Does gravity move at the speed of light? For instance, if I were able to change some energy into matter, how long would it take for other matter to begin to feel the attraction of the matter I created? I assume someone out there knows the answer.
If you could convert energy to matter and back again in an amazingly concise manner, and do it at a specific frequency, and if you could detect such changes in gravity at a specific frequency, you could conceivably use gravity as a communications mechanism. That's not likely to happen during my lifetime, but it does make for some interesting science fiction.