The general direction may be possible, but the source itself would not be. The compression is smallest parallel to the direction of the wave's travel, and is largest perpendicular to the direction of the wave's travel. LIGO measures a change in the difference between the lengths of the two arms, ad so it would be more sensitive to waves moving along one of the arms. It would not be at all sensitive to waves that are moving perpendicular to both arms.

Since there are two such facilities, located on different parts of the Earth, they may be able to compare the relative size measured by each facility, and narrow down a part of the sky. Since the facilities are relatively close to each other (only 2000 miles, 30 degrees along Earth's circumference), the margin of error would be very large.

Ideally, to localize the direction, you would have three facilities, each located at 90 degrees away from the other two, so that you have one facility "pointed" in each direction. Even then, it would only be able to narrow it down to 2 possible origins, as the direction of travel of the wave would not be measurable.

Anything that carries information is limited to the speed of light. Gravitational waves carry information about the location of the merging black holes, and so they are limited to the speed of light. If anything that carried information were to travel faster than the speed of light, it would break causality, because you could find some frame of reference in which the effect happened before the cause.

And yes, it is spacetime itself that is vibrating.

How exactly is "speed of light" defined when the spacetime is moving in the precisely the region in which we are trying to measure the speed of light?

Same way an ant walks a constant speed even if the balloon he's walking on is being blown up.

It's deforming (stretching) not translation (moving).

Would the rotation of the earth give some clues? Eg, the detectors are rotating, so the strength of the signal will change over time, assuming the signal lasts long enough.

Unfortunately, no. The signal only lasts for a few seconds, not long enough for the Earth to rotate any significant amount.

It's possible, there are three detectors hundreds of miles from each other, so they can use triangulation to get a fairly accurate read of the source.

It's two detectors, each of which has two tunnels at right angles to each other (and generate an interference pattern when a gravity wave distorts the tunnels length).

Oops, I meant two.

I'm no expert, but wouldn't you need an extremely long baseline to triangulate on distant stars? I mean, too long to fit on planet earth?

Only if your interested in the distance, parallax measurements do that by measuring when the Earth in on different sides of the sun.

If you're only after direction you can measure the time skew between signal hitting detector one and two.As you know the speed (same as light), and that the wavefront is parallel, you have a pretty good idea where in the sky the wave came from.

For the first case, you would need a signal that lasts for at least half a year. Does that cover two black holes merging?

Well, technically, they black holes circle each other for billion of years. So with instruments sensitive enough, you could do that. Unfortunately, LIGO and the others are only sensitive enough to see the last few parts of a second, when the signal is strongest.

If at the same time* we were to observe a supernova via other telescopes, that would give a pretty good indication. :)

*: And this would also possibly serve to answer another question -- do gravitational waves travel at precisely the speed of light? (also, I'd be interested in hearing about neutrino detectors -- how closely (if nonzero) to the speed of light do neutrinos travel?)

Yes, gravity waves travel at lightspeed, at least if you buy into general relativity's tensor equations.

Neutrinos have rest mass and move slower than light. We know this because neutrinos change their flavor in transit, meaning they experience the passage of time, something that would not be possible if they had no rest mass and traveled at light speed.

https://en.wikipedia.org/wiki/Neutrino_oscillation

Nobody knows how much slower, but they must be moving almost light speed because we see neutrinos from supernova collapse before we see the light emission. (The light emission from supernovae is delayed by several hours, because it takes that long to heat up the gas around the collapsed core before it can radiate out.)

https://en.wikipedia.org/wiki/SN_1987A#Neutrino_emissions

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.

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.

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

You can get the direction of such waves by looking at 2 experiments at once, or experiments with 3-d mirror arrangements instead of 2-d ones.

Yes, the existance of grav waves is only possible if they travel at some top speed, which is presumed to be the speed of light.