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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Blandford-Znajek process: Why/how does the current flow along the magnetic field lines

Related: How would a black hole power plant work? I have put a bit of commentary enumerating my confusions in parentheses I read in Black Holes and Time Warps (Kip Thorne), that quasars can generate ...
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Reconstruction of information stored in an evaporating black hole from the emission spectrum?

For simple setups, where the radiation field deviates not too far from thermodynamic equilibrium (< 10 %), corrections to the Planckian thermal emission spectrum can be calculated (and measured) ...
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Can a super-extremal charged black hole be made out of electrons only?

In a previous Question it was argued that it would be impossible to add enough charge to a black hole to make it pass the extremal black hole limit since adding charge would increase the mass of the ...
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Photons and Black holes

How many photons in one Planck volume would it take to form a tiny black hole? A photon doesn't have mass but it does have energy, $1.0101 \times 10^{-37}$ Joule for red $650$ nm wavelength light if ...
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Does Stephen Hawking believe that General Relativity is wrong? [duplicate]

Stephen Hawking recently said that black holes do not exist. According to Wikipedia, black holes are predicted by General Relativity. So it would seem to follow that Stephen Hawking believes that ...
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What would happen to the Moon if Earth is turned into a black hole?

Assume that all of sudden the Earth is turned into a black hole. And the moon revolves around the Earth (before turning into a black hole). What would happen to the Moon after earth changes to black ...
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Is there a binary black hole system in the middle of the galaxy?

We have observed gravity effects from black holes in the center of galaxies, but galactic centers are dusty so we can’t tell if it’s one black hole or two black holes in a binary system in there. A ...
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Are Stephen Crothers' claims legitimate? [closed]

I came up last night with a talk given by Stephen J. Crothers in which he claims that black holes and the Big Bang have no basis in general relativity. But is he really true? How legitimate are his ...
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What is Hawking radiation and how does it cause a black hole to evaporate?

My understanding is that Hawking radiation isn't really radiated from a black hole, but rather occurs when a particle anti-particle pair spontaneously pop into existence, and before they can ...
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How would you describe a black hole to a complete layman?

Non-astrophysicist visitors to Black Hole, will most likely barely make it past the introduction before getting lost in the technical details. Moreover, the Wikipedia article does not teach laypeople ...
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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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If an anti-matter singularity and a normal matter singularity, of equal masses, collided would we (outside the event horizon) see an explosion? [duplicate]

If an anti-matter singularity and a normal matter singularity, of equal masses, collided would we (outside the event horizon) see an explosion?
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Holographic principle “inside-out view”

From the perspective inside a black hole: Is information about everything outside a black hole - the rest of the cosmos - represented on the inside of the (event) horizon too? NB. I realize it is the ...
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Relaxation time for deviations from spherical shape of a black hole's event horizon (and waves)

A different question about truly spherical objects in nature (Do spheres exist in nature?) made me think of a lecture I had been at where, as I recall, it was mentioned that the most perfectly ...
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From the perspective of an observer inside a black hole's horizon, where does the energy for Hawking radiation come from?

Would energy be seen to "flow" to the outside of the black hole? Through what mechanism?
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How can one reconcile the temperature of a black hole with asymptotic flatness?

A stationary observer very close to the horizon of a black hole is immersed in a thermal bath of temperature that diverges as the horizon is approached. $$T^{-1} = 4\pi \sqrt{2M(r-2M)}$$ The ...
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Is it possible to escape from within event horizon?

I always think that it is not possible to escape from within event horizon. However, some one recently told me with deep conviction that it is possible with sustained energy output. I countered with ...
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What is the “Event Horizon” of a black hole [duplicate]

Can someone please explain what the event horizon of a black hole is? I mean is it the actual surface of the black hole or is it the point of no return where light can no longer escape?
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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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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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Photon Escape Angle From Black Hole

Consider a photon source emitting photons near the surface of a Schwarzschild black hole. What angle, as a function of the source's radius from the event horizon, must the photons be emitted at such ...
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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 ...
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Can we see a spaceship falling into a black hole and entering the event horizon?

Or it pauses in time because the spaceship reaches the speed of light, c?
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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 happens to a photon in a black hole?

