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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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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90 views

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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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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55 views

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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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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47 views

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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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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1answer
186 views

What are the virtual particles generated during the Hawking radiation?

What are the virtual particle pairs generated during the Hawking radiation? If a photon is emitted by Hawking radiation, what is its negative energy partner which fell into the black hole? Does it ...
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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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A ball in the gravity potential field of a black hole — seems a paradox

As illustrated in the following diagram (A, B, C, D are 4 specified space points, and C is close to a black hole), a small ball at distance of a black hole is stationary (suppose now it's mass is m0) ...
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How is the Ricci scalar $R=0$ here?

Given the metric in the form: $$ds^2 =-A(r)dt^2 +B(r) dr^2 dr^2 +r^2(d\theta ^2 +\sin^2\theta d\phi^2)$$ Papapetrou in his book said that $R=0$ But when I performed it I didn't get zero. For ...
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94 views

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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57 views

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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1answer
37 views

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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330 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 ...
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72 views

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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5answers
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The bigger the mass, the more time slows down. Why is this?

If I were to stand by a pyramid, which weighs about 20 million tons, I would slow down by a trillion million million million of second. Don't know if that's exactly right, but you get the point. Also, ...
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149 views

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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4answers
355 views

What would be the view like from inside a black hole looking towards the event horizon?

Ignoring the fact that we would be torn apart by gravitational gradient and assuming we get some time to make some observations before hitting singularity, what would we see looking towards the event ...
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158 views

Rotation of Spacetime => 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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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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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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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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1answer
141 views

Why is the mass of a Kerr black hole proportional to it's angular momentum?

I'm a third year mathematics undergrad, and have just started the module General Relativity and spacetime geometry, I also have a keen interest in black holes. However I would like to know why and ...
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1answer
40 views

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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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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1answer
125 views

Black Hole Matter to Subatomic Exchange

When matter enters a black hole with a singularity the particles smaller than photons released come from within the black hole or do subatomic particles shed from matter just before matter passes the ...
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109 views

Is black hole formation reversible if physics law holds even in time reverse?

As we know many situations still fulfill physics law if time is reversed, such as particle collision. But how about black hole formation? Suppose a star is turning to a black hole and starts to have ...
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1answer
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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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Coordinate Singularity in Metric

Suppose I have some metric $$ds^2=g(t)dt^2+\frac{1}{r}dr^2$$ which has a singularity at $r=0$. However, if I make the coordinate transformation $u=\frac{1}{r}$, then I get: $$ds^2=g(t)dt^2+r^3 ...
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52 views

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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1answer
75 views

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 ...
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Hawking paradox [duplicate]

Over the centuries, physicists "gave up" on conservation of elements, conservation of mass and then conservation of energy, which now we know to be related to symmetries. How can we be sure that ...
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Doesn't the Schwarzschild metric combined with Hawking radiation imply that nothing ever gets past the event horizon of a black hole?

According to the General Theory of Relativity, the coordinate time distance per spacetime distance traveled by a particle freely falling into a black hole gets closer and closer to $0$ as the particle ...
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What does black hole temperature mean? [duplicate]

From the title, the question is a bit open ended. Let me make it a bit simpler. 1. What region of the black hole has temperature? Is it the surface space? the region outside? . . . etc , any precise ...
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330 views

Time Dilation inside a hollow shell

Assuming I have a hollow shell with total mass $M$ and radius $r$. On the surface, the gravitational time dilation would be $$\tau=t \cdot \sqrt{1-\frac{v_{esc}^2}{c^2}}$$ where $$v_{esc} = ...
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When is the event horizon a Killing horizon?

I know the definition of both (event horizon is closure of causal past of future null infinity whilst Killing horizon is a null surface where some Killing vector becomes null e.g. the surface where it ...
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Can a solid object inside the black hole event horizon avoid the singularity? [duplicate]

I will ask this as a simple lay man. Let's consider an astronomical black hole that was creating by in falling matter on a neutron star. As neutron star get heavier it gets smaller and a Schwarzschild ...
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3answers
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When a neutron star accumulates enough mass, does a black hole start at its center?

With less dense bodies, such as the earth and the sun, the center has less gravity/density (since there's an equal amount of mass surrounding the center, pulling out on it from all directions). Does ...
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1answer
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Derivation of Schwarzschild metric using the full machinery of differential geometry [closed]

How would one derive the Schwarzschild metric using the full machinery of differential geometry, using the component approach as little as possible? Something along these lines: Begin with a manifold ...
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2answers
89 views

Gravitation, light and black holes

I have few basic misunderstandings about black holes. I know that heavy mass causes a curved space time around it, what i don't understand is if this mass is a star and it becomes a black hole, why ...
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If gravitons are particles, how do they escape BH gravity if nothing can? [duplicate]

If gravitons are particles, how do they escape BH gravity if nothing can? So are the gravitons really particles or is there really just/only curvature of space-time?
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93 views

Black hole merger orbital speed

If a ship is close to a black hole and we see it getting slower to the point if could see them not for red shift would almost stop moving. Then how fast is the little black hole moving in this video? ...
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Photon gas into a Black hole

Can a photon gas have so much energy that it collapse into a black hole just like a regular star?
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Using a black hole as a mirror

http://www.youtube.com/watch?v=ENd8Sz0AFOk The YouTube video is a good example how the gravity of this merging binary black holes bend light around themselves.