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.
3
votes
2answers
103 views
Is there something like Hawking radiation that makes protons emit component quarks?
If Hawking radiation can escape from black holes, could quarks perhaps become separated from protons despite it being "impossible" for that to happen?
50
votes
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 ...
4
votes
8answers
610 views
Flat space limit of the Schwarzschild metric and Hawking temperature
The Schwarzschild metric reduces to the Minkowski metric in the limit of vanishing $M$, but the Hawking temperature which is proportional to $1/M$ diverges in the same limit. This would imply that ...
11
votes
5answers
466 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 ...
4
votes
1answer
88 views
Where and how is the entropy of a black hole stored?
Where and how is the entropy of a black hole stored?
Is it around the horizon? Most of the entanglement entropy across the event horizon lies within Planck distances of it and are short lived.
Is ...
15
votes
3answers
113 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 ...
9
votes
1answer
410 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 ...
1
vote
0answers
68 views
Black hole entropy from collapsed entangled pure light
Consider the following scenario, very similar to the one proposed in this question, but this time, the pure quantum radiation used for the black hole collapse, is now being split with down-converter ...
2
votes
1answer
72 views
Killing Vectors of BTZ black hole and their calculation in general
I was wondering what are the Killing vectors of BTZ black hole and how to guess them easily? Will it be the same as of AdS? What then will be Killing vectors for AdS-Schwarzschild e.g.?
4
votes
2answers
81 views
What reason(s) exist to suppose that all degeneracy pressures can be overcome in Black-Hole formation?
In models of stellar collapse to a black hole, it is a given that density increases without bound towards a singularity. Electron degeneracy I get. Neutron degeneracy I get. I assume there's some ...
-2
votes
5answers
506 views
Theoretically, can a black hole collapse until it gets smaller than its Schwarzschild radius?
So, the concept is this:
When a supergiant star collapses, its nucleus becomes a black hole. So, what if that black hole collapses again?
I don't know if it is possible, but what if it happens?
2
votes
1answer
102 views
Would dense matter around a black hole event horizon eventually form a secondary black hole? [duplicate]
Possible Duplicate:
Black hole formation as seen by a distant observer
Given that matter can never cross the event horizon of a black hole (from an external observer point of view), if a ...
0
votes
0answers
36 views
Can a black hole actually grow, from the point of view of a distant observer? [duplicate]
Possible Duplicate:
Black hole formation as seen by a distant observer
I've read in several places that from the PoV of a distant observer it will take an infinite amount of time for new ...
1
vote
1answer
214 views
In general relativity (GR), does time stop at the event horizon or in the central singularity of a black hole?
I was reading through this question on time and big bang, and @John Rennie's answer surprised me.
In the immediate environment of a black hole, where does time stop ticking if one were to follow a ...
4
votes
2answers
136 views
Is Brian Cox right to claim that Gravity is a strong force for large masses, is it wrong, or is it only a matter of interpretation?
I watched a program of his in which it was claimed that since mass bends space in accordance to General Relativity, then in the case of very large stars it becomes a strong force to the point of being ...
4
votes
3answers
181 views
Black hole formation as seen by a distant observer [duplicate]
Possible Duplicate:
How can anything ever fall into a black hole as seen from an outside observer?
Is black hole formation observable for a distant observer in finite amount of time? ...
4
votes
0answers
84 views
Information scrambling and Hawking non-thermal radiation states
Could a very small black hole where half of its entropy has been radiated, emit Hawking radiation that is macroscopically distinct from being thermal? i.e: not a black body radiator. Or would the ...
3
votes
1answer
99 views
Does black hole evaporation respect quantum superpositions?
I've confused myself about the following scenario:
Suppose you make a black hole out of states with spin aligned into one direction, say the positive x-direction, and let's call this "up". Then the ...
1
vote
1answer
109 views
Information loss
First time poster!
I just burnt a piece of paper containing a 5 digit number I made up randomly and as far as I am concerned no one else will ever be able to retrieve the information contained on ...
5
votes
0answers
88 views
Geometric entropy vs entanglement entropy (dependent on curvature coupling parameter)
I have a quick question. In hep-th/9506066, Larsen and Wilczek calculated the geometric entropy (which I believe is just another name for entanglement entropy) for a non-minimally coupled scalar field ...
4
votes
0answers
89 views
Alternate geodesic completions of a Schwarzschild black hole
The Kruskal-Szekeres solution extends the exterior Schwarzschild solution maximally, so that every geodesic not contacting a curvature singularity can be extended arbitrarily far in either direction.
...
4
votes
2answers
168 views
Does the Chandrasekhar Limit scale for a Black Hole?
No physicist/astrophysicist I; All I know about the Chandrasekhar limit is that it apparently limits the mass a star may survive, beyond which it degenerates to a neutron star, or a black-hole.
Does ...
6
votes
1answer
62 views
does the background spacetime of a black hole affects its thermodynamic properties?
The question is this: will the thermodynamic properties of a black hole (Hawking radiation spectra and temperature, entropy, area, etc.) depend if the black hole sits in a DeSitter or an Anti-DeSitter ...
2
votes
1answer
140 views
is this generalized Hawking radiation formula right?
Look at equation 11.2.17 in this page. The expression is:
$$ T = 10^{-5} \text{K m} \frac{\xi}{\frac{GM}{c^2} \lbrace \frac{GM}{c^2} + \xi \rbrace - e^2 }$$
where
$$ \xi = (r_s^2 - a^2 - ...
11
votes
2answers
456 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 ...
2
votes
1answer
95 views
Kerr-Newman black holes and infinite charge
Recall the first law of BH thermodynamics
$ dM=\frac{\kappa}{8\pi} dA + \Omega dJ + \Phi dQ $
Now, let's consider the Reissner-Nordstrom solution $J=0$ such that $m>Q$ but only slightly greater. ...
