So why not just create some kind of sub-space, tachyon, ultrawave relay, thingy that allows you to send FTL (faster than light) or even instantaneous signals? Relativity does more than just place a cap on the speed that signals and objects can move. The real problems crop up when you can send instantaneous messages in two or more reference frames. That allows you to bounce signals back and forth, and thus send a message to yourself in the past. So, the fix is to have only one frame with instant communication (magic post offices only send letters in one direction).