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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Accretion disks on neutron star binaries

Why does hydrogen gas from accretion disks not constantly get sucked onto/into a neutron star or into a black hole? I understand that some gets sucked into the black hole and some may come down and ...
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Can the event horizon save conservation laws for black holes?

How reasonable it it to conclude that, from a remote observer’s frame, matter falling towards a black hole never crosses the event horizon, because ∆ t → 0 as v → c (according to the Lorentz ...
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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 ...
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Primordial galaxies and associated mass of blackholes [duplicate]

How could primordial galaxies or quasars at great redshifts like quasar ULAS J1120+0641 (detected as per when the universe was 700 mill years old) have blackholes at their centers with the mass of 2 ...
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Minimum size of black hole

What is the minimum size a black hole could be? I have been told that they were worried that the LHC would create a black hole, yet they say the Sun cannot be a black hole. I understand that the ...
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480 views

Surface waves on Dr. Miller's planet

In the movie 'Interstellar', the crew land on a water world orbiting a black hole. The gravity is greater than that of Earth and there are huge surface waves present in the ocean that they land in. ...
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Mass, energy, and entropy.

I have a seemingly simple question about the relation between these three that for some reason doesn't make sense to me. If entropy is the disorder of a system, then a low entropy state is one of ...
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Generalized Bekenstein-Hawking temperature for Kerr-Newmann-dS black holes

What is the formula for the Bekenstein-Hawking temperature in a Schwarzschild-Kerr-Newmann-de Sitter spacetime, i.e., the temperature for a black hole with Mass (M), angular momentum (J), electric ...
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60 views

How much work can we extract from a black hole?

Recently, I've read an article on scientific regarding the possibility of a stairwell into a blackhole (unsurprisingly, it isn't possible). I've found the following question more interesting: ...
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What will be the gravitational pull of the Sun if the Sun contracts to a black hole?

What will be the gravitational pull of the Sun if the Sun contracts to a black hole, and would the Sun be a Schwarzschild black hole or not?
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Why is $p_\phi$ conserved in a Schwarzschild orbit?

This arises from the question What is the relationship between $a$ and $m$, which I'm afraid I answered just by looking it up in Schutz's book. However Schutz (as he frequently does) glosses over ...
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Who can see the event/apparent horizon and Hawking effect?

As I understand, the Hawking effect is related with the presence both event and apparent horizon, i.e. if some kind of horizon exist, a loss/gain of information (unformally speaking) exists. My ...
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42 views

The formation of black holes and their destiny [closed]

Is anyone else against the current theory on what occurs for a black hole to begin? I know a black hole occurs due to neutron degeneracy no longer creating contraction energy to combat the hydrostatic ...
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501 views

A Theorem Due to Hodge: Hawking/Ellis

This is probably quite an obscure question but hopefully somebody has a simple answer. I'm studying the proof of the topology theorem on black holes due to Hawking and Ellis (Proposition 9.3.2, p. 335 ...
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Do all “normal” black holes rotate?

Can we assume that most (if not all) black holes are rotating, due to conservation of momentum? I am excluding the micro world from this question, just thinking of the range of stars on the main ...
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276 views

Is this derivation of Black Hole entropy viable?

This question is motivated by this one. Suppose $l$ is the minimum measurable unit of length. What is entropy of a spinless particle contained in this interval? We know that entropy of a two-level ...
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112 views

How can the Schwarzschild radius of the universe be 13.7 billion light years?

So i was reading about Schwarzschild radius on Wiki and I found a interesting thing written there link. It says that the S. radius of the universe is as big as the size of the universe? How is this ...
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49 views

Reference frame of high speed rocket [duplicate]

Imagine a high speed rocket, traveling past a star. In the rocket's reference frame, the star's length is contracted. This is a lot a mass in a thin line, so it the rocket's reference frame, could it ...
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96 views

Eternal black holes and Hawking radiation

I have a fairly simple question which is confusing me a lot. As Hawking showed, a black hole originated by collapse will emit Hawking radiation. This process will reduce the mass of the black hole ...
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1answer
42 views

Visibility of a super-massive star

I heard that mass and distance are the only deciding criteria for determining the gravitational pull. Keeping the distance constant, if mass would be the only deciding factor for gravitational pull, ...
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1answer
79 views

Is there a horizontal event horizon on a spinning black hole?

It seems that most of the information one reads about black holes theorises that they are not spinning. The theories then describe all matter approaching a black hole as being absorbed by it. Wouldn't ...
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Which way do black hole jets spin?

The centers of black holes and quasars often have jets coming out the two poles of an accretion disk, say north and south. Is it known if the two jets spin in the same direction or opposite ...
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3answers
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Is the difference between an event horizon and a singularity merely perspective?

I know the question has been asked about how an event horizon is distinguishable from a singularity given that time must come to a stop at the event horizon, but I haven't been fully satisfied by the ...
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Calculate the mass of a Schwarzchild black hole with Komar integral [closed]

In Wald's GR, Komar integral is Eq. (11.2.9): $$M=-\frac{1}{8\pi}\int_S\epsilon_{abcd}\nabla^c\xi^d$$ $S$ can be chosen as a 2-sphere, the boundary of a spacelike hypersurface $\Sigma$ such that the ...
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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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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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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 there a map which shows all the black holes in our galaxy?

