time) does Hawking radiation originate?
According to my understanding of black hole thermodynamics, if I observe a black hole from a safe distance I should observe black body radiation emanating from it, with a temperature determined by its ...
17
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6answers
2k views
How can a black hole emit X-rays?
Considering that a black hole's gravity prevents light from escaping, how can a black hole emit X-rays? Visible light and X-rays are both electromagnetic radiation, shouldn't the black hole's gravity ...
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3answers
443 views
Direct observations of a black hole?
I'm not very knowledgeable about physics generally, but know that nothing can escape a black hole's gravitational pull, not even light (making them nearly invisible?).
My question is: What has been ...
16
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4answers
911 views
Why do we care about black hole interiors' physics?
Whatever happens in there is not falsifiable nor provable to the outside. If for (amusing) example the interior consisted of 10^100 Beatles clones playing "Number Nine" backwards, do we know how to ...
15
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3answers
118 views
Information Retrieval
This question is motivated by the issue of information retrieval from black holes, but it is essentially a question about quantum information.
It is widely believed (in certain circles) that the ...
15
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8answers
2k views
How can anything ever fall into a black hole as seen from an outside observer?
The event horizon of a black hole is where gravity is such that not even light can escape. This is also the point I understand that according to Einstein time dilation will be infinite for a ...
15
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2answers
412 views
is there a way to split a black hole?
Classically, black holes can merge, becoming a single black hole with an horizon area greater than the sum of both merged components.
Is it thermodynamically / statistically possible to split a black ...
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5answers
471 views
Why can't you escape a black hole?
I understand that the event horizon of a black hole forms at the radius from the singularity where the escape velocity is c. But it's also true that you don't have to go escape velocity to escape an ...
13
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2answers
418 views
Detection of the Electric Charge of a Black Hole
By the "No Hair Theorem", three quantities "define" a black hole; Mass, Angular Momentum, and Charge. The first is easy enough to determine, look at the radius of the event horizon and you can use the ...
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7answers
1k views
How precisely does a star collapse into a black hole?
I think we all heard general statements like "once big enough star burns out there is nothing to prevent the gravitational collapse ending in a black hole". But I can't remember even seeing the ...
12
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1answer
693 views
Can one black hole suck in another black hole?
In the recent news, scientists at NASA have found “unprecedented” black hole cluster near Andromeda’s central bulge. I wonder why doesn't all these black holes merge and such each other in until just ...
12
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3answers
1k views
If the universe were compressed into a super massive black hole, how big would it be?
I understand only a little of general relativity, but that's why I'm here! :)
Consider the hypothetical situation of some extra-terrestrial intelligence pushing all the mass in the universe, every ...
12
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3answers
713 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 ...
12
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5answers
778 views
Are elementary particles ultimate fate of black holes?
From the "no hair theorem" we know that black holes have only 3 characteristic external observables, mass, electric charge and angular momentum (except the possible exceptions in the higher ...
12
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3answers
793 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 ...
12
votes
6answers
782 views
Do all massive bodies emit Hawking radiation?
It is known that any accelerated observer is subject to a heat bath due to Unruh radiation. The principle of equivalence suggests that any stationary observer on the surface of a massive body should ...
12
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3answers
3k views
How do we determine the mass of a black hole?
Since by definition we cannot observe black holes directly, how do astronomers determine the mass of a black hole?
What observational techniques are there that would allow us to determine a black ...
12
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5answers
945 views
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 ...
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5answers
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How exactly does time slow down near a black hole?
How exactly does time slow down near a black hole? I have heard this as a possible way of time traveling, and I do understand that it is due in some way to the massive gravity around a black hole, but ...
11
votes
5answers
472 views
Binary Black Hole Solution of General Relativity?
This is rather a technical question for experts in General Relativity. An accessible link would be an accepable answer, although any additional discussion is welcome.
GR has well known solutions ...
11
votes
2answers
416 views
What is the mass density distribution of an electron?
I am wondering if the mass density profile $\rho(\vec{r})$ has been characterized for atomic particles such as quarks and electrons. I am currently taking an intro class in quantum mechanics, and I ...
11
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5answers
802 views
Double slit experiment near event horizon
What happens if you perform a double slit experiment near an event horizon, if one of the slits is outside, one is inside the event horizon?
11
votes
2answers
471 views
If a Kerr-Newman black hole is like a charged, spinning, heavy magnet, what kind of magnet is it like?
I was reading up on De Sitter spaces, which states that the gravitational effects from a black hole is indistinguishable from any other spherically symmetric mass distribution. This makes a lot of ...
