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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Evolution of black holes ensemble
If the Universe contained only black holes with a certain mass and velocity distribution, how would it evolve over time? Is it enough to know the mass/velocity distribution to predict the general ...
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1answer
47 views
Entropy of Black Hole
What is the relation between the entropy of rotating and non rotating Black hole?
Which one's entropy is greater?
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1answer
45 views
How does gravity effects both time and light if they have no mass [duplicate]
I've been reading about how black holes can effect both time and light with gravity. So I was wondering, doesn't something have to have mass to be effected by gravity? And if so, does this mean both ...
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0answers
22 views
Is a dynamical extension of non-commutative black holes feasible?
Non-commutative (sometimes called "fuzzy") black holes are solutions of Einstein's equations obtained with a previous basic assumption of non-commutativity of the coordinates $[x^{\mu},x^{\nu}]=i\, ...
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4answers
331 views
Which gets you first when you are falling into a black hole, the black hole singularity or the cosmic background radiation?
If you look up while you are falling into a black hole you see the universe blue shifted, that is, you see the universe moving quickly forward in time compared to your local time. Since this effect ...
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0answers
23 views
Black hole and photons [duplicate]
I've read in a book that even light can't escape the gravitational pull of a black hole after a certain distance. But this seemed impossible to me.
Imagine a photon, moving directly in the opposite ...
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1answer
282 views
Do spacelike junctions in the Thin-Shell Formalism imply energy nonconservation and counterintuitive wormholes?
The Thin Shell Formalism (MTW 1973 p.551ff) is used to properly paste together different vacuum solutions to the Einstein equations. At the junction of the two solutions is a hypersurface of matter – ...
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2answers
355 views
Are there experiments that are banned from being done at the LHC?
Are there experiments that are banned from being done at the LHC because they are too dangerous?
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1answer
28 views
Can we build a synthetic event horizon?
If we imagine ourselves to be a civilization capable of manipulating very heavy masses in arbitrary spatial and momentum configurations (because we have access to large amounts of motive force, for ...
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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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0answers
55 views
Entanglement and Black holes
If you have two entangled quantum states, One state falls into a black hole and you measure the other state, What can you say about the state that has fallen into the black hole?
If you have billions ...
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1answer
37 views
What's the criteria for black hole thermodynamically stability? (And dynamical?)
It looks like usual criteria (positivity of Hessian; what geometrically means a cancave of entropy) is no useful, becouse entropy is not additive and not extensive for black hole. Then what is the ...
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Gravitational redshift of Hawking radiation
How can Hawking radiation with a finite (greather than zero) temperature come from the event horizon of a black hole? A redshifted thermal radiation still has Planck spectrum but with the lower ...
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1answer
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Is it possible (theoretically) to divide Black Hole into two parts? [duplicate]
I have read that it's not possible.
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2answers
395 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 ...
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0answers
40 views
What is the physical meaning of fact, that Reissner-Nordstrom black hole is thermodynamically unstable?
It is known, that Reissner-Nordstrom black hole is thermodynamically unstable [1].
Does it mean, that there is no Reissner-Nordstrom black hole in physical world?
Does it mean, that there may be ...
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0answers
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Dark Energy, Space Time and Black Holes [closed]
Since space and time are both one and the same would that mean that as time passes and accumulates after the big bang that space is forced to grow? This would explain why all the galaxies are speeding ...
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3answers
803 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 ...
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13answers
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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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2answers
42 views
Do black holes have charges?
Do black holes have charges? If so, how would they be measured? Also, does electricity behave the same way? Black holes affect photons, which are carriers of EM radiation, so do black holes have any ...
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5answers
681 views
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 ...
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5answers
281 views
Theoretical physics and education: Does it really matter a great deal about what happens inside a black hole, or about Hawking radiation? [closed]
I stumbled across this article http://blogs.scientificamerican.com/cross-check/2010/12/21/science-faction-is-theoretical-physics-becoming-softer-than-anthropology/
It got me thinking. Why do we ...
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3answers
637 views
Black holes and positive/negative-energy particles
I was reading Brian Greene's "Hidden Reality" and came to the part about Hawking Radiation. Quantum jitters that occur near the event horizon of a black hole, which create both positive-energy ...
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1answer
77 views
Will the black hole evaporate in finite time from external observer's perspective?
There is the problem that is bothering me with the black hole evaporation because of Hawking radiation.
According to Hawking theory the black hole will evaporate in finite time because of quantum ...
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30 views
How connected thermodynamical stability and dynamical stability for black holes?
Criteria for thermodynamical stability is the convex of entropy. But for black hole entropy is non-additive.
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2answers
967 views
How would a black hole power plant work?
A black hole power plant (BHPP) is something I'll define here as a machine that uses a black hole to convert mass into energy for useful work. As such, it constitutes the 3rd kind of matter-energy ...
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Pair production intefering with gamma-ray laser black hole fabrication
A common "proposal" to make a micro black hole is to use on the order of 10^12 kg of gamma-ray lasing medium and focus all the light at a small point. However, intense light will interact with itself ...
