Tachyons just go faster than light in trek (warp speed), not nessesarily backwards in time.
I think DS9 had one ep. where sisko went to cardassia in a sailboat, cause he caught a tachyon crossbreeze, which pushed his poor little ship into warp
Cronotons are apparently little "quanta" (smallest unit) of time (
!), so anti-cronotons would logically make the universe go backwards.
Ahhh, treknobabble. You can make any plot device logically derive from the most wacky assumptions
.
[This message has been edited by suicide_junkie (edited 17 May 2001).]