Comment by pdonis

Comment by pdonis 3 days ago

> How do you understand this process of matter escaping a black hole?

No matter escapes. Gravitational waves are not matter. They are spacetime curvature. Nor do they "escape" the black hole; they are emitted from outside the horizon. The reason the mass of the merged hole can be smaller than the combined masses of the original holes, with the difference being emitted as gravitational waves, is that black holes are not made of matter, they are made of spacetime curvature, and when they merge, some of the spacetime curvature doesn't get included in the merged hole. That's just how spacetime curvature works.

LegionMammal978 3 days ago

> Nor do they "escape" the black hole; they are emitted from outside the horizon.

That's interesting, I'd never really thought about that before. Does GR predict that there would be any waves confined to the inside of the merged black hole?

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pantulis 3 days ago

If there is anything combined inside the resulting event horizon it doesn't matter what it was: as far as GR is concerned it has been reduced to the effect of its mass, charge and spin. But we already know that GR isn't the whole story when it comes to BHs. See "soft hair" black holes.

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- pdonis 3 days ago
  
  > we already know that GR isn't the whole story when it comes to BHs. See "soft hair" black holes
  More precisely, most physicists believe that GR isn't the whole story. But we have no actual evidence for quantum gravity speculations like "soft hair". They're just speculations at this point. We don't know that any of them will actually turn out to be right.
  
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  pantulis 2 days ago
  
  You are right. GR being or not the whole story is a thing, "soft hair" black holes is another thing with a much more speculative edge.
  But I would say that the first assertion, that GR is not the whole story, is more or less a given knowing that GR returns non-physical infinities when trying to describe what's inside the BH.
  
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pdonis 3 days ago

> Does GR predict that there would be any waves confined to the inside of the merged black hole?
Yes, but we would of course never observe those, and they would not reduce the externally measured mass of the merged hole.

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- LegionMammal978 2 days ago
  
  Yes, of course there'd be no external effects. I was just curious, since the region between the event horizon and the singularity is an interesting place to think about, even if we're forever confined to predictions rather than real observations.
  
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akira2501 3 days ago

> matter, they are made of spacetime curvature

Which is caused by matter achieving a particular density. What we observe is not the matter but the _effects_ of the curvature created by that matter but to say they're not "made of matter" seems overly reductive here.

> some of the spacetime curvature doesn't get included in the merged hole

The observable mechanics of a black hole are (probably) controlled by it's surface area and not it's volume. When two spherical objects merge the surface area is less than the sum of the two original objects.

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pdonis 2 days ago

> Which is caused by matter achieving a particular density.
More precisely, by an isolated blob of collapsing matter surrounded by vacuum achieving a particular density.
> to say they're not "made of matter" seems overly reductive here.
No, it isn't, it is making a very important point: that the matter that originally formed the hole is not there any more. The hole itself is vacuum. If you fall into it, you won't see any matter, even though the hole was originally formed by collapsing matter.
> The observable mechanics of a black hole are (probably) controlled by it's surface area and not it's volume.
Only in the sense that the horizon area is proportional to the square of the mass, whereas the volume is not even well-defined. The actual thing that is controlling the "observable mechanics" is the mass (and spin, and charge if present, but in any actual hole it probably won't be).

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