JDude13 said:
The meaning of the phrase "tachyon's time" is sort of ambiguous since a tachyon can't have its own inertial rest frame...it is true that if you use the same method that's used to calculate proper time along a slower-than-light particle's worldline, for a tachyon you'll get an imaginary value of time (but you can also think of this in terms of a real spatial distance rather than a time--see time-like vs. space-like intervals)JDude13 said:
People do say this but again they're speaking in a sort of ambiguous way...what is true is that if a tachyon is moving faster than light but forward in time in the inertial rest frame of one slower-than-light observer, then you can find another slower-than-light observer who measures the same tachyon moving backwards in time (here 'moving backwards in time' is shorthand for the idea that in this observer's frame, the event of a tachyon signal being received happens at an earlier time than the event of the signal being sent, due to the relativity of simultaneity). One consequence of this is that if it were possible to send FTL signals with tachyons, and if relativity's postulate that the laws of physics work the same in all inertial frames was still correct, then it would be possible to for two inertial observers to bounce a tachyon signal back and forth in such a way that observer #1 sends a tachyon message to observer #2, observer #2 sends a tachyon signal in reply, and observer #1 actually receives the reply before he sent the original message, so if the reply was just a copy of his original message then he can send a message into his own past using this method (see this section of the tachyon wiki article).JDude said:
