– how come black holes have different density if they all have a singularity that is infinitely dense?
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I dont think they have different density. They have a mass and that determines its radius, aka the schwarzschild radius.
Singularities aren’t physical, they’re artifacts of incomplete theories. We don’t know of any force that would maintain a structure in that extreme gravity, but “don’t know” doesn’t imply that infinities actually occur in nature. In practice, no-one has ever observed an actual infinity, and I can’t imagine how one could even observe or measure such. There’s no reason from experience or theory to think that infinities actually occur, and we know already that singularities in a theory are discontinuities where the theory fails to accurately describe nature.
That is referring to the density of the event horizon. The singularity is infinitely dense due to zero volume but the event horizon has both a volume and mass so it has a real density. All the mass is still contained within the singularity at the centre of the black hole. The larger a black hole is the less dense it is as the event horizon grows with the cube while the mass grows linearly. Again the singularity just grows in mass as it has no volume.
Singularities are a result of incomplete theories to describe black holes. They are an approximation. Until we have a theory of quantum gravity they are all we have.
This is slightly misinterpreted I think.
The mass of a black hole is concentrated in an infinitely small point, the singularity.
What we call the size of a black hole is in reality the “Schwarzschild radius”, which marks the “event horizon” which is the point where the escape velocity becomes larger than the speed of light, meaning that at this point not even light can escape the gravitational pull anymore, making it appear completely black.
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