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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Does Hawking radiation need an apparent horizon and when does it switch on during stellar collapse?

I've read that Hawking radiation is implicitly linked with the existence of an apparent horizon (1). This seems a slightly less onerous than linking Hawking radiation with a genuine bona fide event ...
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What happens to a particle in the exact center of a Kerr black hole?

Kerr black holes (and Kerr-Newman black holes), instead of the "point" singularity theorized in spherically symmetric black holes, instead have a "ring" singularity, spread along the equatorial plane ...
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Hawking (blackbody) radiation, emission or absorption

A black body is classically defined as a perfect absorber of radiation. That seems to fit a black hole pretty well. The only remaining question, then, How do we know that Hawking radiation is emission ...
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Is relativistic event horizon half of Newtonian event horizon?

Is Relativistic event horizon half of Newtonian event horizon? relativistic escape velocity formula (from $m\phi=E-E_0$) is $v_e=\sqrt{2\phi-(\frac{\phi}{c})^2}$ and the Newtonian version of the ...
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Does space expand locally without restriction in freefall to central region of black hole?

It is often stated that a distant observer will observe a freefalling object as experiencing an infinite journey to the event horizon, but that a traveler in the local frame will experience a ...
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Could undetectable micro black holes floating around the universe cause cellular damage if they passed through a person's body?

What is to say there are not trillions upon trillions of micro black holes floating around Earth passing in and out and occasionally swallowing parts of cells of people, animals, plants, and other ...
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Maxwell's equation for non inertial observer

Applying Maxwell's equation we can prove that light will move at the speed of light for every inertial frame, is it true as well for non-inertial frames? How light moves slowly near a black hole??
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264 views

Black Holes can't exist?

So in this article here, Mersini-Houghton says that Black Holes can't exist. Is there any truth to this? references: http://arxiv.org/abs/arXiv:1406.1525 http://arxiv.org/abs/arXiv:1409.1837
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Can one model a spiral galaxy with a similar manifold to one for a whirlpool?

My question is more of a phenomenological question than literal, since the forces involved are very different. When one looks at the distortions (or wrinkles or whatever you wish to call them) in the ...
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46 views

Are there any difference between gravitational potential of different types of black holes?

There are four possible types of black holes that could exist in the theory of gravitation (called general relativity). Are there any difference between the value of gravitational potential of these ...
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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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Could galactic rotation be similar to an irrotational vortex?

Could black holes' near light speed rotation cause galaxies to move like an irrotational vortices?
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In black hole complementarity, does Bob literally observe Alice's death?

I'm interested in the picture of black hole complementarity where an observer (Bob) far away from the black hole watches another observer (Alice) fall in, and develops a very different picture of ...
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Can a lone black hole in a closed Universe evaporate?

If there is a closed Universe which only has a black hole in it, can that black hole evaporate? As the black hole evaporates, it gives off energy, which will eventually come back and be re-absorbed ...
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How do we determine the temperature of a Black Hole?

How do we determine the temperature of a Black Hole? Since we cannot see a Black Hole, which I presume, is because it absorbs light, would it not also prevent radiation from escaping, making ...
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49 views

Data/signal from a black hole to observe a singularity

I wonder if a situation is possible where, we measure some signal/property concerning a black hole. Supposing the measurement we make with some telescope, gets us the Fourier transform coefficients of ...
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3answers
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What happens to objects pulled in by black hole? [closed]

What happens to the particles/elements/objects that sink into the gravitational pull and ultimately go to the interior part of a black hole? If, according to popular notion, it is crushed by the ...
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230 views

Could strings be geons?

Is it possible that string theory strings are geons? This may be an overly speculative or naive question, but is there an obvious reason why not? Both strings and geons seem to have roughly the same ...
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The bigger the mass, the more time slows down. Why is this?

If I were to stand by a pyramid, which weighs about 20 million tons, I would slow down by a trillion million million million of second. Don't know if that's exactly right, but you get the point. Also, ...
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Is there any evidence that matter and antimatter continuously appear and disappear on the edge of a black hole?

I heard Stephen Hawking got a Nobel prize for this, someone said there was no evidence for it which I find quite strange since he got an award for it.
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Can a black hole bounce?

Is there a limit to the amount of matter that a black hole can accrete per second and if so could a certain sized black hole bounce off a dense enough surface?
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What happens when electron degeneracy pressure is being overcome (right before an object turns into a black hole)?

I was wondering about hydrostatic equilibrium (the balance of radially inward and outward forces) in large (i.e. above the Chandrasekhar limit) stars. It is often said that when the gravitational ...
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194 views

Einstein and the existence of Black Holes

I have seen some papers claiming that Einstein said there is no Black Hole in the real world, here is a phrase from Einstein himself, see this arXiv preprint: The basic result of study is the ...
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Is the Casimir effect also present between two event horizons?

A follow-up to this question: Is the Casimir effect also present between two event horizons?
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118 views

How far has a black hole to be in order for its tidal forces to disintegrate earth?

