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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What happens to orbits at small radii in general relativity?

I know that (most) elliptic orbits precess due to the math of general relativity, like this: source: http://en.wikipedia.org/wiki/Two-body_problem_in_general_relativity I also know that something ...
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Event horizons without singularities

Someone answered this question by saying that black hole entropy conditions and no-hair theorems are asymptotic in nature -- the equations give an ideal solution which is approached quickly but never ...
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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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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) ...
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How can a singularity in a black hole rotate if it's just a point?

I guess nobody really knows the true nature of black holes, however, based on everything I know about black holes, there is a "singularity" at their center, which has finite mass but is infinitely ...
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Would the inside of a black hole be like a giant mirror?

As any light reflected or emitted from objects inside a black hole (if it is possible to be there) does not leave the event horizon and comes back inside, would it be like seeing yourself? What I ...
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The final death of a black hole

What are the different death scenarios for a black hole? I know they can evaporate through Hawking radiation - but is there any other way? What if you just kept shoveling more and more mass and ...
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4answers
732 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 ...
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Evidence for black hole event horizons

I know that there's a lot of evidence for extremely compact bodies. But is there any observation from which we can infer the existence of an actual horizon? Even if we are able to someday resolve ...
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What is the capture cross-section of a black hole region for ultra-relativistic particles?

What is the capture cross-section of a black hole region for ultra-relativistic particles? I have read that it is $$\sigma ~=~ \frac{27}{4}\pi R^{2}_{s}$$ for a Schwarzschild BH in the geometric ...
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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 ...
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666 views

Do singularities have a “real” as opposed to mathematical or idealized existence?

I was thinking of, for example a Schwarzchild metric at r=0, i.e. the gravitational singularity, a point of infinite density. I realise that there are different types of singularities--timelike, ...
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1answer
121 views

Black holes and Time Dilation at the horizon

What is the difference between proper time and the observer time? Whilst thinking about Black holes, when we see the Schwarzschild metric $$c^2\tau ^2 = \left ( 1 - \frac{r_{s}}{r} \right )c^2t^2 - ...
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2answers
491 views

How long does it take for a black hole to form?

The well-known fable of an astronaut sending signals out to an external observer while falling toward an event horizon states that the time lapse between such signals becomes greater even if in the ...
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1answer
301 views

Black hole entropy

Bekenstein and Hawking derived the expression for black hole entropy as, $$ S_{BH}={c^3 A\over 4 G \hbar}. $$ We know from the hindsight that entropy has statistical interpretation. It is a measure ...
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1answer
228 views

When would the proposed black hole at the centre of Milky Way gulp in our solar system? [duplicate]

I've heard and read that our solar system lies near to the peripheral region of the Galaxy. Then accordingly we would have a greater probability of sustaining to eventual gulping down by the ...
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5answers
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Is there a black hole in the centre of the Milky Way?

Is it true that the whole galaxy is actually revolving, and powered by a black hole? Has it been proven, and if it is true, how can our solar systems actually keep up the momentum to withstand the ...
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3answers
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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?
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830 views

Could Dark Matter form black hole?

Some people speculate that the mysterious dark matter in the universe could be tiny black holes. But on the other side, could dark matter particles attract each other by gravity and finally form a ...
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756 views

Are electrons just incompletely evaporated black holes?

Imagine a black hole that is fast-approaching its final exponential throws of Hawking evaporation. Presumably, at all points in this end process there there will remain a region that identifiably ...
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1answer
171 views

Does compression of matter increase gravity?

Black holes are thought to originate when a star burns out and collapses to a smaller physical size. This compresses it's matter, but would that increase it's gravity? And would it be enough to ...
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2answers
665 views

Does the mass of a star change as it collapses into a black hole?

I know (I think!) that when a really big star collapses on itself it creates a black hole. My question: When a star collapses, is the mass equal to the mass of the star when it's not a black hole? Or ...
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1answer
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Formation of supermassive black holes

Scientists have found very bright source of light which they call quasar and the are found to be supermassive black holes. So these black holes are so massive that they cannot be formed by a ...
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2answers
202 views

When does a singularity start to exist during a black hole formation?

Excuse me if the question is naïve. I am not a physicist. Let's imagine formation of a black hole from a homogeneous collapsing star. At certain moment when enough of matter is inside of a small ...
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3answers
109 views

How can black hole increase its mass?

