Ask a Physicist

Is there an estimate of the speed of propagation of gravitational waves?
Submitted by Jaime from Canada

Einstein's answer is that gravitational waves should propagate at the same speed as light. Light in vacuum, that is; not the slower speed light has going through matter. Or you could say that light propagates at the speed of gravitational waves: Gravity determines the structure of spacetime, which in turn determines how things can move. Including light.

But that's not the whole story. Light slows down in matter because of quantum mechanical interactions with the atoms. We usually think of a piece of glass as a continuous medium, but on a length scale a millionth the thickness of a human hair it's not. There are atoms, separated by about that scale, and the light interacts with the atoms. Changing the type of atoms, or even the arrangement of a given type of atoms, can drastically affect the behavior of light. And how big the effect is depends on the wavelength of the light. At some wavelengths light travels almost as fast as in vacuum, at others it can't get through at all.

Something similar might happen with gravitational waves. Einstein treated spacetime as a continuous arena for things to happen in, but at scales less than a trillionth of a trillionth of the spacing between atoms the effects of quantum mechanics should show up. Einstein never worked out how to combine quantum mechanics with gravity. Even today we don't know how to do that, but we have some idea that spacetime at very small scales might have an "atomic" structure like matter. Then gravitational waves would slow down at some wavelengths, just like light in matter. (And light of the right wavelengths would slow down in vacuum too.)

Because this length scale is very small, it would be very hard to observe this effect. Most predictions say that LIGO and LISA (like LIGO, but in space) won't even come close, since the effect should be very small at the wavelengths they are sensitive to. But they'll be looking anyway, and people are already looking for the effect in light from astronomical objects. The odds are pretty long, but if anyone finds such an effect it will be the first observation of something related to quantum gravity. Einstein spent half a lifetime looking for such a thing, so you'll hear it in the news if somebody does find gravitational waves at anything but light speed.