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> I've always wondered how we actually measure the density of black holes. We don't. The density of a black hole is not even a well-defined concept; black hole
by pdonis 3y ago
> I've always wondered how we actually measure the density of black holes.
We don't. The density of a black hole is not even a well-defined concept; black holes are not like ordinary objects.
> My confusion lies in how we can derive the mass at the hyper-quantum state at the center of a black hole
There is no such thing. A black hole is vacuum inside. The "mass" of the hole is not due to "stuff" sitting inside it; it's a global property of the hole's spacetime geometry, that maintains itself because of the nonlinearity of the Einstein Field Equation.
> from the gravitational waves we observe as it loses energy.
An ordinary black hole in our universe will be gaining energy, not losing it--if for no other reason that CMB radiation is shining on it. But the black holes we see all have matter falling into them as well.
As for detecting gravitational waves, except for violent events like black hole mergers, we don't detect any; what few such gravitational waves there are are far too weak for us to detect.
- ptarjan 3y ago> As for detecting gravitational waves, except for violent events like black hole mergers, we don't detect any; what few such gravitational waves there are are far too weak for us to detect. I thought LIGO and VIRGO had been detecting gravitational waves for the last 4 years pretty regularly (i.e. once a week).
- some_furry 3y agoYes, from mergers.
- meowface 3y agoTo what degree would a non-spinning, non-moving black hole necessarily produce even very weak gravitational waves? Would some be produced as matter/energy reaches it? If so, would no gravitational waves at all be produced if nothing was reaching it, hypothetically? (I suppose these same questions apply to all massive objects.)
- scatters 3y agoA radially symmetric object does not produce gravitational waves, whether it is spinning or not, since it has zero quadropole moment.
- pdonis 3y ago> To what degree would a non-spinning, non-moving black hole necessarily produce even very weak gravitational waves? If it's perfectly non-spinning, and nothing is falling into it, then it emits no gravitational waves at all. If matter or energy falls in, and does so in a manner that is not spherically symmetric (which will most likely be the case), then some very weak gravitational waves will be emitted. But they will be much, much too weak for us to have any hope of detecting them. Even a gravitational wave detector like LIGO in the same stellar system would have difficulty detecting them.
- BurningFrog 3y agoA black hole has a mass, and a volume defined by its event horizon. From that you can compute a density number, but I agree it's not quite the same concept as the density of water.
- pdonis 3y ago> A black hole has a mass Yes, but not because of "stuff" inside it. As I said, the mass is a global property of the spacetime geometry. > and a volume defined by its event horizon. No, it has a surface area defined by its event horizon. But you can't compute a meaningful volume from this surface area. You can, of course, plug the surface area into the formulas of Euclidean geometry, but the number you get will have no physical meaning for a black hole. A black hole has no well-defined volume at all--indeed, one can find spacelike surfaces inside that are infinite in extent, even though it has a finite surface area.
- thehappypm 3y agoI don’t know if I really agree that a black hole’s mass is not because it has “stuff” (that is, massive entities) inside it. How do you argue that? In simpler terms, let’s say a star has a mass just below the limit of making it a black hole. Then, a planet collides and it crosses the threshold. Are you saying the “stuff” disappears ?
- pdonis 3y ago> How do you argue that? Because the solutions of the Einstein Field Equation that describe black holes are vacuum solutions. "Vacuum" means "no matter or energy". > let’s say a star has a mass just below the limit of making it a black hole. There is no lower limit to the mass of a black hole. There are maximum mass limits for white dwarfs and neutron stars, and stars more massive than those limits (after they have undergone supernova explosions and probably shed a large portion of their original mass) will collapse to black holes. As for what happens to the matter in the original object that collapses to a black hole, it reaches the singularity at the center of the hole and is destroyed. But anyone falling into the hole after the collapse will never encounter that collapsing matter; all they will see is vacuum.