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Q & A: black hole gravity

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Most recent answer: 08/29/2013
Q:
If gravitons travel at the speed of light, how does the gravity of a black hole escape its event horizon?
- Peter Dubuque
Malden, MA
A:
Think of some stuff falling together to make a black hole. Even when the stuff is sitting around as some sort of star, it has a gravitational field. As it starts to collapse, it doesn't have to send out a new field- the old one is already out there. 
i know this is only a partial answer, but maybe it's a useful start.

Mike W.

Here’s another partial answer:  The gravitons are not "on shell" -- that is, they do not have the relationship between energy and momentum given by special relativity because they are virtual particles, exchanged as intermediate quantum states.  For the same reason, the photons exchanged in the electrostatic force between two stationary charges are virtual particles.

A charged black hole will still have its electric field around it (otherwise you violate Maxwell’s equations and the conservation of electric charge) after it collapses.  The electric and magnetic interactions are mediated by virtual photons.  I guess that the same sort of argument that lets black holes have electric and magnetic fields also allows gravitons to propagate.

One feature of this -- the space and time inside the event horizon is causally disconnected from the rest of the universe.  That is, any event that happens inside the event horizon is incapable of affecting events outside.  So there can be no transmission of information outside of the event horizon.  The static electric and gravitational fields however do not carry such information, and bursts, burps and upsets happen only as stuff falls in and radiates real photons and gravitons as it does so.

Correct me if I’m wrong!  (but I know the answer is incomplete).

Tom

(published on 10/22/2007)

Follow-Up #1: light and black hole gravity

Q:
How a black-hole can attract the zero massed light when the gravity is proportional to product of masses? is this because the black hole possesses enormous mass/density and the mass of light is actually not zero but assumed negligible for mathematical convenience?
- Shakkir (age 22)
Trivandrum
A:

You don't need black holes here. The bending of light near the Sun was observed in 1919. If you follow the earlier part of this thread, you'll see that the key is that the gravitational interaction involves the non-zero inertial mass, not the rest mass of zero. See also:

Mike W.


(published on 08/29/2013)

Follow-up on this answer.