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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Is it mathematically possible for a black hole to be connected to a white hole or a worm hole?
I know that there's no evidence at this point for "white holes" however would it even be mathematically possible for a black hole to be connected to a white hole (total opposite so everything would be ...
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0answers
45 views
Kerr solution for finite collapse time
The Kerr black hole solutions gives an analytic continuation that is asymptotically flat. Some people have argued that this is another universe, but others state that the analytic continuation ...
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9answers
2k views
How does this thought experiment not rule out black holes?
How does the following brief thought experiment fail to show that general relativity (GR) has a major problem in regards to black holes?
The full thought experiment is in my blog post. The post ...
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1answer
342 views
Moving black holes
What happens to the fabric of space in the wake of a moving black hole? Is space permanently deformed by a moving black hole or does it rebound as the black hole passes?
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1answer
89 views
NGCC 1277— a recoil ejection?
Recent calculations agree that a merging pair of supermassive black holes can emit enough gravitational waves to eject themselves from a galaxy.
Could NGCC 1277, a small galaxy with a 17 billion ...
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0answers
43 views
transition between extremal and nonextremal black hole states
Extremal black holes are at zero temperature, hence they do not radiate.
my question is twofold:
1) is extremality of micro black holes a stable property? electric charge is quickly emitted from ...
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0answers
166 views
Nonlinear refraction index of vacuum above Schwinger limit
This question is more about trying to feel the waters in our current abilities to compute (or roughly estimate) the refraction index of vacuum, specifically when high numbers of electromagnetic quanta ...
3
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0answers
152 views
micro black hole forces
A black hole would radiate mass optimally for interstellar-travel applications in the range between $10^7$ and $10^8$ kilograms. Assuming a light-only radiation emission spectrum, with a parabolic ...
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3answers
306 views
An electron falling into a black hole
If an electron falls into a black hole. How can the Heisenberg uncertainty principle hold? The electron has fallen into the singularity now so it has a well defined position which means that it ...
8
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4answers
301 views
At what rate does a rotating black hole lose mass via Hawking Radiation?
I initially thought it was inversely proportional to the mass, but I think that's wrong because temperature is inversely proportional to mass. If someone could give the formula(s) for finding this ...
4
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1answer
127 views
Is it possible for a black hole to form for an observer at spatial infinity?
To my knowledge if you calculate the coordinate time (time experienced by an observer at spatial infinity) it takes an infinite amount of time for an object to fall past the horizon of a Schwarzschild ...
4
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1answer
179 views
Can a blackhole eat a blackhole?
I'm not a physicist and I do not understand maths. But I watch documentaries about "how it all began", "the big bang", "What is time", etc etc just really fascinating.
I was wondering if a blackhole ...
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2answers
66 views
A thought about Quasars
If Quasars are "beams" of energy exiting a super-massive black hole, in order for them to get through the black-hole's event horizon, they'd have to be traveling faster than the speed of light. My ...
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1answer
116 views
Way to escape from a black hole
I’ve had a question on WHY a traveler couldn’t “escape” from a black hole under specific conditions (I have an image I'd like to send to clarify, but the website won't let me)>
The key is for the ...
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4answers
106 views
How do black holes accrete mass?
Thanks to time dilation, a distant observer watching a man fall in to a black hole will only see him asymptotically approach the event horizon. So how do black holes ever get bigger?
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3answers
322 views
Thermodynamically reversed black holes, firewalls, Casimir effect, null energy condition violations
Scott Aaronson asked a very deep question at Hawking radiation and reversibility about what happens if black hole evolution is reversed thermodynamically. Most of the commenters missed his point ...
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1answer
152 views
Kerr geodesics differential equations in equatorial plane
With friend, we are writing an interactive educational simulation of particle falling into a black hole.
Currently we use Schwarzschild geodesics. However, we want to generalize it to the case of ...
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1answer
151 views
Thought Experiments on black holes, (im)possible perpetual motion and minimum mass for photographic plates
Consider a black hole in vacuum at Temperature T. The setup that I am interested is a one that collects thermal photons from a black hole by enclosing in a spherical photographic plate and then ...
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3answers
231 views
What is exactly the density of a black hole and how can it be calculated?
How do scientists calculate that density? What data do they have to calculate that?
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6answers
605 views
What prevents the accumulation of charge in a black hole?
What prevents a static black hole from accumulating more charge than its maximum? Is it just simple Coulomb repulsion?
Is the answer the same for rotating black holes?
Edit
What I understand from ...
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3answers
295 views
Gravity stronger than electromagnetic force in a black hole?
Well, the question has somewhat been answered before, but there's one part missing, which - I'd think - is in conflict with the physical laws.
The earlier reply says that the gravitational pull even ...
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5answers
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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?
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2answers
533 views
If nothing in the universe can travel faster than light, how come light can't escape a black hole? [duplicate]
Possible Duplicate:
How does gravity escape a black hole?
If nothing in the universe can travel faster than light, how come light can't escape a black hole? I mean, Einstein's relativity ...
11
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7answers
990 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 ...
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1answer
148 views
what does holographic principle from string theory say about the possibilities of wormhole travel?
