> 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.

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jfengel 6 days ago | parent [-]

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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aw1621107 5 days ago | parent [-]

> You can't make a teaspoon of neutroniun, either. The neutrons would immediately drift off and quickly decay (half life about ten minutes).

Technically speaking that sure sounds like scooping out a teaspoon of neutronium to me. Nothing said it had to be stable :P

But in any case, I suppose what doesn't work for me is that when the teaspoon illustration is being used it's in the context of picking out some sample/subset of a larger whole - take a whole neutron star and examine the properties of this supposed representative part of it, same way one might scoop out some ice cream out of a container. While that's technically not totally correct for neutron stars since they don't exactly have a uniform density, I feel that it's usefully-close-enough compared to black holes, since as far as we know all the mass of a black hole is concentrated in a point at its center so your "scoop" is either going to get nothing or everything.

> You actually can have a black hole with the volume of a teaspoon, and it's stable.

Sure, but at that point I wouldn't use the wording "a teaspoon of black hole"; something more like "teaspoon-sized black hole" would be more appropriate (though to be fair that's still technically somewhat ambiguous).

	▲	rapnie 5 days ago \| parent [-]
		Saw some Youtube vid years ago about what happened if you accidentally dropped the content of your teaspoon on the carpet of your living room. Earth would be relatively fine for a long time afterwards was the gist, if I remember correctly.