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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Amateur's question on Black Holes [duplicate]

Black holes are caused by massive curvature of the fabric of space-time. Is it right in believing theoretically that forces of electromagnetic origin could also lead to distortion of the fabric of ...
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Violating Cosmic Censorship

Let's say we try to remove the event horizon of a Kerr black hole by throwing in matter with some large angular momentum. If it starts with GM > a, could we increase a at all? Would such a particle be ...
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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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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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Information loss in a black hole

How does the Holographic Principle help to establish the fact that all the information is not lost in a black hole?
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How close can two extremal black holes with the same charge can get?

Here's a puzzle I have been pondering over. If we have two extremal black holes with the same charge, the electrostatic repulsion between them ought to cancel the gravitational attraction between ...
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Have scientists been able to find an example of a possible White Hole?

We went to a planetarium last night last night and watched "Black Holes" narrated by Liam Neeson. If I recall correctly, he said that scientists believe that they have identified a number of Black ...
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Can Black Holes be the Dark Matter?

Seems to fit the definition: interacts with gravity, doesn’t radiate energy (except Hawking Radiation) and could create gravity lensing without absorbing very much of the light. Could 80% of the ...
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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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Schwarzschild Metric coordinate sign change in $0\leq r \leq 2GM$

In the event horizon of the Schwarzschild-metric not only the time coordinate but also the radial space coordinate seems to change sign: ...
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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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How do black holes in merging galaxies find each other to merge?

In many accounts of galaxy mergers, the prompt merging of their central black holes, if any, is stated seemingly as too obvious to need further explanation. While I don't dispute that this may indeed ...
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Why are geons unstable? Are there other problems with geons?

I read in various places geons are "generally considered unstable." Why? How solid is this reasoning? Is the reason geons are not studied much anymore because we can't make more progress without ...
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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 ...
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following up dark matter accretion in supermassive black holes

A while ago, there was some conspicuous evidence that supermassive black holes didn't seem to be eating dark matter at the expected rate of 70%-30%, in fact, only 10% of the black hole mass increase ...
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Is a black hole's surface area invariant for distant intertial observers?

Let's imagine I'm very far from any massive objects, so my local space-time is Minkowskian. Off in the distance is a black hole, far enough away that it doesn't noticeably curve space-time near me, ...
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Hawking radiation: direct matter -> energy conversion?

When a black hole evaporates, does it turn all the matter that has fallen in directly to energy, or will it somehow throw back out the same kind of matter (normal or anti) that went in?
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Do black holes have transient color charge?

In the membrane model, when a baryon hits the event horizon its spatially separated quarks will impact the membrane at different times. Doesn't this necessarily mean that black holes acquire, however ...
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Proof of conservation of information [duplicate]

After listening of some lectures of Leonard Susskind about black holes, he mentioned that conservation of information is one of the foundations of physics. After searching the web I cannot seem to ...
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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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Are black holes (and the universe in general) said to exhibit Ostwald's Ripening?

Ostwald's Ripening is a phenomenon where the surface area to volume ratio of droplets causes small particles to shrink until they disappear and for droplets above a certain volume to continuously ...
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Thermal AdS and the Hawking Page phase transition

I have some difficulty understanding the concept of pure thermal radiation, as described in Hawking and Page's paper on the Hawking-Page phase transition. The four-dimensional thermal AdS solution ...
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Black hole temperature in an asymptotically de Sitter spacetime

I am trying to calculate the Hawking temperature of a Schwarzschild black hole in a spacetime which is asymptotically dS. Ignoring the 2-sphere, the metric is given by ...
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Asymmetric Schwarzschild solution - unequal mass on each side

Look at any Kruskal–Szekeres coordinate plot of the Schwarzschild solution. It shows the same mass everywhere. Yet the two sides cannot talk to each other, in that no information, particles, etc can ...
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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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Do black holes play a role in quantum decoherence?

Sorry for such a vague question but I could have sworn I read somewhere that Hawking proposed the reason we might see a classically appearing universe is due to the possible role of black holes in ...
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What is a black hole?

