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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How to derive the Schwarzschild radius? [duplicate]

I know that the Schwarzschild radius is given by $$r=\frac{2GM}{c^{2}}.$$ but I never seen a derivation for this equation. 1- Does anyone know how to derive this equation from general relativity? ...
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question about Blandford-Znajek original paper

I'm trying to follow the original Blandford-Znajek paper (found here), and I can't find how equations 2.1, 2.2 and 2.3 are obtained: 2.1 refers to "the charge density required to ensure that $E \cdot ...
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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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The age of black holes

I have a very small understanding of space-time however I have been watching some interviews and reading articles featuring theoretical physicist Kip Thorne and I have a few questions that I hope can ...
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Will a black hole cause scattering of a gravity wave?

In my GR textbook, it states that gravity waves can undergo interference but not scattering. I am just starting the chapter on linearised gravity concepts (weak field approximation) and my apologies ...
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“Center of a black hole is a time”

$\newcommand{\d}[1]{\mathrm{d} #1}$In one lecture (around 1:33:15) of the series of lectures "Theoretical Minimum" of Prof. Susskind he talks about black holes and the Schwarzschild metric: $$\d ...
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micro black hole forces

A black hole would radiate mass optimally for interstellar-travel applications in the range between $10^7$ and $10^8$ kilograms. Assuming a light-only radiation emission spectrum, with a parabolic ...
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Hawking radiation for closely orbiting black holes

Suppose we have two black holes of radius $R_b$ orbiting at a distance $R_r$. I believe semi-classical approximations describe correctly the case where $R_r$ is much larger than the average black body ...
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Information scrambling and Hawking non-thermal radiation states

Could a very small black hole where half of its entropy has been radiated, emit Hawking radiation that is macroscopically distinct from being thermal? i.e: not a black body radiator. Or would the ...
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Multipolar expansion profile of Hawking radiation on Kerr black holes

I would be very curious if Kerr black holes emit Hawking radiation at the same temperature in the equatorial bulges and in their polar regions. I've been looking some reference for this for a couple ...
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Surface gravity of Kerr black hole

I'm going through Kerr metric, and following the 'Relativist's toolkit' derivation of the surface gravity, I've come to a part that I don't understand. Firstly, the metric is given by ...
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Taking selfies while falling, would you be able to notice a horizon before hitting a singularity?

I am generally interested in the role of "pings"(0a) between participants (a.k.a. "signal roundtrips"(0b), as familiar for instance from Synge's "five point curvature detector") in the determination ...
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Nonsingularity of the Euclidean black hole metric

How can one show that the nonsingularity of the Euclidean black hole metric is required for thermal equilibrium of the original black hole in Lorentzian signature? It is mentioned in Prof. P. K. ...
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Falling into a black hole

I've heard it mentioned many times that "nothing special" happens for an infalling observer who crosses the event horizon of a black hole, but I've never been completely satisfied with that statement. ...
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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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How can light enter a black hole if it cannot get out?

I have known for a very long time that light cannot exit a black hole. I can even understand some of the simpler reasonning about it, such as escape velocity, or space geometry inside the black hole. ...
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345 views

What happens to the total volume of a chunk of space that is being sucked into a black hole?

Does it increased, decrease, or stay the same? Maybe it explodes to infinity... Here is a similar question: Do black holes have infinite areas and volumes? But it's different because it asks how to ...
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How can we feel the effects of a Black Hole if all the mass is gone?

This question may help me learn more about the subtleties involved between the notions of gravity, in the Newtonian sense and those of curved spacetime, in the General Relativity sense. I will take ...
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Will the rotation of a neutron star prevent it becoming a Black Hole?

This question, to me anyways, is basically a balancing act between 2 possibly opposing effects. Take a neutron star with just too small a mass to overcome it's degeneracy pressure, failing to ...
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Magnetic dipole near black hole

Usually it is said that black holes cannot have electric or magnetic dipole, only electric charge and angular momentum are allowed quantities besides mass So, it would seem that black holes behave as ...
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Is an event horizon absolute to all observers?

Recently I had discussion whether the event horizon of a black hole is absolute or relative to different (outside) observers. Does someone just 1m above the horizon (disregarding effects of tidal ...
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Are moving objects producing stronger gravity fields? [duplicate]

If the strength of gravitational influence exerted by a body is derived from its mass and energy then is it true that a moving object which has some kinetic energy should also produce stronger ...
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What does it mean “Hawking radiation is in a pure state”?

I'm trying to understand black hole paradox but I'm not sure if I understand what does it mean "Hawking radiation is in a pure state". Does it mean if Hawking radiation is in a mixed state then ...
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Difference between anti-matter and matter black holes [duplicate]

Laymen question: Is there any way to determine by observation whether or not a black hole has resulted from the core collapse supernova of a star originally composed of anti-matter versus a star ...
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Evaporation of black holes

What is the current scientific consensus about the evaporation of black holes? Is there any experimental evidence that support it? (I distinguish between Hawking radiation and black hole evaporation)
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What kind of volume does the event horizon of a Kerr black hole enclose?

