Comment by swframe2

Comment by swframe2 3 days ago

We were told that nothing can escape a black hole.

But when black holes merge, the combined mass is smaller.

It is described that the lost mass is converted to gravitational waves.

But gravity is the curvature of space caused by mass, it is not described as a form of mass.

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

Is it that the gravitational waves are caused by a "quantum gravity" particle that can't be converted back to any of the other quantum particles?

cyberax 3 days ago

> But gravity is the curvature of space caused by mass, it is not described as a form of mass.

Gravitational waves carry momentum and energy (and therefore mass), just like electromagnetic waves. Theoretically, you can extract that energy from gravitational waves, by using oscillating masses tuned to the wave's frequency.

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pdonis 3 days ago

> Gravitational waves carry momentum and energy (and therefore mass), just like electromagnetic waves.
No, EM waves do not have mass. They are massless. They carry momentum and energy, yes, but not mass.
In GR, the source of gravity is not "mass", it's stress-energy. EM waves carry stress-energy even though they are massless.
Gravitational waves do have some aspects that are analogous to EM waves, but there is a key difference: gravitational waves do not have any stress-energy. They are pure spacetime curvature in vacuum. So while there is a sense in which they carry momentum and energy, since properly constructed detectors can extract momentum and energy from them, they do not carry any stress-energy and the momentum and energy they carry cannot be localized the way momentum and energy in EM waves can.

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- cyberax 3 days ago
  
  > No, EM waves do not have mass. They are massless. They carry momentum and energy, yes, but not mass.
  OK, EM waves can get _transformed_ into mass.
  
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  pdonis 3 days ago
  
  > EM waves can get _transformed_ into mass.
  In the sense that they can be absorbed by matter and (possibly) increase the invariant mass of the matter, yes.
  
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- cyberax 2 days ago
  
  > gravitational waves do not have any stress-energy
  Wait, what? Of course, they do!
  It's just not localized, like in regular classic EM waves.
  
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  pdonis 2 days ago
  
  > Of course, they do!
  No, they don't. Read what I said carefully. I did not say gravitational waves do not carry "energy". I said they do not have any "stress-energy". In other words, they are vacuum solutions of the Einstein Field Equation--their stress-energy tensor is zero. That is a true statement, and I contrasted it with EM waves, whose stress-energy tensor is not zero.
  > It's just not localized
  This is a consequence of the fact that their stress-energy tensor is zero, so there is no tensor that describes "energy carried by gravitational waves".
  
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SideburnsOfDoom 2 days ago

> We were told that nothing can escape a black hole.

Stephen Hawking told otherwise, actually: https://en.wikipedia.org/wiki/Hawking_radiation

In 1974, and that's now 50 years ago.

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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.

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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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- 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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