# Tagged Questions

An event horizon is a type of boundary such that any information past this boundary is inaccessible to the observer it is defined for. Common examples are the Schwarzschild radius of a black hole (which is defined commonly for all observers outside this radius) and the cosmological even horizon ...

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### What is the radius of the event horizon?

I know that the Schwarzschild radius is given by $$r~=~\frac{2GM}{c^{2}}.\tag{1}$$ However, If we had the metric $$ds^2~=~−A(r,t)dt^2+\frac{dr^2}{B(r,t)}+r^2(dθ^2+\sin^2{θ}dϕ^2),\tag{2}$$ where ...
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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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### 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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### Closed timelike curves in the region beyond the ring singularity in the maximal Kerr spacetime

The region beyond the ring singularity in the maximal Kerr spacetime is described as having closed timeline curves. Why and/or how is the question. Now if you look a Kruskal-Szkeres Diagram (or a ...
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### What would happen if a negative mass crossed the event horizon of a black hole?

If negative mass really existed and somehow a very fast traveling negative mass object reached near the black hole's event horizon. What would happen when it crosses the event horizon? According to ...
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### How close can an observer approach the black hole in an unpowered flyby without falling into it?

In classical mechanics by choosing the right trajectory you can approach a planet arbitrarily closely, if there is no atmosphere or anything to slow you down, you can approach the surface then fly ...
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### Why Hawking radiation emission rate inversely proportional to black hole mass? [duplicate]

The rate of Hawking radiation emission increases as the mass of the black hole decreases, what I don't understand is that if the emission method is related to zero-point energy particles appearing and ...
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### Black Holes: How does a three dimensional object collapse into a singularity & Where does the matter go?

A black hole comes into existence as the result of the core collapse of enormous stars, which lose quite some mass in a supernova explosion. However, supermassive black holes are still by any means ...
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### What does this depiction of a black hole in the movie Interstellar mean?

I was expecting a whirlpool in 3D and the matter glowing from friction as it nears the center, as I expected a event horizon to be negligible visually. How does this depiction work? How big is the ...
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### When do you see the second horizon as you fall into a black hole?

Suppose you are a massive observer falling into a Schwarzschild black hole (geodesically or not, it shouldn't matter). At any point on your personal worldline you can receive light signals from the ...
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### Is the event horizon also the boundary region of the mass contained by a black hole?

I was recently playing around with classic Newtonian mechanics and calculated that the Earth would have to be compressed to a spherical region of $8.8 \ mm$ (Its Schwarzchild radius) to turn its ...
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### Does inbound light slow down (for an external observer) as it approaches the event horizon?

As I understand, for an external observer nothing even enters a black hole because it takes infinite time in his frame of reference for something to actually reach the event horizon. I also read that ...
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### Is the gravitational acceleration at the event horizon constant?

If the escape velocity at the event horizon of a black hole is equal to the speed of light, does this imply that the gravitational acceleration at the event horizon is also constant? For example, ...
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### How can black holes be point-like but have a size?

Presumably all singularities are the same size: a point. What makes one black hole bigger than another? Also, a singularity will only have the same mass (or less) than the collapsed star, so what ...
Is Relativistic event horizon half of Newtonian event horizon? relativistic escape velocity formula (from $m\phi=E-E_0$) is $v_e=\sqrt{2\phi-(\frac{\phi}{c})^2}$ and the Newtonian version of the ...