I apologize if this has been asked before. I searched numerous times and couldn't find any discussion on this specific topic. Every description I've read of the impossibility of FTL signaling uses some variation of this diagram: http://www.theculture.org/rich/sharpblue/archives/000089.html (if the link ends up not working at some point, it's the thought experiment involving two pairs of spacecraft, with each pair sharing a reference frame between its members, but the two pairs having different reference frames, and the result being that communication to the past is allowed) That makes sense to me, but after thinking it over for awhile, it seems to me that the specific paradox described there only rules out FTL signaling which can be used between observers in different reference frames*. That is, it rules out the faster-than-light equivalent of broadcast radio. Is there also a paradox that arises in a hypothetical scenario where the communication mechanism only physically works if the sending and receiving party are in the same reference frame? If I modify the standard thought experiment at the previous link with that limitation in mind, Carol sends her FTL signal to Dave at event Q, but in order to "close the loop" back to Alice, Dave would have to match reference frames with her (or use conventional, non-FTL signaling), and (unless I'm missing something, which is certainly possible) would therefore no longer be sending a signal into the past. Again, sorry if this has come up before and I just couldn't find it. * Obviously this assumes that communication to the past is impossible. For purposes of my question, let's assume that the laws of physics do not allow a mechanism by which causality can be violated, such as communication backwards in time.