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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Redshift of merging black holes

How did they found that the gravitational waves where emitted at redshift $z=0.09$? I understand the measurement of redshift for an electromagnetic wave where we have measured in a lab various ...
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So Black Holes Actually Merge! In 1/5th of a Second - How?

I've read a lot of conflicting answers in these forums. However, today saw the awesome announcement of gravitational waves. Two black holes merged: ...
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How much light is necessary to form a black hole [duplicate]

I suppose that enough light in a small enough volume could create a black hole. What is the good quantity that can tell when light can or cannot make a black hole? Energy density? But there must be ...
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Does the size of a black hole change depending on its charge or spin?

I was talking to this friend of mine who said that the size of a black hole could vary depending on its "charge" and spin. He said the size of a black hole would reduce if it had a spin or charge. The ...
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Why can an electric field escape from a black hole?

I know this is a duplicate question but the other answers didn't explain clearly my problem. The talkes about no hair theory for explaining the communication and information problem of matter inside ...
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Can black holes with the same mass evaporate with different speeds?

I am curious about the following observations: (1) For a normal Schwarzschild black hole, it evaporates according to $dm/dt=-1/m^2$; (2) We have eternal black holes which do not evaporate (though ...
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Question about the Blandford–Znajek process

I've been trying to understand the Blandford–Znajek process which explains how Gamma Ray Bursts form, but it's still not clear to me how it works. In this article, several theories about the formation ...
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Scattering in Black Holes

Let's take your favorite Black Hole and consider the scattering problem for a scalar field around it. Suppose that the Klein-Gordon equation can be separated in the radial and the angular part. Which ...
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Looking out from a collapsing star

From the perspective of the surface of a collapsing star, if you were riding it in as it formed a black hole, what would you observe looking out into the rest of the universe? As you approached the ...
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Merger of old black holes

On the eve of a possible announcement on the production of gravitational waves via a black hole merger, I think this question is quite aptly timed. I have a few questions regarding the evolution of ...
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Is there a limit to the size of black hole?

I have read answer by @John Rennie in regards to the size and density of black hole. In the last sentence he states that super supermassive black hole ...
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If black holes warp space-time infinitely, does that mean space-time goes on infinitely in all directions?

My thoughts are if a black hole warps space-time infinitely then does that mean space-time goes on in all directions infinitely? Maybe (probably) my understanding of space-time is not great, if ...
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Why is it difficult for 2 black holes to merge?

Apparently as part of the merger of galaxies, sometimes the central supermassive black holes of colliding galaxies get really close, however once they come to a distance of about <1 parsec, it ...
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Cold Solid Matter at the End of a Black Hole's Acceleration Disk?

Is it possible that black holes magnetic field are rapidly cooling matter using a type of magneto caloric effect and cold fusion creating new matter only possible with a black hole's gravity? A type ...
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Why can't we see black holes?

Is it all because they are so black? But even our tech is not able to see black holes. We can only tell where black holes are, when they are eating stars. Or it's because they consume light ?
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Black hole starship: Would micro black-hole Hawking radiation emit anti-matter?

My understanding of Hawking radiation is that virtual particle pairs of matter and anti-matter are created from vacuum energy and one of those virtual particles may escape while the other enters the ...
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Black Hole Collision [duplicate]

What would happen if two equally massive black holes directly collided at 5% the speed of light. I'm not talking about slowly orbiting each other for thousand of years. I mean one instant catastrophic ...
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is the potential energy of a black hole infinity

It appears like black holes posses an escape velocity greater than the velocity of light. It takes an atom infinity energy to travel at speed of light.Do black holes posses that energy to retain ...
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Shapes in a black hole

A question around an answer of Timaeus What happens to a particle in the exact center of a Kerr black hole?-answer Outside both is a normal type region of spacetime. In between the two the $r$ ...
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Hawking radiation for an observer at infinity

As an external observer at infinity, I observe a star collapsing. But because the surface of the star will be infinitely redshifted, I will never see the formation of the black hole. For me, there ...
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A problem with ADM mass in the derivation of 1st law of black hole thermodynamics

The definition of ADM mass is $M=\frac{1}{16\pi}\lim_{r\rightarrow\infty}\int(\frac{\partial h_{\mu\nu}}{\partial x^\mu}-\frac{\partial h_{\mu\mu}}{\partial x^\nu})N^\nu dA$ according to Wald. ...
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How has the world remained in balance?

In the center of our galaxy, there is a Supermassive black hole. We were told in early astronomy units of school that black holes are giant vacuums that will absorb/consume the area around it. My ...
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Space-time and gravitational fields

I am reading a book by Carlo Rovelli, Seven Brief Lessons On Physics, and would like to check if I have understood something. Apologies if my question is badly phrased, feel free to edit where ...
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Is there a doppler effect on the images of stars around rotating black holes?

I'm an illustrator working on a project involving rotating black holes like those discussed in "Gravitational Lensing by Spinning Black Holes in Astrophysics, and in the Movie Interstellar" by James, ...
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Evaporating black hole Penrose diagram

The context for this is the diagram from From where (in space-time) does Hawking radiation originate? - the more I think of it, the less it makes any sense ) This particular question is about ...
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Is a larger black hole a faster or a slower processor?

For a remote observer, a black hole with mass $M$ has a temperature $T=1/M$. Now I am confused with the problem: A larger black hole can achieve a task faster or slower if it's regarded as a kind ...
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Does time symmetry still holds when a particle drops into a black hole?