5
votes
2answers
511 views
Why is spacetime near a quantum black hole approximately AdS?
In this link, one of the answers contains the statement
If you examine the space-time near a finite area quantum black hole,
you will see an approximate AdS space.
Presumably "approximate" ...
5
votes
1answer
159 views
Why do the stars in the galaxy core move so fast?
There are a bunch of stars orbiting the black hole in the center of our galaxy.
These stars move at huge speed. Why do we see this? Why do the black hole not impose any noticeable time dilation on ...
4
votes
0answers
81 views
Can a nearly-extremal black hole be stable against Schwinger vacuum breakdown?
I was doing some basic algebra to estimate the range of possible masses $M$ and electric charge $Q$ for a nearly extremal Reissner-Noström black hole. I want to see if the logic is correct
the ...
4
votes
1answer
181 views
What is a virtual photon pair?
When describing a black hole evaporation in the hawking black body radiation it is usually said that is due to a virtual photon pair, is it this what happens? And what is virtual photon pair, does the ...
2
votes
1answer
46 views
Rotating Empty Sphere around stationary black hole
Would it be possible to create a habitable in terms of gravity planet that would rotate with the black hole acting as a center of gravity? The rotation of the structure would lessen the gravitational ...
2
votes
1answer
227 views
Mass of Milky Way Supermassive Black Hole
I am looking at data from Andrea Ghez (2005; ADS link) which states the mass of the supermassive black hole in the MW. Here's what I don't understand: She reports the value as $3.7 \times 10^6 ...
4
votes
2answers
136 views
What is the motivation for assuming “Page” scrambling for Hawking radiation?
What is the motivation for assuming "Page" scrambling for Hawking radiation?
Obviously, at the semiclassical level, we want the outgoing Hawking radiation to look thermal and mixed. However, surely ...
3
votes
1answer
84 views
Are different frequencies of light lensed differently during gravitational lensing a bit like refraction?
So I was wondering about the event horizon on a black hole. And wondering if the point of no return for radio waves vs gamma rays would be different. I guess the logic being, since gamma rays have ...
1
vote
3answers
231 views
Why can't light escape from inside event horizon of Black Holes?
The simple answer: Its because Gravity of Black Hole there doesn't allow it. See also this and this Phys.SE posts.
Isn't it a classical answer? When we're unable to connect Gravity with Quantum ...
12
votes
5answers
901 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 ...
6
votes
0answers
67 views
Is the Hawking radiation of a charged black hole thermal?
Suppose you have a Schwarzschild black hole of mass $M$ and angular parameter $a = 0$ (no rotation).
Question: is it possible to throw a charge $Q$ at a faster rate that it will be reradiated? Will ...
4
votes
1answer
173 views
Will acceleration rate of expansion of space become faster than speed of light?
From watching cosmology lectures, it seems that the space between galaxies is expanding at an accelerating rate, my question is since it is the space that is (acceleratingly expanding), the special ...
7
votes
1answer
95 views
Could micro black holes obey the Eddington limit?
A stellar-mass black hole has recently been discovered in the Andromeda galaxy. One interesting part of the release is that this black hole shines close to its Eddington limit.
Quasars are ...
0
votes
0answers
54 views
Why don't black holes have magnetic hair? [duplicate]
Possible Duplicate:
What happens to an embedded magnetic field when a black hole is formed from rotating charged dust?
It is well stablished that the only hair a black hole can have is:
...
0
votes
0answers
66 views
How to calculate the mass of the Cygnus X-1 black hole?
I have received a question about how to calculate the mass of Cygnus x-1 (black star).
Since we are able to find the the mass through this Wikipedia page I know that we can find the mass through ...
6
votes
3answers
799 views
Why can't dark matter be black holes?
Since 90 % of matter is what we cannot see, why can't it be black-holes from early on? Is is possible to figure out that there are no black holes in the line of sight of various stars/galaxies we ...
5
votes
0answers
51 views
paper about black branes and implications to 4d black holes
This paper makes a case for piezoelectric response (electric dipole moment under mechanical oscillations) of black branes. This paper does not make an implication of their results for 4D black holes ...
3
votes
1answer
171 views
Event Horizon fluctuating due to gravitational waves
Do the interiors of black-holes create gravitational waves and if so do these waves cause the radius of the event horizon to fluctuate as the waves pass the horizon ?
1
vote
1answer
129 views
Where did Karl Schwarzschild derived his solution?
Does anyone know more about circumstances of Karl Schwarzschild at the Russian front in 1915 where he allegedly derived his famous solution of the Einstein equations (describing a black hole)? Sources ...
2
votes
2answers
161 views
What happens to the wavelength/frequency of a photon as it passes through an event horizon?
I've asked a similar question about photons and black holes but wanted to rephrase it more specifically, so here goes...
Ever since I learned how a photon's wavelength and frequency are indivisibly ...
6
votes
0answers
87 views
Hawking radiation for closely orbiting black holes
Suppose we have two black holes of radius $R_b$ orbiting at a distance $R_r$. I believe semi-classical approximations describe correctly the case where $R_r$ is much larger than the average black body ...
15
votes
2answers
407 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 ...
5
votes
1answer
75 views
Is there a black hole interior in black hole complementarity?
According to black hole complementarity, for an external observer, the interior of the black hole is replaced with a stretched horizon at a Planck distance above where the horizon ought to be. Is this ...
0
votes
1answer
82 views
Nothing escapes BHs, gravitons mediate gravity, so why do BHs gravitate? [duplicate]
Possible Duplicate:
How does gravity escape a black hole?
Nothing escapes black holes, gravitons mediate gravity, so why do black holes gravitate?
My question is, "where is the hole (no ...
