What is waving when gravity waves propogate?

Question

My guess is that the answer is "the fabric of space-time", but how can there be "fabric" of anything if there is empty space.

Answer 1

The gravitational field is correct. I wouldn't get too wrapped up in trying to understand gravity as "curvature in the fabric of space-time". It's more straightforward to think of it the same way we think of light--as bunches of particles (ie disturbances in a particle field) propagating through space. Instead of spin-1 photons, we have spin-2 gravitons. Like photons, gravitons are massless, so they must propagate at light speed. Each graviton, though, has much, much less energy than your typical photon. So much less, we've never seen one and may never do so. But we may soon be able to see the wave effects caused by huge supernovae and stuff like that.

Answer 2

The gravitational field.

Think of it this way. Gravity of the Sun pulls Earth toward the Sun. Now if some giant hand grabbed the Sun and pulled it a few million miles to one side, then the pull of gravity would change direction to follow it. But since gravity is believed to travel at the speed of light, Earth doesn't feel the change in direction until 8 minutes later. Venus feels the change sooner and Mars feels it later. There is a wave of change in the direction of gravity moving out through space at the speed of light.

Notice I said gravity is BELIEVED to travel at light speed. This has not been proved, and if it turns out not to be true, and gravity is infinitely fast, so that changes are felt immediately even many light years away, then gravity waves would be impossible. Since gravity waves have not yet been detected, it has yet to be proved that gravity does not move infinitely fast.