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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Is there such thing as imaginary time dilation?

When I was doing research on General Relativity, I found Einstein's equation for Gravitational Time Dilation. I discovered that when you plugged in a large enough value for $M$ (around $10^{19}$ ...
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Deriving Birkhoff's Theorem

I am trying to derive Birkhoff's theorem in GR as an exercise: a spherically symmetric gravitational field is static in the vacuum area. I managed to prove that $g_{00}$ is independent of t in the ...
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Can one black hole suck in another black hole?

In the recent news, scientists at NASA have found “unprecedented” black hole cluster near Andromeda’s central bulge. I wonder why doesn't all these black holes merge and such each other in until just ...
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Universe Expanding

Quick question that I was just hoping to get some clarification on. Current theories state that the universe is expanding; with those planets farther away moving away from us at a faster rate. Is it ...
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55 views

Can a High Enough Temperature Create a Black Hole?

My very basic understanding of GR leads me to think that if a substance has a high enough temperature, it can transform into a black hole without a mass required to create a black hole. The equation ...
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Rainbow Black Hole [closed]

In the photon sphere of a black hole the photons are being trapped in orbit, pulled in and trajectories changed. From a safe distance should we see color from the entire spectrum if light in the ...
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If a 1kg mass was accelerated close to the speed of light would it turn into a black hole?

I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever ...
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Is there a way to move a black hole?

Is it possible to move or transport a black hole? What about a white hole?
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Conservation of Komar Mass

The definition of Komar Mass in GR is associated with one asymptotically flat end. However, a hypersurface may contain more than one end, such as the spacelike Einstein-Rosen bridge in Kruskal ...
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Can the “black hole bomb” instability be made to work with small discrete mirrors instead of a spherical mirror?

Is it possible for the so-called "black hole bomb" instability to work with small discrete, orbiting mirrors instead of one spherical mirror? This would be far more practical for a hypothetical ...
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39 views

Luminosity of an accretion disk?

With reference to Black holes in particular, how can you approximate the luminosity of an accretion disk? It is possible to quantify the temperature at a given point, but as the disk is not a black ...
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Current rate of supermassive black hole merges

I think, it is probably not a very common event, and if it happens, it can be maybe one of the ...most interesting events of the Universe. Considering that the merge of two stellar blackholes were ...
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What would happen if a black hole merged with a white hole of the same size?

I learned about white holes a while back, this was the first question that came to mind.
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Density of states from the Retarded Green's function for a rotating black hole

I have been studying the scattering of a scalar field around a rotating black hole in the near-horizon extremal limit. The radial solution provides the retarded Green's function, just by taking the ...
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How do we know the Schwarzschild solution contains an object of mass $M$?

The Schwarzschild metric is $$ds^2 = - \left( 1 - \frac{2GM}{r} \right) dt^2 + \left(1-\frac{2GM}{r}\right)^{-1} dr^2 + r^2 d\Omega^2.$$ In Carroll's GR book, it is claimed that $M$ is the mass of the ...
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If Black holes are maximal entropy how can they evaporate?

According to Hawking/Bekenstein a black hole represents the highest amount of entropy for a given volume, (actually the entropy is related to the surface area of the black hole but the fact that they ...
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Why are black holes black? [duplicate]

If heard two explanations for this. One explanation is that the gravity is so strong that space is being stretched inward faster than the speed of light and thus no photons could possibly escape. The ...
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scalar field boundary condition in background with U(1) isometry

I have a simple question about Lewkowycz and Maldacena's paper In section 2, they consider the scalar field in BTZ background ground and require boundary condition of the scalar field, $\phi \sim e^...
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Are black holes really just neutron stars? [closed]

I was curious if it would be possible for blackholes to actually just be neutron starts that are so large that they trap light and gain mass from nearby objects. If this were true that would mean that ...
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What is the mass density distribution of an electron?

I am wondering if the mass density profile $\rho(\vec{r})$ has been characterized for atomic particles such as quarks and electrons. I am currently taking an intro class in quantum mechanics, and I ...
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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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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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539 views

An explanation of Hawking Radiation

Could someone please provide an explanation for the origin of Hawking Radiation? (Ideally someone who I have been speaking with on the h-bar) Any advanced maths beyond basic calculus will most ...
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Is it possible for a particle to orbit a singularity inside the event horizon [duplicate]

Event horizon just means a distance from singularity, where there is no "going-away" from the black hole. But is it possible, that there are particles with enough momentum to actually orbit the ...
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How would one attach black hole to starship in the Black hole Starship model? [closed]

A black hole starship is a theoretical idea for enabling interstellar travel by propelling a starship by creating an artificial black hole and using a parabolic reflector to reflect its Hawking ...
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Rotation of Spacetime $\Rightarrow$ Change in orbit/path

Along the idea of frame-dragging; Will the rotation of a black hole, which has some velocity v and angular momentum, influence its path in 3D space? I've seen the fact that depending on the ...
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Energy conservation around a black hole

In the Schwarzschild black hole, the Killing vector "time translation" $k^a$, so that the following quantity is conserved along a geodesic: $$E = -g_{ab}k^au^b = (1 - \frac{2GM}{r})\frac{dt}{d\tau}.$$...
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What happens if I slowly lower a dangling object into a black hole?

