What is a tachyon?

Because a tachyon always moves faster than light, we cannot see it approaching. After a tachyon has passed nearby, we would be able to see two images of it, appearing and departing in opposite directions.

Tachyon or tachyonic particle is a hypothetical particle that always moves faster than light. The word comes from the Greek: tachys, meaning "swift, quick, fast, rapid", and was coined by Gerald Feinberg in a 1967 paper. Feinberg proposed that tachyonic particles could be quanta of a quantum field with negative squared mass. However, it was soon realized that excitations of such imaginary mass fields do not in fact propagate faster than light, but instead represent an instability known as tachyon condensation.