From my amateur readings in relativity, one of the arguments against tachyons is that causality would be violated locally. But how? Let's say we have observer A and B with synchronized clocks that are separated by a reasonable distance d known to them. A sends B a photon at A's clock t0, and B sees it at his clock t0 + cd. A sends a tachyon to B at A's clock t1, and B sees it at his clock t1 + x, where x < cd. But regardless of the speed of the tachyon, x is always >= 0. How can A or B violate causality with tachyons? I.e., how can x ever be less than 0?