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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 t

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 2y 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.
- pdonis 2y 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.
  - cyberax 2y 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.
    - pdonis 2y 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.
  - cyberax 2y 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.
    - pdonis 2y 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".
pdonis 2y 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 2y 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?
  - pantulis 2y 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.
    - pdonis 2y 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.
      - pantulis 2y 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.
SideburnsOfDoom 2y ago
> We were told that nothing can escape a black hole. Stephen Hawking told otherwise, actually: https://en.wikipedia.org/wiki/Hawking_radiation https://en.wikipedia.org/wiki/Hawking_radiation In 1974, and that's now 50 years ago.

cyberax 2y 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.

pdonis 2y 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.
- cyberax 2y 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.
  - pdonis 2y 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.
- cyberax 2y 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.
  - pdonis 2y 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".

pdonis 2y 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 2y 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?
- pantulis 2y 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.
  - pdonis 2y 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.
    - pantulis 2y 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.

SideburnsOfDoom 2y ago

> We were told that nothing can escape a black hole. Stephen Hawking told otherwise, actually: https://en.wikipedia.org/wiki/Hawking_radiation https://en.wikipedia.org/wiki/Hawking_radiation In 1974, and that's now 50 years ago.