Blitzfrog wrote:
First off, Blackholes don't have mass. Not directly. They're not anything, how can they have mass? Black holes are not objects, they're what happens when you delete a region from the fabric of spacetime. We just talk about them like mass because they, similar to a massive object, exert gravitatational properties to other objects. For example, replace the sun with a black hole the size of the schwartzchild radius of the sun, and nothing about the gravitational effects of nearby objects are effected. Earth will still orbit fine, so will Venus and any other planet in the solar system.
That is naive thinking. Alright, yes space is bent infinitely in, but that doesn't mean there is a whole of nothingness there.
Let's examine what happens when a photon gets captured be a blackhole. As it passes the event horizon, time no longer ticks, right? Yea. So there are no dynamics going on, right? No. If you see the
PBS Spacetime episode on how spacetime behaves inside a blackhole, you would know space starts acting like time and time starts acting like space. Instead of oscillations per second, you need to measure it in oscillation per meter. Let's demonstrate this using an intuitive example, a wave falling towards the singularity. That wave gets pulled toward the singularity, stretching it indefinitely, however it never quite reaches there. It just keep on going there, forever due to the infinite curvature. It is not stretched indefinitely all at once either. You see, if you take a point inside a blackhole intersect by a wave falling in, that point will experience those waves oscillations. However, those oscillations will pass slower and slower as the wave gets stretched more and more. At that point you would get maybe 10 oscillations per meter, then 7, 3, 1, 0.5, etc, approaching to 0 but never quite getting there. You actually need to stay still or move away from the singularity through the space inside a blackhole to have any resemblance of time. This is what it means to have time and space reversed. And as you can see, there is still much happening in there.
Blitzfrog wrote:
Now you might say "But where does all the mass of the original star go?", well first off, this question was what made Stephen Hawking famous. It was originally thought that the materials of the star are just gone, deleted. But if you do some quantum physics you will know that information cannot be destroyed, and therefore mass cannot just be deleted. What got Stephen Hawking famous was his Hawking radiation, which is literally Black Holes "radiating mass out of itself".
Yes, mass isn't deleted because the matter is still there, albeit disintegrated into its component field, may it be the electric field, magnetic field, strong force field, etc and is stretching indefinitely toward the singularity. When the mass radiates out, it's not what inside gets radiated out, but rather due to effects of the strong gravitation field ripping apart virtual particles near the event horizon into matter and antimatter. One matter/antimatter into blackhole, the other half out.
Blitzfrog wrote:
About the singularity, which is what the media says is "Mass squashed to an infinitesimal point". I think what you may be thinking is that black holes are massive objects, collapsed into an infinitesimal point called the singularity, therefore have infinite density.
We, again, have to be careful. The singularity isn't anything. It is not an object, not an event nor a location in spacetime. If you look at the diagrams you find on the internet, it might show the singularity as a point stretching the fabric of spacetime really far down, but that isn't it. Singularities are a hole in spacetime, and this hole makes the geodesics(you can think of geodesics like the "straight lines" of a curved geometry, basically the path an object would follow given by Newton's first law of motion in a curved space) so distorted that it is basically undefined.
Something to clear up. All the energy is being attracted to other energy inside the blackhole, which leads it all towards one point. However, since the energies are concentrated so much, they bend spacetime so much so that the energy as waves have to travel toward the one point indefinitely. Hence the singularity. Another thing, I would say that it's infinite density only relative to an observer outside the event horizon. Relative to an observer inside the event horizon, there is no such point. It's an infinite journey to the center, much like an infinite journey towards the edge of the universe.
Blitzfrog wrote:
Black holes are formed when a sufficiently massive object is collapsed into a volume smaller than the original object's Schwartzchild radius.(I realised this is rather jargon so you can search up about it) This usually means the "mass" of the black hole is equal to the original star that collapsed in the first place. However the horizon of the black hole first forms inside the star, and then expands. This means to external observers, the mass of the object never crosses the horizon. (Nothing ever crosses a black hole to an external observer as light never allows external oberservers to "see" and object crossing). So in this scenario, the mass of the black hole is redundant.
You see it redshifted as it is sucked in. It effectively crosses the event horizon. Though, the actual wave is being stretched from the event horizon to toward the singularity, so some part of the wave is indeed stuck on the event horizon. Time slows down indefinitely there, so the wave becomes highest freq gamma on the event horizon relative to an observer outside the event horizon, though you will never observer that. Relative to an observer inside the event horizon, the wave gets stretched indefinitely toward the "singularity" (which doesn't exist relative to there, just a direction) from an unreachable, external, source.
Blitzfrog wrote:
Another problem here is that the equation used to calculate a black hole (Einstein's Field Equations), also allows an empty universe with an eternal blackhole. If you're gonna talk about blackholes, you must include all of them, including this one. But in this case, what are you suppose to say about the mass of the black hole??? There is no mass, as defined by the equation, anywhere.
This would be true if dark energy expands spacetime such that the cosmic microwave background gets redshifted slower than the massive blackhole's Hawking Radiation vaporizes it, allowing the blackhole to get larger as time goes on. Since dark energy seems to be accelerating the universe's expansion, I don't think this will be the case unless universal expansion decides to take another, slower, curve in the next trillion years or so.