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.
2
votes
0answers
51 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 ...
1
vote
1answer
35 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 ...
4
votes
0answers
51 views
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 ...
-3
votes
1answer
80 views
Is it possible (theoretically) to divide Black Hole into two parts? [duplicate]
I have read that it's not possible.
10
votes
2answers
392 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 ...
3
votes
0answers
39 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 ...
-3
votes
0answers
27 views
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 ...
20
votes
3answers
776 views
+50
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 ...
91
votes
13answers
5k views
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 ...
0
votes
2answers
40 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 ...
10
votes
5answers
676 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 ...
1
vote
5answers
276 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 ...
4
votes
3answers
627 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 ...
3
votes
1answer
75 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 ...
0
votes
0answers
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.
5
votes
2answers
949 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 ...
1
vote
0answers
15 views
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 ...
2
votes
2answers
87 views
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?
2
votes
1answer
48 views
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 ...
13
votes
5answers
441 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 ...
0
votes
0answers
69 views
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 ...
5
votes
3answers
105 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 ...
1
vote
1answer
199 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 ...
3
votes
2answers
217 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?
3
votes
2answers
94 views
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 ...
7
votes
2answers
104 views
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.
0
votes
2answers
88 views
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 ...
1
vote
3answers
54 views
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. ...
0
votes
1answer
358 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 ...
2
votes
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 ...
8
votes
8answers
2k views
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 ...
0
votes
0answers
54 views
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 ...
2
votes
1answer
251 views
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
votes
1answer
111 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 ...
0
votes
1answer
102 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 ...
2
votes
1answer
42 views
“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 ...
1
vote
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 ...
0
votes
0answers
157 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 ...
2
votes
1answer
55 views
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 ...
1
vote
0answers
48 views
Singularities in Schwarzchild space-time
Can anyone explain when a co-ordinate and geometric singularity arise in Schwarzschild space-time with the element
$$ ...
38
votes
2answers
575 views
Analog Hawking radiation
I am confused by most discussions of analog
Hawking radiation in fluids (see, for example,
the recent experimental result of Weinfurtner et
al. Phys. Rev. Lett. 106, 021302 (2011), ...
9
votes
5answers
325 views
What happens to light and mass in the center of a black hole?
I know that black holes are "black" because nothing can escape it due to the massive gravity, but I am wondering if there are any theories as to what happens to the light or mass that enters a black ...
1
vote
2answers
90 views
What happens to things when things get crushed in a blackhole [duplicate]
When a black hole destroys things until they are smaller than molecules, where does it go and what happens when it clogs up?
4
votes
1answer
141 views
Does the curvature of space-time cause objects to look smaller than they really are?
What's the difference between looking at a star from a black hole and looking at it from empty space?
My guess is that the curvature of space-time distorts the wavelength of light thus changing the ...
3
votes
1answer
79 views
Why does the Schwarzschild radius become excessively large after a certain point?
Here's something that I've found difficult to wrap my head around. The relationship between the Schwarzschild radius and mass is linear. It's generally known that if you take an object in the universe ...
1
vote
3answers
166 views
Where 2 comes from in formula for Schwarzschild radius?
In general theory of relativity I've seen several times this factor:
$$(1-\frac{2GM}{rc^2}),$$
e.g. in the Schwarzschild metric for a black hole, but I still don't know in this factor where 2 comes ...
6
votes
1answer
214 views
How much of a star falls into a black hole?
http://blogs.discovermagazine.com/badastronomy/2011/04/05/astronomers-may-have-witnessed-a-star-torn-apart-by-a-black-hole/
A lot of the star in the disc, a lot of the star in the jets, precisely how ...
1
vote
1answer
50 views
Does non-mass-energy generate a gravitational field?
At a very basic level I know that gravity isn't generated by mass but rather the stress-energy tensor and when I wave my hands a lot it seems like that implies that energy in $E^2 = (pc)^2 + (mc^2)^2$ ...
9
votes
2answers
303 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
1answer
105 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 ...
