# Black Holes and Gravitational Waves

1. Apr 4, 2009

### Magister

As far as I know, a black hole is a singularity in the space-time. The space-time becomes so curved that the geodesics can't get out, i.e., the gravitational field becomes so strong that information can not get out.
OK! But what about gravitational waves? As far as I know, gravitational waves are perturbations in the space-time and hence I suppose that they can climb the space-time curvature of the black hole.
I believe that this must be wrong since otherwise the black hole would emit information from inside of the event horizon which I never heard of.

So thinking about this I have come across with some questions. Do the gravitational waves also follow a geodesic in space-time? How can one define a geodesic for the motion of a perturbation of the space-time along this very same space-time? Do the trajectory of a gravitational wave also bend due to the presence of a gravitational field?

Last edited: Apr 4, 2009
2. Apr 4, 2009

### JANm

Hello Magister
That is an interesting question. As far as I know one could see, no i'd rather say look at a Black Hole as some object as one with a radius smaller then its Schwartschild radius, but on larger scale say 10 R_S a Newtonian attractor the same as any other of the same mass. So were the black hole suddenly moved because: "hit by a train or so" this information of gravitation change should be send in to space as if it were a classical object. You are right, but also light which is emitted at a right angle from the surface also comes out, but gravitational frequencyshift of a black hole or a small neutron star nullify respectivily almost nullify this. Tangentialy emitted light at the schwartschild radius rotates around the black hole and from a small neutron star spirals out many times before being freeed from the surface. I don't know to much about the question if gravitational waves are bent through gravitation in the same way as light. Recent probe send to space measuring gravitational waves gives perhaps answer to that?
greetings Janm

3. Apr 7, 2009

### DrGreg

Gravitational waves follow exactly the same geodesics as light does, and therefore cannot escape from within the event horizon. Note, however, that gravitational waves transmit a change in gravity, not gravity itself.

A black hole cannot just spontaneously move, or change its mass, without some external source causing that change; the external source would cause gravity waves.