I am looking for a map which shows all black holes in the Milky Way. See this exoplanet map for what I mean. If it is not existing (yet). Is there at least an online catalogue which summarizes for ...
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Formation of the event horizon seems impossible with singularity inside seems impossible [duplicate]

According to what I was reading, the formation of an event horizon appears if the mass becomes compact enough. Unfortunately, due to time dilation the time for mass to fall into the event horizon ...
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Why the center of our galaxy doesn't absorb us?

Depending on the theories, the center of our galaxy is a super massive black hole, this is easy to accept as a truth, but what I couldn't simply devour is how the solar system is orbiting around it ...
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Are black holes an infinite source of energy? [duplicate]

Black holes are considered particularities because in a determined point in time they are pointless, as consequence there are some mass in a null space so the density become infinite. Finally if ...
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Ricci tensor of Metric black holes with nils and solv geometries of Horizon

The metric of black holes with nils and solv geometries of the horizon is generically represented by $$\mathrm{d}s^2=-r^{2z}\mathrm{d}t^2+\frac{\mathrm{d}r^2}{r^2}+\sum_{I=1}^3 r^{2q_I}(w^I)^2$$ How ...
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3answers
114 views

Why can't light escape a blackhole? [duplicate]

Gravity attracts objects which have mass right. We know that light is massless so why does a black hole's gravity attract light?
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Travelling through a worm hole

In every documentary I've seen about worm holes, I have noticed that they usually say that according to the General Theory of Relativity, there could exist worm holes called traversable wormholes; ...
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64 views

What is the Kerr factor for Sagittarius A*?

I have searched for it, but everything what I found is that A0620-00 (the current closest black-hole to Earth) is a slow spinner with Kerr factor $a=0.12$. How about the Kerr factor for Sagittarius ...
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Black Hole singularity in Loop Quantum Gravity

How is the singularity of a Black Hole treated in Loop Quantum gravity ? Does it go away ? And if it does, what's after the event horizon ?
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Quantum Gravity Singularity Question?

I am not a expert in QG (Quantum Gravity) or GR (General Relativity) for that matter so please forgive if I make any small mistakes, its just a curious question but I know that a singularity is a ...
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Is there a black hole in the centre of the Milky Way?

Is it true that the whole galaxy is actually revolving, and powered by a black hole? Has it been proven, and if it is true, how can our solar systems actually keep up the momentum to withstand the ...
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Is gravitational time dilation the same for an object at $x$ radii from a black hole no matter what size the black hole is?

Assuming you have two non-rotating black holes. Black hole 1 is 1 meter in radius while black hole 2 is 10 meters in radius. If you are standing 10 meters from black hole 1, would you experience the ...
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Can Black Holes Maintain Data Entropy When Formed By a Data Storage System?

It's been shown that black holes don't destroy information. There's a little-known scenario in computer science (Specifically AI) where an advanced civilization creates an artificial intelligence that ...
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Why does Stephen Hawking say black holes don't exist?

Recently, I read in the journal Nature that Stephen Hawking wrote a paper claiming that black holes do not exist. How is this possible? Please explain it to me because I didn't understand what he ...
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Acceleration due to Gravity: Free Fall [duplicate]

If we are in a free fall which implies we are accelerating at 9.8 m/sec every sec. And let's say that we are falling into a pit that has enormous depth. So isn't this be possible that we may ...
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How can matter reach the singularity of a black hole if time stops at the event horizon? [duplicate]

Black holes are said to be composed of two parts: A singularity at the center, where all the mass of the black hole is condensed in a point of zero volume. A black and empty ball delimited by the ...
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Given that matter cannot escape a black hole, how did the big bang produce the universe we see today?

Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. If the matter contained within our ...
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Firewall solution for black hole paradox and creation of black event horizon

AMPS paper [1] proved that black hole complementarity explanation of blackhole paradox requires there is a "firewall": high-energy wall which destroys everything entering event horizon. However, I'm ...
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115 views

Could gravity be repulsive at short distances?

I was wondering if it would be consistent with empirical data if gravity became extremely repulsive at short distances. For example (please bear with my use of classical formulas, I know little of ...
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160 views

What are the implications for the AdS/Cft program if AdS is unstable?

To my understanding recent progress in the study of the non linear stability of AdS spacetime suggest that $AdS$ might be unstable. If this is true, what are the physical and mathematical ...
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Smallest Black hole nessesary to devour Earth?

While looking into black holes I can across the idea of hawking radiation, and the fact that microscopic black holes would near instantaneously evaporate before doing any damage. However larger black ...
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Eddington-Finkelstein coordinates: Why $\ln(r-2m)$ instead of $\ln|r-2m|$?

If one considers the Schwarzschild metric $$ \text d s^2 = -V(r)\text d t^2 + \frac{1}{V(r)}\text d r^2 + r^2 \text d \Omega^2\;,\qquad V(r) = 1-\frac{2m}{r}\;, $$ and introduces the ...
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Location of singularity in moving black hole

Say a black hole is travelling at $c/2$, does the shape of the event horizon change? What about the location of the event horizon? If it is travelling at a hypothetical $c$, does the event horizon ...