A theoretical type of
subatomic particle that is supposed to move
faster than light. For ordinary particles,
the speed of light (a constant denoted as
c) is the
upper limit; for tachyons,
c is the
lower limit. According to
relativity theory, such particles move
backward in time; that is, their path and energy are determined by
causal conditions in
the future.
(FTL limerick: A young lady named Bright)