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2answers
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Why don't black holes within a galaxy pull in the stars of the galaxy
visit http://www.nasa.gov/audience/forstudents/k-4/stories/what-is-a-black-hole-k4.html
If black holes can pull even light, why cant they pull the stars in the galaxy?
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1answer
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Why does the Kruskal diagram extend to all 4 quadrants?
Why is it that the Kruskal diagram is always seen extended to all 4 quadrants when the definitions of the $U,V$ coordinates don't seem to suggest that the coordinates are not defined in, say, the 3rd ...
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5answers
445 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 ...
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0answers
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Is there any proof that the speed of gravity is limited? [duplicate]
I must warn that though I'm argumenting with black holes I'm not asking how does gravity escape the black hole!. I want to know if the absolute speed of gravity waves were proven bu an experiment.
We ...
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3answers
109 views
Is it possible to have a singularity with zero mass?
A singularity, by the definition I know, is a point in space with infinite of a property such as density.
Density is Mass/Volume.
Since the volume of a singularity is 0, then the density will thus ...
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1answer
201 views
Special relativity paradox and gravitation/acceleration equivalence
One of the features of the black hole complementarity is the following :
According to an external observer, the infinite time dilation at the horizon itself makes it appear as if it takes an ...
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2answers
219 views
Hawking radiation and black hole entropy
Is black hole entropy, computed by means of quantum field theory on curved spacetime, the entropy of matter degrees of freedom i.e. non-gravitational dofs? What is one actually counting?
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2answers
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Is time going backwards beyond the event horizon of a black hole?
For an outside observer the time seems to stop at the event horizon. My intuition suggests, that if it stops there, then it must go backwards inside.
Is this the case?
This question is a followup ...
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2answers
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What is a sudden singularity?
I've seen references to some sort of black hole (or something) referred to as a sudden singularity, but I haven't seen a short clear definition of what this is for the layman.
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2answers
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How can we detect a black hole? [duplicate]
If black holes are phenomena of very high density (gravitational singularities) which don't emit radiation how can we detect them so far away from us where so much other radiation can hide the black ...
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3answers
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Spaceship split near event horizon
Lets say there's two astronauts, Alice and Bob, going on a space trip to a super-massive black hole. So large that they wouldn't notice any significant spaghettification forces at the event horizon. ...
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1answer
364 views
Firewall's grandfather paradox
See What are cosmological "firewalls"?.
Alice is in freefall in her spacecraft just above the horizon of a gigantic black hole. She measures whether or not the near modes of the horizon ...
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1answer
107 views
Why doesn't the firewall argument also apply to far away ingoing modes?
Gidom Mera's answer at http://physics.stackexchange.com/a/45511 is illuminating, but on closer analysis, it brings up further puzzles.
Backscattering works in both directions. Let's see what we get ...
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8answers
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What are cosmological “firewalls”?
Reading the funny title of this talk, Black Holes and Firewalls, just made me LOL because I have no idea what it is about but a lively imagination :-P (Sorry Raphael Bousso but the title is just too ...
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What is the physical mechanism for the subjective rapid vanishing of the firewall on such a short notice?
Suppose there is an astronomical sized black hole. There is an observer Alice. She jumps into the black hole after it has emitted 2/3 — or 3/4, the exact number doesn't matter — of all the ...
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1answer
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Gravitational Redshift around a Schwarzschild Black Hole
Let's say that I'm hovering in a rocket at constant spatial coordinates outside a Schwarzschild black hole.
I drop a bulb into the black hole, and it emits some light at a distance of $r_e$ from the ...
3
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1answer
112 views
How would you detect Hawking radiation?
Hawking theorized that a black hole must radiate and therefore lose mass (Hawking radiation). According to classical relativity though, nothing can escape a black hole, the hawking radiation would ...
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1answer
104 views
Do all black holes spin in the same direction?
My question is as stated above, do all black holes spin the same direction?
To my knowledge, the spin in the direction of the spin of the matter that created them. Another similar question was asked ...
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1answer
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“WLOG” re Schwarzschild geodesics
Why, when studying geodesics in the Schwarzschild metric, one can WLOG set
$$\theta=\frac{\pi}{2}$$
to be equatorial? I assume it is so because when digging around the internet, most references seem ...
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1answer
47 views
Would the universe get consumed by blackholes because of entropy?
Since the total entropy of the universe is increasing because of spontaneous processes, black holes form because of entropy (correct me if I'm wrong), and the universe is always expanding, would the ...
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0answers
159 views
Spacetime around a Black Hole
If we consider the sun, then space-time is curve around it. My question is that what is the kind of curvature of space and time around the black hole. Is that space and time more curved around the ...
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1answer
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Relativistic Computation?
Is it possible to employ relativity to develop computational technology?
Here is a really basic example:
Build a Computer and Feed it the Problem (say the problem is projected to take 10 years to ...
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Singularities in Schwarzchild space-time
Can anyone explain when a co-ordinate and geometric singularity arise in Schwarzschild space-time with the element
$$ ...
