Comment by dweinus
1 day ago
Using the formula for black hole density, a black hole of this mass would have an average density about the same as the near-vacuum atmosphere of Mars(!)
https://physics.stackexchange.com/questions/26515/what-is-ex...
1 day ago
Using the formula for black hole density, a black hole of this mass would have an average density about the same as the near-vacuum atmosphere of Mars(!)
https://physics.stackexchange.com/questions/26515/what-is-ex...
That calculation of density is nice, but since we don’t know what’s inside a black hole, it doesn’t mean anything.
Passing the event horizon doesn’t mean you’ve reached the potentially ultra dense singularity, but it does mean you won’t escape.
we don't know how the mass inside a black hole is doing, but we have a pretty solid understanding of the volume and the total mass of black holes
And it would take 10 days from event horizon to the singularity.
from which perspective? I have yet to wrap my head around it(this usually means I am wrong about something), but there may be no singularity because it takes matter an infinite amount of time to reach the center due to time dilation effects.
https://modern-physics.org/time-dilation-near-massive-bodies...
This is the origin of my favorite science fiction theory. (little to no actual science but you could write a fun space romp around it) If you get a large enough black hole where the tidal forces will not rip you to shreds instantly, you could just scoot across the event horizon right, now what happens? you can still move around, everything feels normal, but really you have lost half a dimension, everything "out" from the center is completely gone from the universe. Now the theory, back to our universe, What happened to time? why does time only go one way? we can accelerate and decelerate along the time axis, but can't reverse it. Where has our missing half of a time dimension gone?
>To an external observer, an object falling into a black hole appears to slow down and never actually crosses the event horizon, seemingly freezing in time.
It takes infinite time to reach event horizon, not the center.
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> but there may be no singularity because it takes matter an infinite amount of time to reach the center due to time dilation effects.
Wouldn't that just mean that the singularity is located infinitely far into the future?
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It's not quite true that everything feels normal. If I am standing with my feet toward the singularity, my hand cannot move above my head, the best it can do is fall toward the singularity slower than my head does. Especially at very slow speeds this has some very weird physical effects, not the least of which is the immediate impossibility of all systems that make you 'you' continuing to function.
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How so?
The black hole has two conceptual parts - the event horizon and the singularity. The event horizon is a one-way imaginary shell where once you pass it, you will end up at the singularity which is a point at the center of the event horizon. It’s the hole in black hole. Because the radius of the spherical horizon grows linearly with mass, but the size of the hole is fixed at effectively 0, it allows for a bit of sightseeing on your way to impending doom if the mass of the hole is large enough.
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Definitely a dumb question but I had read "a teaspoon of black hole is more dense than Mt Everest" or something like that.
The near-vacuum atmosphere of Mars seems very light...? What fundamental concept am I misunderstanding?
> but I had read "a teaspoon of black hole is more dense than Mt Everest" or something like that.
That sounds more like a description of the stuff neutron stars are made of. I don't think that description really works for black holes - how exactly do you take a teaspoon out of a black hole?
> The near-vacuum atmosphere of Mars seems very light...? What fundamental concept am I misunderstanding?
The linked Physics.SE answer does a decent job at explaining it, but the short of it is that for Schwarzchild black holes mass ~ event horizon radius, so if you define density as mass / (Schwarzchild volume) you get density ~ 1/(mass^2) - in other words, the more massive a black hole the less dense it is by that measure.
You can't make a teaspoon of neutroniun, either. The neutrons would immediately drift off and quickly decay (half life about ten minutes). It's just a way of illustrating the density.
You actually can have a black hole with the volume of a teaspoon, and it's stable. It will eventually decay by Hawking radiation, but not for umpteen gazillion years until the CMB gets cold enough.
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Black holes become less dense as they get bigger.
Radius is linearly proportional to the mass: r = 2GM/c²
(So volume grows faster than mass)
Small black holes are light, a large black hole with the mass of our visible universe would have an event horizon larger than the visible universe, because the area, not volume, scales linearly with the contained mass.