[Space] Ditching Relativity, with minimal collateral damage

[Anaraxes]

But this isn't really time travel, because if you tried to send a message back to Sol telling yourself not to set out, it would always arrive after you left (due to the fact that the backward-propagating effects always go slower than their rated speed in the preferred frame).

I don't quite understand. Does this impose a speed limit on the FTL? Suppose you have a path in the preferred frame that gives you negative displacement in time. I can see that going back by the same route moves you back forward. But what about a return path not in the privileged frame? That is, you FTL to Alpha Centauri, pick up -5 years, then radio a message back through normal space. That takes four years, but would arrive before you left. If the latter is forbidden, then it must be that you can't get -5 years on that particular trip.

[Anthony]

The requirement for a privileged frame is that all FTL trips take non-negative time when measured from the privileged frame.

[Anaraxes]

Yes, but Tev's earlier post seems to say that you can get negative time displacement in some reference frames. So what happens when your reverse trip occurs in that reference frame, rather than the PRF?