[joolze]

you can determine direction with multiple experiments, or with 3d mirror arrangements, see LISA

and yes, they can only exist if they travel at the speed of light. it's a major test of the limiting speed of light.

[MereInterest]

Multiple experiments wouldn't help very much, because it will be a new source each time. When two black holes merge, it is only during the final few seconds that very large gravitational waves are emitted.

[evanb]

I believe gp meant multiple laser interferometers, not multiple gravitational-wave-emitting events.