system
Given the following post, respond as if you're commenting on Reddit:
human
Since light can't escape from a black hole, can gravitational waves escape? If not, how do black holes have gravity?
gpt
>Since light can't escape from a black hole, can gravitational waves escape?

No, they can't, assuming they originate inside of the event horizon.  Waves originating outside the event horizon and directed outwards will of course escape, the same way light would.

>If not, how do black holes have gravity?

Don't think of gravity (or electromagnetism) as a continuous exchange of gravitons or gravitational waves; it's not.  Gravitational waves are produced when masses accellerate.  If you have two masses which are stationary with respect to eachother, they won't suddenly emit gravitational waves, but they *will* be attracted to eachother.  It is the gravitational *field* that causes them to be attracted; even without any gravitational waves at all, the attraction will still occur.

(Above, replace "gravitational wave" with "electromagnetic wave", "graviton" with "photon," and "mass" with "charge" and the same statements apply for electromagnetism.)

Objects which fall into a black hole are being accelerated and will emit gravitational waves on their way towards the event horizon.  For an outside observer, these waves escape just fine, and the object never crosses the event horizon anyway from their perspective.

You can think of a gravitational wave as a moving *change* in the curvature of spacetime.  Even if the curvature is not changing, there may still be curvature present (and massive bodies will tend to "roll together").  Same with electromagnetism -- photons are propagating *changes* in the electromagnetic field potential; even if you have a static field, the potential difference between two points may not be zero (and opposite charges will attract).

Hope that helps.