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The radius of the event horizon of a black hole of mass $m$ is given by: $$r_s = \frac{2GM}{c^2} \tag{1}$$ Let's consider your idea of taking $n$ black holes of mass $M$ and arranging them into a sphere. The total mass is $nM$, and the radius of the event horizon corresponding to this mass is: $$R_s = n\frac{2GM}{c^2} \tag{2}$$ Now let's see how ...

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It is in principle possible, at least for some time, to have a collection of black holes gravitating along the surface of a sphere such that one can still escape from the inside of that sphere. In other words, the inside of the sphere need not be hidden behind a gravitational horizon. However, as soon as the density of black holes exceeds a critical value, ...

5

Understandably, Einstein would probably have been uncomfortable with the idea of singularities being consequences of GR. Your quote seems to indicate that he thought the singularity of a Schwarzschild solution was an accident caused by exact spherically symmetry, and that a more generic configuration would somehow not result in a singularity at all. However, ...

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If you stick to the theory of general relativity then what happens to the matter is fairly straightforward. As the matter falls inwards it experiences increasing tidal forces. The matter reaches the singularity in a finite (short!) time, and at the singularity it is compressed into a point with zero size and infinite density. Note that nothing special is ...

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The distance at which the tidal forces froma primary start tearing apart a satellite is known as the Roche limit. In calculating the Roche limit we assume that the yield stress of the rock making up the planet is small compared to the gravitational forces at work so it can be ignored. The question is then simply whether the gravity of the body (in this case ...

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Direct experimental evidence of Hawking radiation is going to be exceedingly difficult to obtain. The radiation from stellar mass black holes is so small as to be undetectable, and we haven't (yet) worked out how to small black holes in the lab. At the moment there is no direct experimental evidence, and we have to accept we may not see any in our lifetimes. ...

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Here's an answer from a nonspecialist who's reasonably comfortable up till the end and no further of "The Big Black Book" (Misner Thorne and Wheeler) and with a general lay reading other than this. Although often asked and thought about a great deal by physicists, your question is an excellent one because it leads straight to the edge of our knowledge of ...

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This is actually more subtle than you would think it would be. First, remember that a fourier transform is defined, for some time-dependent signal $F(t)$, as${}^{1}$: $$F(\omega) = \frac{1}{\sqrt{2\pi}}\int_{-\infty}^{+\infty} dt\, e^{i\omega\,t} F(t)$$ Well, this is great in special relativity, but in general relativity, what time do we actually use? ...

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There is a known example where it looks like a black hole on the outside but is flat spacetime on the inside. Imagine the funnel shaped exterior of a black hole and take the entire part outside of the event horizon, which is a spherical shell of surface area $4\pi r^2$. Then take a spherical ball of Minkowski space of radius $r$ and sew the two together ...

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The difference between your bungee cord analogy and the train is that the bungee cord is secured to the centre of the merry go round while the train is falling freely. So unlike the bungee, the far end of the train is not supporting the weight of the parts of the train nearer the singularity. The tidal acceleration between two points separated by a small ...

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The equation you cite is the Newtonian potential energy, and as BMS suggests I'd guess whoever is presenting the video is just being careless with the sign. Potential energy is not a well defined concept in general relativity. If you do a naive integration of $Fdr$ along a radial line to the event horizon it goes to (minus) infinity at the event horizon. ...

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General Relativity allows black holes of any size (though making a small one might be hard or worse than hard). So as a thought experiment that means that you can consider a small black hole that curves spacetime exactly as much as the sun does. This black hole would be much smaller than the sun, but to us out here spacetime would look the same (except we ...

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This is covered by a number of existing questions, but I think it does no harm to present a fresh summary. Firstly the analogy of the black hole absorbing one member of a pair of virtual particles is just an analogy, and actually a rather poor one, but let's go with it for now. In this analogy you're quite correct that equal numbers of particles and ...

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If we take the Milky Way as an example, the black hole at the centre, Sagittarius A$^*$, has a mass of about 4 million times the Sun. However the mass of the Milky Way is somewhere around a trillion Suns. So the central black hole makes up 0.0004% of the total mass. While the central black hole may have been important in the formation of the Milky Way, its ...

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There is plenty of evidence for the underlying quantum field theoretical description of the vacuum. The (complete) quark content of the nucleons has been measured, and includes both flavors not in the valence content (strange, charm (?)) and lots of anti-quarks. Everything that isn't valence content ($uud$ for a proton or $udd$ for a neutron) is called the ...

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If the event horizon is really a one-way surface, the answer is that whatever happens inside the event horizon is irrelevant to our universe. An object that crosses the event horizon will appear to an outside observer to take an infinite amount of time to do so, and no model of the interior of a black hole can ever be tested. Whether the event horizon is ...

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This answer expands on @Rex s answer, on the part elementary particles have in a black hole creation. It is often said that when the gravitational force exceeds any outward forces or pressures, mainly the electron degeneracy pressure I'm thinking, the star collapses into a black hole. But how can this happen without the Pauli exclusion principle being ...

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