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?
The near-vacuum atmosphere of Mars seems very light...? What fundamental concept am I misunderstanding?
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 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.
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).
Radius is linearly proportional to the mass: r = 2GM/c²
(So volume grows faster than mass)