Changes

::<math>t = 2(1 - \frac{2c}{1} \frac{.866}{c}) = 2 - 4(.866) = -1.464</math>

::<math>t = 2(1 - \frac{2c}{1} \frac{.866}{c}) = 2 - 4(.866) = -1.464</math>

so <i>negative</i> time has passed; the tachyon seems to have arrived before it left, and so has moved back in time from one of the available reference frames. This creates obvious potential for paradox.  

so <i>negative</i> time has passed; the tachyon seems to have arrived before it left, and so has moved back in time according to one of the available reference frames. This creates obvious potential for paradox.  

This possibility of effect precluding cause indicates that, if tachyons truly existed, then there may be no free will, and actions would always be pre-determined. Otherwise, one observer may have an "early warning" of something that will happen in another observer's frame and somehow change it; but then special relativity has it that all reference must agree on whether a spacetime event occurring at all or not: no in-between.  

This possibility of effect precluding cause indicates that, if tachyons truly existed, then there may be no free will, and actions would always be pre-determined. Otherwise, one observer may have an "early warning" of something that will happen in another observer's frame and somehow change it; but then special relativity has it that all reference must agree on whether a spacetime event occurring at all or not: no in-between.