I'm sorry if this is a naive question, I'm not too good with General Relativity. I'm aware that a rotating black hole is described by the Kerr Metric, and black holes of this kind have ring ...
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Does a body have to compress to its Schwarzschild radius to become a black hole?

When something is compressed to within its Schwarzschild radius, it will compress to a singularity within finite time regardless of what forces are involved. But is there some lesser point at which ...
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Thought experiment - would you notice if you fell into a black hole?

I've heard many scientists, when giving interviews and the like, state that if one were falling into a black hole massive enough that the tidal forces at the event horizon weren't too extreme, that ...
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Charged black holes and AdS/CFT

People generalize the statements of AdS/CFT correspondence by adding black hole (charged black hole) in the gravity theory to provide the dual gauge theory finite temperature (finite density). I have ...
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Do all paths leading into a black hole lead directly at the exact center of the singularity? Or could they go very near it and curve back?

I only have a very basic understanding of general relativity, so this might sound stupid. But as I see that gravitational objects orbiting each other, even if one of them is massive, as long as their ...
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Could the universe's antimatter be hiding in black holes?

As far as we know almost all the mass in the universe is matter, not antimatter. There are three scenarios: The universe started off with more matter than antimatter and the small amount of ...
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Does it make sense to model the Universe from an outside perspective? [closed]

I was reading some questions and answers about black holes and whether matter can actually pass through the apparent horizon, in particular this one: How can anything ever fall into a black hole as ...
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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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Resources for Entanglement Entropy in Condensed Matter Systems and Black Hole Thermodynamics

Over the last month or so, I have set it upon myself to teach myself the AdS/CFT correspondence. In particular, I am interested in the connection between black hole entropy and entanglement entropy in ...
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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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Does GR put a theoretical lower limit on the radius of a black hole event horizon?

Within GR theory, without going to the extreme r/0 as a radius, (but approaching that as an asymptotic case), is there any theoretical limit as to how small the event horizon of a rotating and/or ...
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Can we assume black hole is just two big stars revolving around each other? [closed]

I am just curious by watching all the black hole theories.I think every phenomenon of the black hole can be explained just by this theory that Black hole is just some bunch of stars(possibly BIG ...
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Gravitational waves as information carriers

Is it possible to utilize gravitational waves as a delivery system for information between two observers straddling the event horizon of a black hole? And why ?
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Does dark matter originate from matter falling into a black hole? [closed]

Is it possible that due to the fact that dark matter is clumped around black holes, that the dark matter used to be matter, fell into a black hole, and simply became dark? Furthermore, if the total ...
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How fast does a black hole appear to decay from infalling observer's perspective? [duplicate]

Black holes eventually decay by emitting radiation. It can take a long time. But from an outsider's perspective. How long does it take from the perspective of someone falling into it? I understand ...
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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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Neutron stars and black holes

The official limits for a neutron star is $1.4 - 3.2\;M_\odot$. But I read that the limit depends on the particular structure of a star to estimate which mass it must have. I also read that neutron ...
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How do black hole jets form? [duplicate]

A black hole, neutron star or any object that has accumulated an accretion disc, sometimes features opposing jets, positioned perpendicular to the accretion disc. I understand that these jets are ...
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Can a neutron star compress until it's converted to a black hole?

The Universe's densest objects are black holes. In the second place, there are neutron stars. So, if a neutron star compresses to its Schwarzschild radius, would it appear as a black hole? That black ...
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What happens when an object passes through the event horizon of a black hole in GR?

I've heard/read many times that in the general relativistic description of a black hole an object would pass through the event horizon unharmed (ignoring tidal forces) while quantum mechanics predicts ...
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Link between Hawking-Bekenstein Black hole entropy and entanglement entropy

I'm currently doing a project on two sided Ads-Schwarzschild black holes in the context of Ads/CFT. I want to show that the entanglement entropy between the two CFTs corresponds approximately to the ...
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Could an ultra-relativistic particle tunnel directly through a stellar mass black hole?

It occurred to me in passing that the Lorentz contraction of a black hole from the perspective of an ultra-relativistic (Lorentz factor larger than about 10^16) particle could reduce the thickness of ...
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What does white hole look like on a graph paper?

Like a black hole looks like this, what does a white hole looks like? The reason I ask is because I would've imagined that it would look like the exact opposite, i.e. piercing out upwards. But it ...
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How many bits are encoded on the surface of the smallest black hole?

Here's my guess ($157\, \textrm{bits}$) and how I got there. Please feel free to disregard completely and give your own answer. My understanding (please correct any wrong assumptions as there may be ...