While I was thinking about the previous entry, and about the fact that gravity's apparent non-polar property may be why they haven't detected gravity waves yet, I found myself explaining to Mom why I was going "Eeew! Eeew!" while heading to the bathroom for a shower: I had realized that non-polar gravity might make gravity waves undetectable.
She said, "It's like they're trying to weigh something with a spectroscope." I wasn't sure about that, but when I had time to think about it, I realized she's right, sort of.
They're using lasers--photons--in an interferometry arrangement. The expectation is that gravity waves change the shape of space, and since light follows the shape of space, a light beam should be able to detect a change in the shape of space.
But thinking about I realized something: a gravity wave isn't the same thing as a gravity field, much the same way a magnetic wave isn't the same as a magnetic field.
A magnetic wave is a photon. A magnetic field is not. We don't know what, exactly, a magnetic field is, but we know it's not photons. An electric field isn't photons but an electic wave is. In fact you can't have a magnetic wave without an electric wave operating at 90° to it. A photon is an electromagnetic wave: a moving electromagnetic field.
Moving electrons generate an EM field, but the field itself doesn't move; the electrons do. The field may change in intensity and direction depending on the motion of the electrons but the field itself doesn't move in space.
On the other hand, photons can't exist without moving. They have zero rest mass.
Robert Forward made the point that a photon, existing--as it does--at the speed of light, exists in a universe of no time: from the photon's perspective, it exists everywhere along its path from source to receiver, simultaneously.
We have seen that a photon that passes through a static gravity field--such as that generated by the mass of the sun--takes a curved path through space-time. But this is due to the shape of space; the photon doesn't realize that space is curved and believes itself to be moving in a straight line. (Photons don't have steering wheels.)
But the gravity wave is a different phenomenon. It's not the same thing as a static field; it's a dynamic one--and that may make all the difference. Yes, it's a change in the curvature of space-time, but it's transient, and the photon may not notice the change!
I think these guys should try using an electron beam instead of a photon beam. That might yield different results, and you can use an electron beam in an interferometer arrangement.