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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Why does time stop in black holes?

Everyone says that time stops in the black hole. It's a "fact". However, I have never heard everyone explaining that. Of course, I know that observer in weaker gravitational field sees that something ...
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What makes a supermassive black hole move through space?

In this example of a Neutrino jet, it seems that the supermassive black hole moves around the space and meets up with a interstellar object sometimes. Source of the above image and a larger ...
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What is the radius of the event horizon?

I know that the Schwarzschild radius is given by $$r~=~\frac{2GM}{c^{2}}.\tag{1}$$ However, If we had the metric $$ds^2~=~−A(r,t)dt^2+\frac{dr^2}{B(r,t)}+r^2(dθ^2+\sin^2{θ}dϕ^2),\tag{2}$$ where ...
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Do black holes have infinite areas and volumes?

How to calculate the area / volume of a black hole? Is there a corresponding mathematical function such as rotating $1/x$ around the $x$-axis or likewise to find the volume?
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Why is it hard to detect a black hole

I've read in some texts that we can't directly observe a black hole in space because not even light can escape from its gravity. Some of the indirect observational methods mentioned are, gravitational ...
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Is it possible that black holes are also neutron stars, but so dark that we cannot see them?

Since the concept of the singularity in a black hole leads to infinite densities, I wonder if it is really certain that black holes exist? Is there a possibility that massive objects (which are ...
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What is the escape velocity of a Black Hole?

The escape velocity of Earth is $v=\sqrt{\frac {2GM}{R}}$, where $M$ is the mass of the Earth and $R$ it's radius (approximating it as a sphere), and is much less than light speed $c$. But I want to ...
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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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Hawking's alternative to Higgs Boson

I have seen in popular media, claims that Hawking does not believe the Higgs boson exists due to microscopic black holes and even made a bet against it. This is based on something published in ...
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Why is it that black holes emit Hawking radiation? [duplicate]

Black holes are basically neutron stars with such a gravitational force that even light cannot escape from it. But what causes it to emit Hawking radiation?
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Do neutrinos of any flavor get trapped in black holes?

This question has been bothering me a bit. I know that neutrinos have super small mass and they interact via the weak force. Since they have a non-zero mass, they should be affected by black holes and ...
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Will acceleration rate of expansion of space become faster than speed of light?

From watching cosmology lectures, it seems that the space between galaxies is expanding at an accelerating rate, my question is since it is the space that is (acceleratingly expanding), the special ...
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Why is spacetime near a quantum black hole approximately AdS?

In this link, one of the answers contains the statement If you examine the space-time near a finite area quantum black hole, you will see an approximate AdS space. Presumably "approximate" ...
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Charged black holes in equilibrium

Consider a pair of (possibly rotating) charged black holes with masses m1, m2 and like charges q1, q2. It seems that under certain conditions gravitational attraction should exactly cancel ...
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What happens to light that falls into a black hole?

When light enters a black hole, what happens to it? I imagine the photons will either fall into the singularity, or the light will orbit just inside the event horizon indefinitely. (Some background ...
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Can colliding gravitational waves create a black hole?

Whether gravitational waves are real or just a coordinate freedom was argued in the early days of GR. Eventually the conclusion was that they were real. And if they are 'real' then I'm curious if... ...
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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 $E^2 = (pc)^2 + (mc^2)^2$ ...
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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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Charged particle close to a charged black hole - what happens?

Let's assume the Reissner–Nordström metric (charged black hole, non-rotating), for simplicity. The black hole is charged with a powerful electric charge. There's a particle nearby, of non-zero mass, ...
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What's the relation between the Euler $\psi$ function, the digamma function, and the hypergeometric function?

Can somebody help me out with the intermediate details of eqn. (2.5) in this paper? Generalized gravitational entropy. Aitor Lewkowycz and Juan Maldacena. arXiv:1304.4926. Is the Euler $\psi$ ...
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Does cosmic censorship rule out stable toroidal black holes? How?

I'm having a hard time understanding what the arguments against stable toroidal black holes are saying. For many of these, I can't figure out if they're talking about: A non-rotating toroidal event ...
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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 ...
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Why isn't black hole information loss this easy (am I missing something basic)?

Ok, so on Science channel was a special about Hawking/Susskind debating black holes, which can somehow remove information from the universe. A) In stars, fusion converts 4 hydrogen into 1 helium, ...
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Lower limits for steady-state black holes

Stellar mass (and larger) black holes emit Hawking radiation below the temperature of the cosmic microwave background; thus, they should always absorb more energy from space than they emit, and always ...
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How legit is this paper claiming to have observed Hawking radiation? [closed]

I recently stumbled upon this paper. In it, the author claims to have found a signature for Hawking radiation in a condensed matter system. I know that experimentalists have been trying to find ...
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Quarks falling in a black hole?

