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New answers tagged black-holes

1 vote

Why can't dark matter be black holes?

tl; dr: it can, but needs more observations/theoretical reasons to be convincing. Black holes can come in a variety of mass ranges, from stellar-mass ones to supermassive black holes. One feature for ...
• 18.1k
3 votes

Why light can't escape a black hole but can escape a star with same mass?

There are a lot of ways to explain this, with various levels of accuracy. I'm going to choose a way to justify this, in the spirit that the original question seems to not come from a place with a lot ...
• 39.8k
8 votes

Why light can't escape a black hole but can escape a star with same mass?

yes, it is because the black hole is much more dense which means it packs the same mass & gravitational pull into a much smaller diameter. This means its surface gravity yields an escape velocity ...
• 80.5k
3 votes

Why light can't escape a black hole but can escape a star with same mass?

Yes, it's just because the black hole is smaller. The only mass factor relevant to the trajectory of a particle in a spherically symmetric mass distribution is the total mass that is closer to the ...
• 10.7k
0 votes

Why apparent horizon area provides a lower bound for event horizon area?

Let me add a comment. Penrose inequality arise from this collapsing shell set-up; however, later this inequality become of interest independent of collapsing shell, as Gibbons put it in "...
10 votes

If massless objects ALWAYS travel at the speed of light and gluons are massless, how are they trapped within hadrons without a need for event horizon?

Now, to make the light trapped within a small region of spacetime, we need curvature so big that it causes an event horizon, so now we have a Black Hole Photons do not carry an electric charge. So ...
• 7,661
13 votes

If massless objects ALWAYS travel at the speed of light and gluons are massless, how are they trapped within hadrons without a need for event horizon?

That quarks and gluons are trapped inside hadrons is called color confinement. As far as I know: it is unrelated to black holes but I don't read Phys Rev D... it's possible it has been suggested ...
• 26.9k
0 votes

Star gets eaten and spit out?

You are talking about Hawking radiation, which makes black holes evaporate. While General Relativity explains how BHs form through gravitational collapse, it's quantum mechanics that explains their ...
• 10.4k
2 votes

Star gets eaten and spit out?

Anything that passes into the event horizon of an astronomical black hole is gone from our observations. It won't be seen again. But material can orbit a black hole without falling in. If there is ...
• 36.4k
2 votes

Typically energy of black hole compared to a planet or star of the same mass

You ask about "the typical energy of a black hole". All the black holes that we know about have formed from the collapse of massive stars or the merger of stellar black holes, so they have ...
• 39.6k
0 votes

Is there any evidence black holes can grow (gain mass) by accretion?

Mass accumulation rates typically are limited by the Eddington luminosity (where the light pressure starts blowing away the accreting material) giving a rate $\approx 1.5\cdot10^{17}(M/M_\odot)$ g/s ...
• 29.3k
0 votes
Accepted

Why apparent horizon area provides a lower bound for event horizon area?

In Penrose's setup, the spacetime interior to the null hypersurface $H$ is always Minkowski.Since the area of a cross-section $S$ of $H$ has to be equal when measured from the interior and exterior ...
• 9,158
0 votes

Where is Schwarzschild solution valid?

Schwarzschild solution is valid only outside of matter's sphere and it is static - there is no time dependence. In case of black hole it ends on the even horizon. Behind it, time and space change ...
• 1,400
3 votes

Where is Schwarzschild solution valid?

However, when we use kruskal coordinates, the coordinate system covers the inside of the spherical object (usually taken to be a black hole). The interior (by which I mean, $r<r_S$) of a ...
• 60.6k
0 votes

How compute the mass of AdS-Schwarzschild by ADM mass formula?

I think the issue is that you’re using a formula for the ADM mass which assumes unit lapse. Consider a spherically symmetric spacetime of the form \begin{align} {\rm d}s^2 & = -f(r) \, {\rm d}t^2 +...
• 43
0 votes

Does black hole entropy change as a gravitational wave passes it?

Yes, gravitational radiation can change the entropy of a black hole, but it will depend on the behavior of these radiation on the event horizon. One way to study these effects is by considering the ...
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1 vote

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19 votes
Accepted

How do we know the assumptions of the Schwarzschild solution are valid?

This is a very long-winded answer, so let me state the punchline up front: the boundary condition you pointed out really is physically suspect, and probably not realized in Nature -- however using it ...
• 41.2k

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