I could've sworn I've seen this question before, but I couldn't find it. Suppose I have an object on the end of a really long string. I can slowly lower it near the event horizon of a black hole, ...
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Why does imaginary periodic time of the Rindler space of black hole give Hawking temperature?

The metric of a Schwarzschild black hole in Rindler coordinates is $$ds^2= -\frac{\rho^2}{(4MG)^2} dt^2 +d\rho^2 + d\mathbf x_\perp$$ where $\rho$ gives us the distance to the horizon. If we switch ...
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Can a black hole have negative temperature?

Stephen Hawking said that black hole also have temperature and it is related to its mass so in other words a black hole can also be shown to have a negative temperature! I know that nothing is colder ...
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Does a black hole ever fully form to an outside observer? [duplicate]

According to general relativity, I had understood that time appears to slow down when looking into high gravitational fields from afar, so that as a black hole forms, the light from a collapsing star ...
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Could dark energy and Hawking radiation be the same thing? [closed]

Specifically, could black holes effectively emit some kind of negative mass or other form of dark energy, so that the mass of the black hole itself would actually increase rather than decrease (...
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Another layman blackhole question, pulling one end of a string out from behind the event horizon

No long explanation is needed, What would happen if I were to allow one end of a rope to fall past the event horizon of a black hole while I held the other end? Would I be able to pull it out? Would ...
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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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What will happen to Earth if it's tied to a black hole thru a really strong string?

Imagine a very strong string (with infinity as as its breaking point) attached to earth very strongly and the other end is put inside a black hole, What will happen to Earth? And the string? Will ...
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Could a black hole maintain a stable orbit around the Earth?

Could a rotating Black Hole of mass $1.24\times 10^{10}$ kg maintain a stable orbit around Earth, without significantly altering the path of the Earth or Moon? In addition to this, would the presence ...
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A doubt regarding Black Hole Complementarity

A friend was explaining Black Hole Complementarity to me, and at one point he said that to get a (horrendously) mixed quantum state, i.e. a thermal density matrix without a heat bath, one takes a ...
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Why would a black hole evaporate? [duplicate]

Hawking radiation doesn't make sense to me, with respect to black holes getting smaller. It would seem that any particle (or anti-particle) leaving the Schwartzchild radius would have a similar anti-...
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Pair Production via Black Hole

I think this should be a straight forward question. Is it possible for a photon to pass near a black hole and be turned into a matter and anti-matter pair according to current theory? Edit: From the ...
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Utilising Black Holes as a potential energy source

I'm aware of the Penrose process and the basic physics behind that. Also, I know that the Blandford-Zjanek process (That is potentially responsible for the relativistic jets). Aside from these two, ...
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Are black holes hollow? [closed]

So when enough matter to create a black hole falls towards the center of its collective mass, at one point the mass/area ratio becomes high enough to form the event horizon of the black hole. Because ...
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Non equilibrium black hole radiating away energy and angular momentum, with total energy and angular momentum conserved

The question refers to whether mass (i.e., energy) and angular momentum can be considered to have been carried away by the gravitational radiation in the hole settling down. And whether those entities ...
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Black hole gravity vs parent star gravity

In the cases of black holes that form from supernova and collapse of a massive star, I understand that in most of these cases, the star loses significant amounts of mass from the explosion. Presumably,...
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1answer
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Big Bang not really the beginning of a completely new universe? [closed]

From what I know about the origins of the universe and the big bang, it is stated that it all started from an intensely hot and dense mass. This sounds like a singularity to me, which means a black ...
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How can super massive black holes have a lower density than water?

I heard on a podcast recently that the supermassive black holes at the centre of some galaxies could have densities less than water, so in theory, they could float on the substance they were gobbling ...
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How can black holes be so dense?

It is said that if the Earth were a black hole, it would be the size of a peanut!? How is this density possible, are atoms really that sparse that they can be compressed so tightly? Is there some ...
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4answers
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Binary black hole merger viewed from inside the event horizon

How did the metric evolve inside the event horizons of the black holes whose merger caused the GW150914 signal? In principle the Schwarzchild metric of a non-rotating black hole is known inside the ...
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Is this analogy of Hawking Radiation correct?

Through reading of textbooks and other research papers, I have settled on the analogy of hawking radiation below (Written completely by myself) Within the ergosphere of the black hole, virtual ...
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If black hole is a “hole”, how can its position be pinpointed from every position in universe? [closed]

Ok, this is going to sound lame but, here goes: When we say that a black hole is situated at a distance of 1 light year(let) FROM EARTH, that means it may situated at a distance of 100 light years ...
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Eddington-Finkelstein coordinate

The Eddington-Finkelstein coordinates in case of Schwarzschild metric are defined as \begin{align} u&=t-r^*\\ v&=t+r^* \end{align} where $$r^*=r+2GM\ln\left|\frac{r}{2GM}-1\right|$$ The ...