All I know about quarks is that they make up protons and neutrons and you can't really pull a quark pair apart, you just end up with 2 quark pairs because of all the energy you added becomes 2 new ...
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Equilibrium for a rope hanging in a Schwarzschild spacetime

Update: Trimok and MBN helped me solve most of my confusion. However, there is still an extra term $-(2/r)T$ in the final result. Brown doesn't write this term, and it seems physically wrong. Update ...
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Why does Hawking radiation cause black holes to die? [duplicate]

If a particle is being expelled from a black hole and an antiparticle is being driven into it, shouldn't the opposite occur as well and in the same frequencies? I mean, black holes should emit ...
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Time dilation at a black hole [duplicate]

According to the Wikipedia article on black holes: Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer sees the infalling ...
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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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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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Inside a huge sphere, which is being converted to a black hole

Suppose the following scenario: You reside inside a huge stable spherical star with non-lethal environment at its core. The object is so huge and massive, that its radius is only slightly above its ...
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Role of the canonical ensemble and electric charge in AdS/CFT

If we consider a charged black hole in AdS spacetime, we can either do thermodynamics in the grand canonical or the canonical ensemble. In the former, we fix the electrostatic potential ...
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Should all theories of gravity have Schwarzschild solution?

A consistent theory of gravity must include the Newton's classical theory of gravity as a weak field approximation. Moreover, to satisfy the experiments in the solar system, the Schwarzschild ...
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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 ...
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Significance for LQG of Sen's result on entropy of black holes?

Sen 2013 says, ...we apply Euclidean gravity to compute logarithmic corrections to the entropy of various non-extremal black holes in different dimensions [...] For Schwarzschild black holes in ...
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What really are exotic supersymmetric black holes?

I have just read (in the black holes chapter 14 on p244 of this book Ref.1) that in string theory, when one adds an (electric?) charge $Q$ to a static black hole, one can arrive at an exotic ...
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what happens to information when black holes merge

When a small black hole mergers with the larger black hole, Would an observer trapped in the smaller black hole be able to detect any difference? Does the observer still exists in the smaller black ...
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Can we detect gravitational waves generated from inside the event horizon of a black hole? [duplicate]

General relativity prevents light from escaping a black hole, but does it also apply to gravitational waves?
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Event Horizon of Supermassive Black Holes

I'm going to ask/explain this as best I can; I'm sure I have some fundamentals wrong here. Spaghettification is a phenomenon which occurs only in stellar-mass black holes owing to the immense gravity ...
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Black holes in a head-on collision

Assume two uncharged non-rotating black holes traveling straight at each other with no outside forces acting on the system. What is thought to happen to the kinetic energy of these two masses when ...
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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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Charging a black hole?

What would happen if we have a black hole and we start shooting at it a single electron at a time, and go on doing it forever? Would the electrons start to bounce off eventually?
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Why do the stars in the galaxy core move so fast?

There are a bunch of stars orbiting the black hole in the center of our galaxy. These stars move at huge speed. Why do we see this? Why do the black hole not impose any noticeable time dilation on ...
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What prevents stars in globular clusters from merging over time to form a black hole?

Globular clusters are apparently very very old, and the density of these clusters appears to increase as one approaches the center of a cluster. Orbits are bound to be chaotic, since there is no ...
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Why is BTZ black hole asymptotically $AdS_3$?

The metric for the BTZ black hole is $ds^2=-N^2dt^2+N^{-2}dr^2+r^2(N^\phi dt +d\phi)^2$ where $N^2=-M+\frac{r^2}{l^2}+\frac{J^2}{4r^2}$ and $N^\phi=-\frac{J}{2r^2}$. It is often said that BTZ black ...
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If there was a black hole on earth, what would it look like?

If an answer does exist, I'd love to hear it. I'm trying to incorporate a doomed earth story in something I'm writing, and the end of the world I'm going for is a black hole. Let's say the black hole ...
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Are black holes eternal?

The question might sound very easy - Hawking radiation, however I was pondering that as you get closer and closer to a black hole, time dilates exponentially where the surface of the black hole is ...
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Gravity on supermassive black hole's event horizon

M = black hole mass Gravitation is about 1/r^2 Schwarzschild radius r is ~ to M Greater BH -> weaker gravitation on its horizon. Lets take black hole so enormous that the gravitation on its ...
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Why does my apple not weigh 500 tons?

Related to this question: What is potential energy truly? $E=mc^2$ - energy equals mass. So, if an object has gravitational potential energy relative to another object, it should have additional mass ...