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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Gravitational singularity

Is it possible that the gravitational singularity actually turns out to be a genuine singularity once we have a true theory of quantum gravity in place. There is a lot of talk about singularity but ...
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Why do black holes have a finite size? [duplicate]

There is one piece of information that makes me wonder why finitely sized black holes are even possible: to an observer from afar, anything that approaches a black hole's event horizon is never ...
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If nothing ever falls into a black hole, why is there a puzzle about information?

From an outside perspective, nothing can ever pass the event horizon. It just scooches asymptotically close to the event horizon. So (from our perspective on earth), when a black hole reduces in mass,...
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Two different expressions for the Schwarzschild Radius? [closed]

This might sound as quite an obvious question at first, but in most places the expression I've seen for this radius is: $$\frac{2GM}{c^2}$$ but in other places, I've seen the expression as:$$\approx \...
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creating black holes

Black holes are created usually when something massive (usually a dying star) collapses on itself to create a hole in spacetime. However, I hear that a black hole can be made with objects that aren't ...
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6answers
855 views

Mass without Matter?

My cursory studies of general relativity had introduced me to the concept of how gravity is associated with the warping of spacetime and that the presence of matter in space warps space and this ...
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Why would a black hole explode?

It is common in popular science culture to assume that Hawking radiation causes black holes to vaporize. And, in the end, the black hole would explode. I also remember it being mentioned in A Brief ...
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3answers
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How do we expect distance measurements to compare inside and outside the event horizon of a black hole?

I've read that as one approaches the event horizon of a black hole, time is dilated relative to time measured farther away from the event horizon (clocks tick slower near the event horizon). I've ...
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315 views

If perfect maximal entanglement is never true, does a remainder invalidate the monogamy of entanglement?

If something is only very nearly (and/or observed to be) maximally entangled, does that remainder allow for a menage trois of hybrid correlation (as it relates to AMPS)?
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Could we have a black hole on Earth? [duplicate]

There was once a Far Side cartoon in which a black hole had opened in someone's living room and was sucking the furniture, people, pets, etc to the middle of the room. It was a funny picture, but ...
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112 views

Schwarzschild Solution

I'm able to derive the Schwarzschild solution under the assumptions that the metric is (1) static (2) spherically symmetric and that the space is the vacuum. However, I have read that the ...
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1answer
119 views

Strange Airy Black Hole

Imagine a sphere, with a radius about the size of the Solar system. Somehow fill it with air at 100KPa (Earth surface density). $1 km^3$ of air is about 1.25 billion kg. A Solar system radius is about ...
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Would a supermassive black hole accretion disk really vaporize solid objects?

Both the movie Interstellar and Greg Egan's Incandescence involve worlds deep inside accretion disks of large holes, kept at a comfortable temperature. Is this (remotely) realistic? Although ...
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Supermassive Black holes at centers of galaxies [duplicate]

Why there is super-massive black holes at the center of our galaxy and other galaxies ?
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2answers
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Spacetime geometry around two black holes

For space-time geometry we all know images like he one below. But if I were to take a neutron star and put it right next to but not touching a black hole what effect would that have on the geometry of ...
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2answers
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If photons have no (rest) mass, why would black holes attract light? [duplicate]

I was told that photons have no (rest) mass. However I thought that black holes are called "black" because no light can go escape the gravity force in their vicinity. I somehow think that, if light is ...
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2answers
469 views

Does the sun have a black hole inside of it?

The Schwarzschild radius of the sun is approx. 3 km. It would be a black hole if all the mass of it were within the 3km. However, even with the mass spread out couldn't the sun technically have a or ...
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1answer
70 views

Black holes “regurgitate”

Do Black holes "regurgitate" their contents at some point or do they simply grow larger as they "suck" in more matter? Also, is their bottom (if they have one) "capped" or is open to expel the matter ...
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174 views

Theory of black holes producing matter or antimatter [closed]

One of my friends gave a theory about the formation of black holes. He said that: Black holes are formed when an extremely massive star collapses in its own gravity to produce a particle of ...
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Thought Experiment - Poking a stick across a Black Hole's Event Horizon

The classical explanation of a black hole says that if you get to close, you reach a point - the event horizon radius - from which you cannot escape even travelling at the speed of light. Then they ...
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2answers
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Is space stretched with no limits by a black hole?

Some depictions of black holes show space being warped into a singularity, with no end, e.g. as pictured below. Moreover, in Cosmos, Neil Tyson speculates with the possibility that Black Holes contain ...
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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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1answer
388 views

Time coordinate inside black hole horizon [duplicate]

I am new to physics and was trying to learn more especially about general relativity. The Schwarzschild metric, changes the sign of the time and radial parts of the metric once we cross the event ...
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1answer
85 views

Can the density of quarks be more than density of black hole? [duplicate]

I have found this list of densities here: http://wiki.chemprime.chemeddl.org/index.php/Density_of_Black_Holes and I am looking for the density of quarks but I have not found it. I was wondering if ...
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194 views

Are a black hole really connecting 2 parallel universe?

What is beyond the point of singularity in a black hole and universe expands in all the 3 $x$, $y$ and $z$ axis? So how can one thing be on top of another when we don't know where the end point of ...
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What happens at the center of a black hole according to holographic theory?

