system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we think gravitational waves travel at the speed of light?

I went to a talk by Kip Thorne the other day and I've been thinking about gravitational waves a lot since then. I understand we assume they do travel at the speed of light for triangulation in LIGO. But what if they don't? Could we measure it somehow? If so, when? Will LISA help? I would guess we'd probably need some very characteristic event to be measured at two different places (LISA and LIGO in the future maybe?). Why can't we already measure it this way with the instruments we have?
gpt
The answer is we aren't 100% sure, but it seems like they should. The closest thing to evidence we have (afaik) is observing binary pulsars orbiting each other whose orbits are decaying due to gravitational radiation. The rate of this energy loss is measurable and is dependent on the speed of gravity. Afaik, looking at these kinds of systems has provided some strong evidence that gravity travels with a few percent of c (so most likely c).

edit: also to answer your question about why can't already measure it. Gravity is incredibly weak. Consider that all of the combined mass of the earth is trying to pull you down through the floor, but the only reason you don't fall through the floor is because the electrons in your feet are pushing against the electrons in the floor. We can easily create electromagnetic fields and measure their strength, but to try and measure the gravitational waves created by something terrestrial (Like a big weight or something moving up and down) would take levels of precision we are no where even close to achieving. Basically our only chance of observing a gravitational wave would come from massive astronomical events.