[LeifCarrotson]

To get density you need to know the volume in addition to the mass. As far as I understand, the volume is hard to measure and mostly theoretically derived, amd lensing and orbiting bodies would behave the same regardless of whether it was a point mass or low density sphere.

[pdonis]

> the volume is hard to measure and mostly theoretically derived

Not even that. The "volume" of a black hole is not even well-defined. A black hole doesn't work like an ordinary object.

> lensing and orbiting bodies would behave the same regardless of whether it was a point mass or low density sphere.

First, a black hole is definitely not a "point mass". Second, the density of a black hole is also not well-defined.

Lensing and orbiting bodies do exhibit unique properties close enough to the black hole's horizon, but unfortunately we would have to be relatively close to the hole (meaning, in the same stellar system, not light years away) to distinguish them.

[dumpsterdiver]

> A black hole doesn't work like an ordinary object.

Black holes are ordinary objects, if they are anything at all. Take any normal object and give it the mass of a black hole, and what do you have?

To claim that a black hole “doesn’t work like normal matter” seems to ignore that these entities have come to exist “normally”, or rather, through a natural course of events.

It’s not that the black hole “doesn’t work like normal matter”, but rather that you refuse to accept that normal matter with the mass of a black hole (and hence, the observable properties we associate with black holes) are still “normal objects”.

When objects of a certain class display certain properties, it’s not accurate to say “that’s not normal.” Instead, it would be accurate to say, “that is the normal behavior of objects once they reach this class or state.”

[pdonis]

> Black holes are ordinary objects, if they are anything at all.

No, they aren't. Their spacetime geometries are very, very different from those of ordinary objects.

> Take any normal object and give it the mass of a black hole

There is no such thing as "the mass of a black hole". What distinguishes a black hole from other objects is not its mass, but its spacetime geometry. That's particularly true for the holes studied by this paper, which are far smaller than the ones we actually see with our astronomical observations.

The rest of your post is simply wrong. A black hole is not made of normal matter; it's vacuum. I'm sorry, but you simply don't understand the physics of black holes. A good free online reference is Sean Carroll's lecture notes on GR:

https://arxiv.org/abs/gr-qc/9712019

It has an entire chapter on black holes.

[dumpsterdiver]

> What distinguishes a black hole from other objects is not its mass, but its spacetime geometry

Fair enough. Primordial black holes are a good theoretical example of environmental density conditions being perhaps a larger factor than mass even. My point was that it was still matter. It doesn’t matter if you squish it all down - it’s still matter.

> A black hole is not made of normal matter; it's vacuum.

When we talk about black holes, some might be referring to the event horizon, and others the singularity which makes the event horizon possible. But I’m not sure what you mean when you say that a black hole is made of vacuum. Are you saying that the singularity of a black hole is a vacuum? And not matter?

[taneq]

I don’t think black holes have a well defined “density” in that sense? I think we just use the event horizon as the “size” (which therefore has a simple relation to the mass.)

[kolinko]

Yes, but black holes are bigger on the inside than on the outside, right? :)