Ok so I've had a busy couple of days, but while thinking about what we've been talking about, I do have another related question...

Gravity depends on mass, and, crucially, distance between masses. The problem here is that on the one hand, distance is relative, depending on your frame of reference. But on the other hand, the gravitational attraction between two objects is absolute, which heavily implies the distance between them is absolute.

How is this paradox resolved?

I've got some time tomorrow, so I'm going to get round to my homework of learning about phase and group velocity.