There's nothing more constant than every scientist's conviction regarding the speed of light. It's a universal speed limit. Nothing can go faster. Even gravity itself travels at the speed of light (or super close, anyway). This means if the sun goes dark, we won't know until enough passes for the last photon to reach us. If the sun winks out of existence, the loss of gravity won't be experienced until the same amount of time passes. How's that for nuts? We're traveling in a big circle around a center of gravity; even if that center of gravity disappears, we'll continue traveling in a circle as though it was still there. For a few minutes, at least. On larger scales, it could continue for much longer periods of time. Reality changed way over there, but reality hasn't changed right here. Yet. So, the speed of light, aka speed of gravity, is also the speed of reality. However, I'd like to point out that saying gravity "travels" bothers me...