Is travel through stable macroscopic wormholes between remote points of spacetime going to be possible in a definitive theory of gravity, be it string theory or something beyond it?
Physicists level ...
2
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2answers
197 views
Is the Schwarzschild black hole unphysical?
To obtain the Schwarzschild metric from Einstein equations of general relativity, we suppose that the energy density is a distribution :
$$ \rho (\vec{r}) = M \delta(\vec{r})$$
The Schwarzschild ...
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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 ...
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3answers
182 views
How many pieces of toast would you need to make a black hole?
I am trying to find the following:
How many pieces of toast would you need to make a black hole?
From what I've learnt so far I need to find an equation for the compression force the massive amount ...
2
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2answers
259 views
Hawking radiation from point of view of a falling observer
This paper tells that Hawking claimed that the falling to a black hole observer will not detect any radiation. But only because the frequency of the Hawking radiation will be of the order $1/R_s$ so ...
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1answer
99 views
Light orbiting a massive body [duplicate]
Possible Duplicate:
Can a photon be made to orbit a known (or undiscovered theoretical) body?
How massive would a black hole have to be for light to orbit it at 1km away from the ...
5
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2answers
760 views
Black hole analog experiment?
This question is directed mostly at people giving lectures on black holes, but input by other physicists or students is very much appreciated.
Do you know a good (home)-experiment with a black hole ...
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0answers
36 views
What is some analogous experiment about the black holes by using the diary product like eggs, milk? [duplicate]
Possible Duplicate:
Black hole analog experiment?
I will explain my situation a little bit:
My teacher assign a experimental project that must be including the diary product: egg and milk ...
9
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5answers
466 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 ...
10
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3answers
327 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 ...
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1answer
194 views
What is the speed for an object that travel close to the speed of light?
I have some questions regarding Einsteins theory of Relativity that should be fairly easy to answer. Lets say we make an experiment where we have a rocket (with an astronaut inside) that travels very ...
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1answer
140 views
Why are black hole singularities stable?
The Friedmann equations says that huge matter densities lead to huge expansion rates. In Newtonian gravity, two massive point particles separated by an infinitesimal distance will experience an ...
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1answer
78 views
What it is like to walk on black hole? Is it hot inside the horizon? How big stars form into black holes? [closed]
What it is like to walk on black hole? Is it hot inside the horizon? How big stars form into black holes? thank you for your answers.
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1answer
200 views
When a neutral star with a magnetic field collapses to form a black hole, what happens to the magnetic field?
By the no-hair theorem, black holes are only characterized by mass, charge and angular momentum. If the star is neutral, the black hole will have only mass and angular momentum - and therefore it ...
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2answers
161 views
Information Loss in annihilation
The concept of information loss is usually discussed with respect to a black hole. My understanding is that whatever matter you put into the black hole, it has only 3 "hairs" and so one doesn't know, ...
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2answers
295 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 ?
6
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1answer
199 views
What happens to an embedded magnetic field when a black hole is formed from rotating charged dust?
Black holes have no-hair so there are uniquely specified by a mass, charge and angular momentum. Imagine a cloud of charged rotating dust. There will be a magnetic field associated with the current ...
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1answer
153 views
If a magnetic monopole falls into a schwarzchild black hole, what happens to the magnetic field?
By the no-hair theorem, black holes can only have mass, charge and angular momentum. Does "charge" include "magnetic charge" (such as from a magnetic monopole)? Can black holes have magnetic charge ...
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2answers
257 views
How can black holes be so dense?
It is said that if the Earth were a black hole, it would be the size of a peanut!?
How is this density possible, are atoms really that sparse that they can be compressed so tightly? Is there some ...
8
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2answers
496 views
What is the physical size of a black hole?
Something that's always confused me. How large is a black hole's physical size - not mass?
From descriptions, it would seem that the 'singularity' is a single point, but is it really?
Say for ...
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4answers
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How can super massive black holes have a lower density than water?
I'm new here, so go easy!
I heard on a podcast recently that the supermassive black holes at the centre of some galaxies could have densities less than water, so in theory, they could float on the ...
4
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1answer
157 views
Are “typical” black holes rotating, or stationary?
From my (somewhat limited) understanding of GR I know that there are two different kinds of solutions that produce a black hole, some that rotate and some that do not. What I can't figure out from my ...
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1answer
123 views
Is a new universe created when dark energy rips open a singularity?
Dark energy will tear everything apart, even black holes. Is a new universe created when dark energy rips open the singularity of a black hole ? And does the topology of the universe depend on the ...
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1answer
350 views
Spaghettification of humans near black holes
A few months ago I was discussing the spaghettification phenomenom with my wife, just for the fun of it. This was when the mass of the super massive black hole from M87 hit the news. The black hole ...
5
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6answers
314 views
Charged Black holes - What is the Interest in those?
Here in this forum (and elsewhre) I read about "charged" black holes.
What is the background for this questions? Is this more a question of
theroretical interest?
In "practice" :=) black holes ...
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2answers
201 views
What is the 'apparent horizon' of a black hole?
The Wikipedia article is full of general relativistic and differential geometry jargon, and its accuracy is questionable. So, without such (or explaining said jargon without more jargon), what is the ...