When a particle drops into Earth, it hits the ground and rebounds, if time reverse, it is equivalent to another particle moves with same speed but opposite direction. But at the case that a particle ...
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Worldlines in Schwarzschild geometry

I have an observer and a photon on a hypersurface $ \theta=\pi/2$ . My observer has $e, l$ constants of motion (energy and angular momentum divided by mass) and photon has $e',l'$. What conditions ...
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Big Bang Quarks Travelling at Light Speed [closed]

When I think of an explosion it lasts a fraction of second. When the entire mass of the universe explodes how much time passes before the perfect sphere slows from traveling at/near the speed of light ...
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Black hole evaporation, mass or bit?

Usually when we talk about black hole evaporation, mass or energy is taken as the target and the evaporation time is computed based on mass loss in a given time period. Is there any work to analysis ...
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What does it look like for a ball falling to the event horizon observed by distant static observer?

Here is the picture used in susskind&Lindesay's book ''An Introduction to Black Holes, Information and String Theory Revolution'' I understand very well that the ball will be contracted at the ...
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42 views

Conditions at black hole's event horizon

This question had, at least partially, been discussed here before, but I feel that the record has not been set straight. There seem to be lack of agreement regarding conditions (like gravitational ...
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Has a stellar blackhole more mass than previously?

What is the coefficient of mass gained/lost by a star in its first phase transitioning into a blackhole. Does the blackhole have more or less mass than the star it was made from? Thank you Regards
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Two black holes prepared from different initial states

I have asked a similar question, I would like to reformulate it in more details. Here is a thought experiment: Assuming we can create black holes by squeezing photons, we can then prepare two ...
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light in black holes and space time

I watched videos, read articles, learned a lot about black holes and got confused: Lets say that a light is doing its way inside a black hole toward the singularity. If it's inside the event ...
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Dimensional inconsistency in first law of black hole thermodynamics

The first law of black hole mechanics (let's simplify by considering a uncharged and non-rotating black hole) can be written as $$\delta M = T \delta S$$ If I use the definition of Hawking ...
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Hawking radiation: why do only negative mass particles fall into black holes? [closed]

So I have read other questions asked about this topic, but they focus more on positive or negative energies of the virtual particles, and I want to ask specifically about the negative mass issue. ...
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Mass and time relativity [duplicate]

In the space between galaxies in the absence of matter, would time run faster than inside a galaxy? A black hole can slow time. Is there a cosmic opposite of that effect on time? Like the following ...
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Does matter stacks up as it approaches Black hole?

When something approaches Black hole it'll experience time dilation with respect to a frame away from the black hole. So to an observer away from the hole the object would seem to slow down until ...
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What is the global Virasoro symmetry generators in BTZ spacetime?

This is the case of AdS3, how about BTZ? The picture is from arxiv:1506.01353.
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How does a black hole's gravity affect other dimensions? [closed]

Given the extreme gravitational force of a black hole, can it have affects on other higher dimensions? Does it continue to warp space-time regardless of the number of dimensions? In fact, can a ...
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If black hole is equivalent to a planet of same mass for a distant observer, then why does 'excess radius formula' require uniform mass density?

I understand that the spacetime curvature of a non-rotating, uncharged black hole is identical to that of a planet with same mass/energy for an observer at a distance farther than the radius of the ...
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Can a massive imploding sphere smash the holographic principle and conservation of information into oblivion?

Let $\text{BH}_1$ be a black hole of mass $M_1$, zero charge and angular momentum, and event horizon radius $R_1=\frac{2GM_1}{c^2}$. Alice is loitering at distance (all distances here are from the ...
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Super massive Black Hole and photon reduction [closed]

This is a picture of 2 galaxies taken from The Hubble. The arrow shows a smaller galaxy's black hole starving of the usual stars because of the binary rotation about the bigger galaxy that is pulling ...
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Black Hole Heat Capacity

Why is the specific heat capacity of a black hole given by $C=\frac{\partial M}{\partial T}$? Heat capacity should be ratio of heat change to temperature change so, denoting heat by $Q$, we have ...
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Quasar redshift

The quasar 3C 273 has a redshift z=0.158. A question in a textbook: could this be a gravitational redshift instead of cosmological (=resulting from the expansion of space)? My answer: no. Firstly, ...
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Could the recent evidence of a gravitational force orbiting the solar system indicate not the existance of a ninth planet, but rather a black hole?

In case you haven't heard already, researches at the California Institute of Technology discovered mathematical evidence of large object orbiting the sun at a distance about 20 times further than that ...
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Hawking radiation and black hole evaporation [duplicate]

As I understand it, Hawking radiation predicts black hole evaporation because of quantum effects. After the creation of a particle/anti-particle near the horizon of the black hole, if the one gets ...
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Will the CO–0.40–0.22 black hole companion of Sagittarius A merge with it?

According to "Signature of an Intermediate-Mass Black Hole in the Central Molecular Zone of Our Galaxy" by Oka, Mizuno, Miura and Takekawa, there is a second black hole, with a mass of ~ $10^5$ Sun , ...
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Black hole with NO radiation

I've been recently thinking about black holes and..., ...let's say we have a black hole that doesn't radiate anything. Then you will drop this black hole to some solid surface (let's say the black ...