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2answers
412 views
Extremal black hole with no angular momentum and no electric charge
A black hole will have a temperature that is a function of the mass, the angular momentum and the electric charge. For a fixed mass, Angular momentum and electric charge are bounded by the extremality ...
11
votes
1answer
133 views
General definition of an event horizon?
Horizons are in general observer-dependent. For example, in Minkowski space, an observer who experiences constant proper acceleration has a horizon.
Black hole horizons are usually defined as ...
11
votes
1answer
287 views
Quantum uncertainty of particle falling in black hole
A stationary observer at infinity sees a particle of mass m falling in a supermassive Schwarzschild black hole. He observes an increasing redshift and sees the particle ceasing to progress when it ...
11
votes
4answers
939 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 ...
10
votes
6answers
1k views
Does any particle ever reach any singularity inside the black hole?
I am not a professional physicist, so I may say something rubbish in here, but this question has always popped in my mind every time I read or hear anyone speak of particles hitting singularities and ...
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votes
4answers
315 views
What is Hawking radiation and how does it cause a black hole to evaporate?
My understanding is that Hawking radiation isn't really radiated from a black hole, but rather occurs when a particle anti-particle pair spontaneously pop into existence, and before they can ...
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5answers
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Anti-Matter Black Holes
Assuming for a second that there were a pocket of anti matter somewhere sufficiently large to form all the type of object we can see forming from normal matter - then one of these objects would be a ...
10
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3answers
353 views
Hawking radiation and reversibility
It's often said that, as long as the information that fell into a black hole comes out eventually in the Hawking radiation (by whatever means), pure states remain pure rather than evolving into mixed ...
10
votes
4answers
683 views
Can black holes form in a finite amount of time?
One thing I know about black holes is that an object gets closer to the event horizon, gravitation time dilation make it move more slower from an outside perspective, so that it looks like it take an ...
10
votes
5answers
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Are we inside a black hole?
I was surprised to only recently notice that
An object of any density can be large enough to fall within its own
Schwarzschild radius.
Of course! It turns out that supermassive black holes at ...
10
votes
3answers
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Why is there a flux of radiation in the Hawking effect but not in the Unruh effect? (and other questions)
This question is slightly related to this one Do all massive bodies emit Hawking radiation?, which I think was poorly posed and so didn't get very useful answers. There are several questions in this ...
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votes
5answers
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How to get Planck length
I know that what Planck length equals to.
The first question is, how do you get the formula
$$\ell_P~=~\sqrt\frac{\hbar G}{c^3}$$ that describes the Planck length?
The second question is, will any ...
9
votes
3answers
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How would you describe a black hole to a complete layman?
Non-astrophysicist visitors to Black Hole, will most likely barely make it past the introduction before getting lost in the technical details. Moreover, the Wikipedia article does not teach laypeople ...
9
votes
6answers
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Why can't light escape from a black hole?
Photons do not have mass (that's why they can move at speed of "light").
So, my question is how the gravity of black hole can stop light from escaping?
9
votes
2answers
322 views
Black Hole Photon Sphere
The photon sphere is a spherical region in space where photons are forced to travel in an orbit at $r = \frac{3GM}{c^{2}}$.
Is it possible to detect these spheres?
What happens if I fall through ...
9
votes
5answers
508 views
Is it possible for one black hole to pull an object out of another black hole?
Suppose we have a spacecraft just inside the event horizon of a black hole, struggling to escape, but slowly receding into it. Another (bigger) black hole expands until its event horizon includes the ...
9
votes
5answers
388 views
What happens to light and mass in the center of a black hole?
I know that black holes are "black" because nothing can escape it due to the massive gravity, but I am wondering if there are any theories as to what happens to the light or mass that enters a black ...
9
votes
2answers
233 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) ...
9
votes
1answer
70 views
Global symmetry in string theory
It is often stated that in quantum gravity only charges coupled to gauge fields can be conserved. This is because of the no hair theorem. If a charge is coupled to a gauge field then when it falls ...
9
votes
1answer
399 views
Second Law of Black Hole Thermodynamics
I've been looking for a satisfying proof of this, and can't quite find it. I read the brief proof of the black hole area theorem in Wald, which is similar, but doesn't quite come down to the actual ...
9
votes
1answer
421 views
If you are carrying a magnet, can you tell when you cross the event horizon of a black hole?
By the no hair theorem, a black hole is completely characterized by it's mass, charge and angular momentum. Therefore dipole and higher pole magnetic fields are completely expelled when a black hole ...