As far as I understand, the AdS/CFT correspondence proposed by Maldacena is an exact duality to a four-dimensional theory, which interpolates between one well-defined conformal field theory in the UV ...
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301 views

Particles inside black hole does not hit singularity

We say that if any particles from outside fall into the black hole, they eventually hit singularity. Then why not particles already inside the black hole are at singularity? Or are they? If yes, then ...
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2answers
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Can we add two singularities and make a single singularity? [duplicate]

My question is if we have two black hole and they will merge into each other, then where will be the singularity?
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How does a sonic black hole (or dumb hole) work?

I came across this term and I heard they used sonic black holes to detect something analogous to hawking radiation, but I have failed to find sources which explain how exactly they work and how they ...
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1answer
123 views

What has been observed as the “Hawking radiation” emitted by a black hole analog?

I've noticed this paper which explains that they have observed the "Hawking radiation" emitted by a black hole analog. In which sense the Bose-Einstein condensate described by the paper can be ...
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425 views

Can gravitational waves orbit each other to form a standing wave?

Since gravitational waves are a type of propagation of energy of some sort, they ought to induce their own gravitational field. I'm assuming this extra gravitational force / curvature is independent ...
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1answer
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Can a fuzzball collapse into a singularity?

I've been reading Kip Thorne's Black Holes and Time Warps and got curious about extreme states of degenerate matter; a little research reveals that since the book was written quark and preon ...
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Is FTL information transfer possible in an experiment involving entangled particles and an “available” black hole?

We consider the classical entanglement experiments involving Alice and Bob, and their entangled particles. It is proved that nothing that happens at Bob's end has any immediate effect on Alice's ...
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1answer
122 views

A Theory of Almost Everything? [closed]

Before I present this theory, I ask that you take the time to read this disclaimer. I do not claim to have an excellent knowledge of physics, it is actually rather limited. Therefore, I am not ...
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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 r<...
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How does the Hawking Radiation mechanism cause a black hole to lose its mass? [duplicate]

Correct me if I am wrong: in the Hawking Radiation mechanism, when a virtual particle-antiparticle pair gets created at the edge of the black hole, a black hole could sometimes eat up one of the ...
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1answer
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If I kept getting closer and closer to the speed of light, what would be the gravitational effects appearing like to an observer? [duplicate]

Now, with special relativity applied to the scenario of me getting closer and closer to light speed, my mass would increase with respect to the observer, and also my length would contract in the ...
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What was the entropy of the universe at the time of the Big Bang?

(I asked this question in Philosophy.SE; but I was advised to direct it here, despite it is, in my opinion, somewhat too speculative for physics.SE). High entropy generally means high disorder; and ...
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Time functions in general relativity

In my general relativity notes a function $f$ is called time function, if $\nabla f$ is time-like past-pointing. Say that we are in Schwarzschild spacetime and I want to check if $f=t$ is a time ...
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Why is the first term of First Law of Black-Hole Thermodynamics in other unit than in joule? [closed]

http://en.wikipedia.org/wiki/Black_hole_thermodynamics#The_First_Law http://www.physics.umd.edu/grt/taj/776b/lectures.pdf (p.13) The 2 sources have various forms of the same law. I found both are ...
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1answer
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How possible is it that the Large Hadron Collider (LHC) on its second try, could disrupt the gravity of Earth? [duplicate]

How possible is it that the Large Hadron Collider, on its second try, could disrupt the gravity of Earth?
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Conformal Killing fields on Schwarzschild

I am trying to understand which are the conformal Killing Fields on the Schwarzschild spacetime. I say that $X$ is a conformal Killing field on $S$ ($S$ is Schwarzschild) if there exists a function $f:...
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Is it possible to assign a physical radius to a black hole?

The Schwarzschild metric is given by: $$c^2d\tau^2 = \left(1-\frac{r_s}{r}\right)c^2 dt^2-\left(1-\frac{r_s}{r}\right)^{-1}dr^2 - r^2 \left(d\theta^2 + \sin^2 \theta \, d\varphi^2\right).$$ The ...
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1answer
458 views

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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Dark matter and black hole [closed]

Is black hole a kind of dark matter.
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1answer
1k views

Shape of accretion disk surrounding the black hole in Interstellar (film) [duplicate]

There is a black hole, called Gargantua, featured in the recent blockbuster Interstellar. I understand why the accretion disk is bright (friction), but why does it seem to be flowing in two ...
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2answers
1k views

Is Hawking radiation a theory or a hypothesis?

There are lots of articles, calling Hawking radiation a theory, but doesn't the definition of a scientific theory state that a theory is substantiated by a repeated testing and an overwhelming amount ...
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Manifold for Schwarzschild and Bertotti-Robinson

In short: what is the manifold in discussion for Schwarzschild metric $$ ds^2 = -(1-\frac {2M}r)dt^2 + \frac1{1-\frac{2M}r} dr^2 + r^2 (d\theta^2 + \sin^2 \theta d\phi^2)$$ and Bertotti-Robinson ...
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How close can an observer approach the black hole in an unpowered flyby without falling into it?

In classical mechanics by choosing the right trajectory you can approach a planet arbitrarily closely, if there is no atmosphere or anything to slow you down, you can approach the surface then fly ...
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Why Hawking radiation emission rate inversely proportional to black hole mass? [duplicate]

The rate of Hawking radiation emission increases as the mass of the black hole decreases, what I don't understand is that if the emission method is related to zero-point energy particles appearing and ...