Assume a photon enters the event horizon of a black hole. The gravity of the black hole will draw the photon into the singularity eventually. Doesn't the photon come to rest and therefore lose it's ...
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Time inside a Black hole

If time stops inside a black hole, due to gravitational time dilation, how can it's life end after a very long time? If time doesn't pass inside a black hole, then an event to occur inside a black ...
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Hawking Radiation: how does a particle ever cross the event horizon?

The heuristic argument for Hawking Radiation is, that a virtual pair-production happens just at the event horizon. One particle goes into the black hole, while the other can be observed as radiation. ...
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What happens to the wavelength/frequency of a photon as it passes through an event horizon?

I've asked a similar question about photons and black holes but wanted to rephrase it more specifically, so here goes... Ever since I learned how a photon's wavelength and frequency are indivisibly ...
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Do black holes accelerate in spin as they obtain more mass?

It is known that - When a star collapses during the formation of the black hole, the black hole obtains the spin of the star which it collapsed from... What I'd like to know is, If this spin ...
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Black-holes are in which state of matter?

Wikipedia says, A black hole grows by absorbing everything nearby, during its life-cycle. By absorbing other stars, objects, and by merging with other black-holes, they could form supermassive ...
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Angular momentum of a rotating black hole

Is there an upper limit to the angular momentum of a rotating (Kerr) black hole?
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What would happen in a collision of an antimatter singularty and a matter singularity? [duplicate]

Would energy be released if a black hole made out of antimatter and another of matter were to collide?
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Calculating the gravity gradient of a black hole [duplicate]

Given a black hole of, say, $10^8 \odot$ (solar masses), how can I calculate the gravity gradient ($\frac{\Delta g}{m}$?) at a given distance from it?
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Is there some special cutoff density after which spacetime “collapses” and forms a black hole?

With crude calculations following densities can be approximated: Given that radius of proton is $1.75×10^{−15} m$ and it's mass is $1.67 × 10^{-27}kg$, this gives density of proton to be $\dfrac ...
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Dipping a Dyson Ring below the event horizon

The basic assumption about inescapability beyond the Event Horizon is that the necessary escape speed (orbital speed) would exceed speed of light, therefore no object can achieve it. Now, would it be ...
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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 ...
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The role of dark matter in black holes and star formation

In my understanding, there exists a critical mass for which a star needs to be in order for it to collapse into a black hole. This also applies to a certain critical density of gas in order for stars ...
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What does velocity dispersion (sigma) reveal about a galaxy?

I'm getting hung up on this term. In studying SMBHs, I see that velocity dispersion strongly correlates with mass. Just what is the velocity dispersion? How can the velocity dispersion of the galaxy ...
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What is the relationship between the Schwarzschild radius and Black hole Singularity?

What is the relationship between the Schwarzschild radius and Black hole Singularity? Can the Planck length be the length of singularity? Or is the length of the Schwartzschild shorter than the ...
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Photons emitted at the event horizon?

While looking through the questions, a came across a section about black holes. I immediately though; what would happen if an atom is orbiting a black hole and emitted a photon perpendicular to the ...
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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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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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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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about the 1D singularity of black hole

I saw some responses here saying that the singularity into the black hole is one dimension object so my question is : is it possible that the singularity is simply a merger of the 4 dimensions of the ...
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Can a black hole actually grow, from the point of view of a distant observer? [duplicate]

Possible Duplicate: Black hole formation as seen by a distant observer I've read in several places that from the PoV of a distant observer it will take an infinite amount of time for new ...
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Why can't light escape from inside event horizon of Black Holes?

The simple answer: Its because Gravity of Black Hole there doesn't allow it. See also this and this Phys.SE posts. Isn't it a classical answer? When we're unable to connect Gravity with Quantum ...
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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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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 ...
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is gravity always the weakest force [duplicate]

Possible Duplicate: What does it mean to say “Gravity is the weakest of the forces”? Strongest force in nature It is usually taught that the gravitational force is the weakest ...
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Could gravity hold electron charge together?

Could the gravitational force be what holds the charge of the electron together? It seems to be the only obvious possibility; what other ideas have been proposed besides side-stepping the issue and ...