If gravity is instantaneous, it could theoretically be used for faster-than-light communication, travel, and I think causality violations. General relativity depends heavily on the speed of gravity not exceeding C, so it would have to be essentially rewritten. Relevant: https://physics.stackexchange.com/questions/26742/does-gravity-travel-at-the-speed-of-light As an engineer, I kind of hope gravity is instantaneous. But I doubt it. Reminds me of Time for the Stars, in which telepathy is found to exist, and be instantaneous, and is used for communication at near-light speeds, eventually being the foundation of FTL. Good story.The rate of orbital decay of binary pulsars is, among other factors, dependent on the speed of gravity. The in-spiral rate of one binary pulsar system has been measured and found to agree with the rate predicted by general relativity to within a 0.2% margin of error.