Is there a definition of a black hole in a generic spacetime? In some books, for example Wald's, black holes are defined for asymptotically flat spacetime with strong asymptotic predictability, ...
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Hayden-Preskill informational mirrors and decryption

I do have a question about an assumption made in the very interesting Hayden-Preskill paper of black holes as informational mirrors. Alice throws her top secret quantum diary which is $k$ qubits long ...
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BTZ Black Hole Central Charge and Conformal Weight

I have been trying to reproduce a calculation (equation 4.12) in this paper http://arxiv.org/pdf/1107.2678v1.pdf by Carlip reviewing the derivation of the effective central charge of the BTZ Black ...
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Doesn't the Schwarzschild metric combined with Hawking radiation imply that nothing ever gets past the event horizon of a black hole?

According to the General Theory of Relativity, the coordinate time distance per spacetime distance traveled by a particle freely falling into a black hole gets closer and closer to $0$ as the particle ...
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Why can apparent horizon be computed based on its local geometry?

Why can apparent horizon be computed based on its local geometry? In the paper titled Black Holes, Geometric Flows, and the Penrose Inequality in General Relativity by Hubert L. Bray, has been ...
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Euclidean black hole extrinsic curvature

I have read that the extrinsic curvature at the horizon of a euclidean black hole is zero? Does anybody know how this can be shown?
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Singularity and Black Hole Complementarity

When looking at a (eternal) Schwarzschild Black Hole, we may identify two worlds. The region $R_1$ (right) - our world -, and the region $R_2$ (left) - an other world. The "black hole interior" ...
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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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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 ...
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Black hole - white hole (collision)

A non-spinning, equally massive black hole and white hole experience a direct collision. What shall happen? What shall be the result of such a collision?
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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 ...
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Ray tracing in General Relativity

I would like to find out what one would see at the Schwarzschild radius of a massive non-rotating black hole, if the black hole is surrounded by a bright ring. For that, I would place the observer at ...
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Do new universes form on the other side of black holes?

I have four questions about black holes and universe formations. Do new universes form on the other side of black holes? Was our own universe formed by this process? Was our big bang a black hole ...
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Can black holes be created on a miniature scale?

A black hole is so powerful to suck everything into itself. So is it possible that mini black holes can be created? If not then we could have actively disproved the rumors spread during LHC ...
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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 ...
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Where does information go if thermodynamic death?

Often read that until Hawking the black holes were introducing a problem with information. Allegedly information should be unable disappear, while this was happening in classic black holes. What ...
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Seeing one's back on the event horizon

If we would hypothetically be exactly on the event horizon, we should see our own back, because of the circular motion of photons on the event horizon, right? But what would be the image size, or $-$ ...
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What would a rotating black-hole look like to a “geo-stationary” observer orbiting the black hole

A rotating black hole is believed to contain a ring singularity rather than a point. However, if an astronaut is orbiting the black hole at exactly the same angular velocity as the blackhole (in ...
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What would happen if a negative mass crossed the event horizon of a black hole?

If negative mass really existed and somehow a very fast traveling negative mass object reached near the black hole's event horizon. What would happen when it crosses the event horizon? According to ...
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I need help understanding a step in the derivation of the Schwarzschild solution

I am looking at Wikipedia's article on deriving the Schwarzschild solution. In the section "Simplifying the components", it says, On the hypersurfaces of constant $t$ and constant $r$, it is ...
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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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What is the Schwarzschild metric with proper radial distance?

Reading the marvellous book "The Membrane Paradigm" I stumbled upon a suggested change of variable that I'm not able to deal with. Starting with the usual Schwarzschild metric for the spatial ...
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What would happen if a supermassive blackhole at the center of a galaxy evaporates?

Well the title says it all but I will just explain it in detail a bit. We know that a blackhole eventually has to evaporate due to the hawking radiation. It is also said that the blackholes at the ...
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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 ...