[El_Phil]

When has it been used correctly? Very rarely is the answer and when it's bad it's awful.

Besides this is about a consistent Trek. Time travel cannot be consistent in any causal universe.

Tachyons can only got so quick and conform with relativity if they have negative mass. This makes interaction with them (like detecting them) somewhat tricky. What you end up doing is trying to detect the result of collisions and other such things. Hence the lack of evidence (bar one debated incidence).