From observer point of view an object, which falls into black hole never crosses its horizon. Then how does black hole appears and grows its mass? Or does any black hole looks (and feels by all other ...
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275 views

How can things at the event horizon slow down and appear to stop to a remote observer?

So they say the remote observer will never see anything fallen to the black hole, because any object will slow down as it gets closer to the event horizon and eventually stop to stay there forever. Am ...
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2answers
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ergosphere treadmills

suppose you place a number of rotating black holes in linear sequence (rotating around the same axis) between two stars at distance $d$ (assume as tightly packed as practical for purposes of ...
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1answer
103 views

The arrow of time and the cosmology of a black hole interior

When solving the Einstein field equations in Schwarzschild metric for an observer falling into a black hole the radial coordinate r of the black hole and time t switch roles in the equations when ...
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1answer
176 views

Is black hole singularity a single point?

General relativity expressed in terms of differential geometry. And it lets you to do interesting things with the coordinates: multiple coordinates may refer to a single point, eg. the equirectangular ...
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3answers
176 views

Photons straight into black hole

What happens to a photon shot straight into a black hole? Does it gain infinite momentum before it crosses the horizon? If it has a finite momentum going in, then it would seem that a photon of the ...
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159 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 ...
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2answers
713 views

Why do photons add mass to a black hole?

Why do photons add mass to a black hole? When photons are taken irreversibly into a black hole does the mass of the BH increase?
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1answer
125 views

interaction between mathematical structures [closed]

From a physicist's perspective there are several situations in which somehow arbitrary choices of mathematical structures can be made. One can describe a system from different perspectives, etc. ...
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5answers
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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 ...
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2answers
183 views

Speed of gravity in cosmological codes and ephemeris generation

There are few questions in Phys.SE concerning the speed of gravity, and the answers are traditionally that the speed of gravity equals to the speed of light. But in that case I have three more ...
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Falling into a black hole

I've heard it mentioned many times that "nothing special" happens for an infalling observer who crosses the event horizon of a black hole, but I've never been completely satisfied with that statement. ...
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1answer
301 views

How small are the smallest black holes?

How little mass can a black hole contain and still be a "stable" black hole? What would the diameter be, in terms of the event horizon?
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1answer
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Is this derivation of Black Hole entropy viable?

This question is motivated by this one. Suppose $l$ is the minimum measurable unit of length. What is entropy of a spinless particle contained in this interval? We know that entropy of a two-level ...
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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 ...
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Smolin on Cosmological selection and neutron stars

Regarding the cosmological selection hypothesis and testable predictions, Lee Smolin asserted the following: "Smolin: I did make two predictions which were eminently checkable by astrophysical ...
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476 views

What are the final particles emitted from an evaporating black hole?

Hawking radiation predicts that black holes can slowly evaporate through the effective emission of a particle. This particle is a real particle, as in, it is not a black hole itself. I'll write this ...
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5answers
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Why is the intensity of Hawking radiation dependent on the size of the black hole it comes from?

I am asking for a non mathematical answer. I think I have a pretty good understanding of physics without the maths, which unfortunately eludes my understanding. It may be the answer would be obvious ...
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2answers
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Why does a star beyond a certain mass limit (Chandrasekhar limit) only become a black hole?

Why does a star beyond a certain mass limit (Chandrasekhar limit) only become a black hole? A star is first made of hydrogen, it undergoes nuclear fussion reaction combining into helium and ...
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1answer
150 views

What happens when a black hole dies?

It just vanishes into space leaving nothing behind or does it expel some material? Also, talking more about black holes, as far as i understand the term "temperature", it is defined by the amount of ...
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2answers
201 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?
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1answer
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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 ...
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About the Hayden-Preskill circuit

Can someone summarize as to what are the problems and/or the open questions with the Hayden-Preskill circuit? (in the context of understanding black-holes or as a computer science question)It gives a ...
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1answer
187 views

Finding the Schwarzchild radius of a star of solar mass 30

I am currently trying to determine the Schwarzchild radius of a star with solar mass 30. I am calculating it both with respect to solar mass, and w.r.t kilograms, however I am getting conflicting ...
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2answers
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Calculating the size of the “lensing sphere” of a black hole

Given a black hole of some size, say $10^8$ solar masses, how can the size of its sphere of influence of light be calculated? To clarify, ultimately I'd like to be able to calculate the apparent ...