I don't know if this question can be answered to honest but I though I might try ask just in case someone can calculate that. What I need to know here is how strong should the tidal forces be to start ...
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What is the metric of Vaidya black-hole horizon?

The metric of a Vaidya black hole in outgoing/retarted null coordinates are $$ds^2=-\left(1-\frac{2m(u)}{r^2}\right)du^2-2dudr+r^2\Big(d\theta^2+\sin^2\theta d\phi^2 \Big)$$ The eveolving horizon ...
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Can every particle be regarded as being a combination of Black holes and White holes?

Can the statement be regarded as true? That every particle, or element in the universe can be regarded as a combination of black hole and white hole in variable proportion.
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The Effect of Tortoise Coordinates

Referring particularly to http://arxiv.org/abs/hep-th/9909056 in regard to the wave equation for Schwarzschild-AdS black holes (p.4), I'm trying to understand tortoise coordinates. So starting ...
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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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862 views

A Sphere of Black Holes

Imagine a sphere of black holes surrounding a piece of space. Will this piece be separated from the rest of normal spacetime (at least for some time, till these black holes finally attracted ...
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63 views

Why doesn't Hawking radiation cancel itself out?

Hawking Radiation is formed when particle, anti particle pairs formed by the uncertainty principle are separated by the event horizon of a black hole. It seems like an equal amount of particles ...
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678 views

Spacetime around a Black Hole

If we consider the sun, then space-time is curve around it. My question is that what is the kind of curvature of space and time around the black hole. Is that space and time more curved around the ...
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Am I right in saying that $gmm/r$ is motion due to potential energy?

I was watching a video about the swarzchild radius and it said that potential energy is $gmm/r$. This cannot be right though because potential energy goes up with distance not down. I'm assuming he ...
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Why exactly is it that light cannot escape a black hole? [duplicate]

I understand that the gravity has to have a pull greater than the speed of light, but I thought that when gravity pulls light it doesn't accelerate it only blue-shifts it. So then why does gravity ...
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Information of things inside a black hole [duplicate]

Can we get the information of things that are gone in black hole?
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When a star becomes a black hole, does its gravitational field strength become stronger?

I've seen in a documentary that when a star collapses and becomes a black hole, it starts to eat the planets around. But it has the same mass, so how does its gravitational field strength increase?
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Speed of light versus pull of gravity - Is $c$ really the limit? [duplicate]

The understanding I have is that the speed of light is considered to be the highest attainable speed in physics. Of course there are theories of tachyons but since those haven't been proven we'll ...
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54 views

Black Hole mass increase from infalling body

When a body falls into a black hole, the black hole's mass increases. Does this mass increase depend only on the rest mass of the infalling object, or is it affected by the velocity of the infalling ...
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Why is the Schwarzschild radius the radius of an event horizon?

I've been searching the web and many references without much success. My question is how do we know that, in the Schwarzschild black hole solution, the surface with coordinate $r=2M$ (in the geometric ...
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If MOND theory doesn't explain gravitational lensing, couldn't well placed black holes explain it?

When I first heard about star velocity as a function of its distance from the center of the galaxy and the difference between prediction and observation I immediately thought "there must be a ...
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Why aren't all large stars black holes?

Like all stars, large ones are stable as long as there is a sufficient amount of hydrogen (or helium) to fuse. This fusion process is what prevents them from collapsing in on themselves. However, ...
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Does the mass distribution matter in (Schwarzschild) black holes?

Is it possible that from the same initial mass different black hole radius will be created due to different mass distribution during black hole creation? If mass is concentrated more on the outside ...
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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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Why does Matter occupies space? Matter and space are two things… If Space does not has mass, why should matter have space-ful nature?

This thought has completely changed my perspective towards matter. If the matter in a star can collapse to a point to form a Black hole, surely the true nature of matter should be able explain this ...
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Black hole (classical or quantum?)

Theoretically we can convert a body into black hole by compressing its mass below some radius (known as Schwarzschild Radius). Suppose such object after becoming black hole has a radius which is ...
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Another faster-than-light question

Imagine we have something very heavy (i.e supermassive black hole) and some object that we can throw with 0.999999 speed of light (i.e proton). We are throwing our particle in the direction of hole. ...
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90 views

At what speed will objects hit a singularity? [duplicate]

I have read in numerous places like here that an object in free fall into a black hole will be travelling the speed of light when it passes the event horizon. How is it possible to go this fast? ...
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Blackholes gravity, why it has effect outside the event horizon? [duplicate]

If gravity is a curvature of space time, and the event horizon is the collapse of this curvature on itself, then if the blackhole gets more matter after it is formed we should see no effect at all ...
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A two-dimensional event horizon?

Today, I came across an article which says that the Universe began with the collapse of a 4-dimensional [spatial] Universe. Well, I don't know if there's any way to confirm these like it is with all ...
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What is the power of the most powerful quasar found?

Trying to find an answer to this question, I came across many sources that are in complete in contradiction. For example Wikipedia states that a typical quasar has a power of $10^{40}$ watts while ...