# B Can gravitational wave escape a black hole

1. Feb 17, 2016

### kodama

Last edited by a moderator: Feb 17, 2016
2. Feb 17, 2016

### phinds

No, they can't but they don't need to. Why would you think they might? And by the way, please don't use HUGE fonts when it is totally unnecessary to do so.

Last edited by a moderator: Feb 17, 2016
3. Feb 17, 2016

### kodama

feel free to correct the formatting.

gravity waves is spacetime waves so its different in nature from lightwaves which are bosons

4. Feb 17, 2016

### bcrowell

Staff Emeritus
It's irrelevant that light waves are bosons. (And if gravity is quantized, then their quanta are bosons as well.) Light waves are also irrelevant here because light waves don't play any fundamental role in GR or in the properties of black holes.

Low-amplitude gravitational waves obey a wave equation. It's a consequence of this wave equation that these waves propagate along the light cone. For an event inside the event horizon of a black hole, the future light cone lies inside the event horizon. Therefore the answer in classical gravity is no, gravitational waves cannot escape from a black hole.

I don't see how string theory or any other theory of quantum gravity is relevant here. This is a purely classical question. Nothing is happening at the Planck scale, so we don't need a theory of quantum gravity to discuss this. Similarly, we don't need QED to explain the radiation pattern of a radio antenna.

5. Feb 17, 2016

### kodama

so what happens to the spacetime curvature wave as it approaches the event horizon from inside the black hole?

6. Feb 17, 2016

### bcrowell

Staff Emeritus
It doesn't approach the event horizon. It propagates toward the singularity and ends there.

This is exactly the same as the case of a radio wave. A radio wave emitted from inside the horizon doesn't approach the event horizon. It propagates toward the singularity and ends there.

You might find it helpful to learn about Penrose diagrams in order to reason about this kind of thing. I have a simple, nonmathematical treatment of Penrose diagrams in section 11.5 of this book: http://lightandmatter.com/poets/

7. Feb 17, 2016

### phinds

what is a "spacetime curvature wave" ? Sounds like you are just stringing words together.

8. Feb 17, 2016

### bcrowell

Staff Emeritus
I think this just indicates a gravitational wave. That's what a gravitational wave is: a wave of spacetime curvature.

9. Feb 17, 2016

### phinds

OK. I just had not heard the term before but your interpretation seems reasonable.

10. Feb 17, 2016

### Staff: Mentor

Done.

11. Feb 20, 2016

### stevebd1

Gravitational waves originate outside the event horizon of a black hole. It's predicted that when a BH collapses with rotation, some of the gravitational wave (the radiative tail) falls through the EH which then inflates in a Dirac Delta like function at the inner/Cauchy horizon contributing to something called mass inflation (a weak singularity at the inner/Cauchy horizon). This is part of the reason why the inner horizon is considered to be unstable and 'the boundary of predictability'.

Old library entry with a number of links to various paper-
What is mass inflation

12. May 22, 2016

### jeremy974

We could detect gravitational waves produced by a merger of 2 BH; but if this waves meet an other BH, will they be absorb by this last one?

13. May 22, 2016

### Staff: Mentor

Not sure. I've heard once from someone here on PF that a gravitational wave coming across a black hole will cause changes in the event horizon that would end up creating another gravitational wave identical to the first one. So it would be like the original gravitational wave simply passed through the black hole.

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