[John Hancock]

John Hancock  asked the Naked Scientists:
   
Dear Chris, I have sent this question to New Scientist several times without getting an answer - it's driving me mad. Perhaps you can explain !
 
Science textbooks insist that the effect of gravity is not instantaneous, but travels at the speed of light. If that is true, how does gravity "escape" from a black hole to cause bodies to orbit round it ?
 
John Hancock.

What do you think?

[Vern]

That is a very good question. We have known for a long time that it is impossible to model the formation of a black hole when you consider relativity phenomena in the model. When you consider adding matter incrementally to a local area of space you find that you can never quite get there.

But we know that super-massive things exist at the centre of galaxies; we suspect that they are the black holes that General relativity theory permits; we do not yet know whether the internals of the super-massive objects at galactic centres exactly match black-hole predictions. The observable stuff orbiting outside the super-massive objects do fit the black-hole predictions pretty well.

As the OP implies; if gravity affects gravity the way gravity affects light, then gravity could not escape the theorized black hole. There is also the well known problem that stars within galaxies do not follow normal orbital patterns. Our ideas about gravity might need a little work.

[peppercorn]

It IS a good question.
But I suspect the answer is not out of line with quantum-gravity theory (incomplete as it may be).
In QM quanta of the gravitational force is carried by gravitons (at c) between matter.
Gravitons (like photons) are massless energy carrying particles and as such no mechanism exists for interactions between two gravitons.  Thus a graviton whilst transferring gravity would not be effected by it.

Edit:
Although photons are clearly effected by gravitons - as near a star.  So I'm not sure, after all!

[Mr. Scientist]

Gravitons do not escape black holes, if indeed gravitons exist.


Instead, the gravitational-attraction of a black hole before the point of no return (the event horizon) is caused by curvature. A black holes quite literally drags space and time around it, which may give the impression that some how black holes gravitationally-radiate from inside, but this is not the case. It's the spacetime distortion/curvature which causes the attractive field around it.

[peppercorn]

Gravitons do not escape black holes, if indeed gravitons exist.
Instead, the gravitational-attraction of a black hole before the point of no return (the event horizon) is caused by curvature. A black holes quite literally drags space and time around it, which may give the impression that some how black holes gravitationally-radiate from inside, but this is not the case. It's the spacetime distortion/curvature which causes the attractive field around it.

I was under the impression that gravitons were a necessary addition to the sub-atomic particle family in an effort to square QM with gravity.
Before QM, gravity and infinite regions of it where allowed as part of general relativity.  However, I am still not sure at what speed a star collapsing to form a black hole is still predicted to send out a ripple of increased pull under GR.

Gravitons must not be self reacting - ie. In the QM model a graviton would travel from the singularity, over the event horizon unaffected by other gravitons.  Either gravity is viewed in terms of geometry or as particle interactions (two sides of the same coin), but to talk about gravitons (QM) being pulled back by S-T curvature (GR) is mixing metaphors.

[Vern]

Everybody seems to have signed on to the absurd notion that space and time can be distorted. Einstein assumed that but it is not necessary in any theory of relativity including GR. It is much more reasonable to assume that it is the material things that distort when they move. With the later assumption we can understand why it is that the distortion happens.

The most elemental things in matter are held together by stuff that must always move at the invariant speed of light. Given this Quantum theory assumption Quantum theory predicts relativity phenomena unless we thwart it by inventing virtual particles.