From another thread on the subject (I only considered information transfer, but you can replace this with any mode of transmission you want; additionally, I’ve covered only the limiting case of instantaneous transmission, but basically the same story works for all speeds greater than c):
Special relativity: Say two spaceships meet each other at time t[sub]0[/sub]. They travel at a relative speed of 0.87c. On both spaceships, there’s an instantaneous transmitter, i.e. a device that transmits information at ‘infinite’ speed (this works for any speed greater than c, but the math gets more complicated for finite speeds). Now, say, a minute after the meeting, people on ship A notice that ship B’s back light is broken, so they instantaneously send a message to ship B informing them of their gross violation of intergalactic traffic codes, which as everybody knows are enforced with extreme rigidity. However, since ship B moves at a speed of 0.87c relatively to ship A, it is subject to time dilation, and thus, only thirty seconds have elapsed since the two ships met.
So far, so good. The engineer on ship B, being of the swift sort, fixes the back light, mumbles something about how he cannae change th’ laws o’ physics, Captain, and then sends back a thanks, at about 45 seconds after the ships met. However, again, since ship A is moving at 0.87c relatively to ship B, it is subject to time dilation; thus, if 45 seconds have elapsed on ship B, only 22.5 seconds have elapsed on ship A. Thus, the thank you arrives on ship A 22.5 seconds after the ships have met, and thus, 37.5 seconds before ship A ever sent its message to ship B, which of course, seeing as how the problem’s already fixed, they now don’t have to.
Or, with more detail, see the story here .