Tachyons

Tachyons are hypothetical particles with the property of moving faster than light. Because of this, they can move backward in time.

Take two observers, Observer A and Observer B, with Observer B moving away from Observer A at a constant velocity, Observer A sends a tachyon to Observer B. From Observer A’s frame of reference (let’s say), the tachyon travels forwards in time, as in diagram 1.

Observer B’s space-time diagram, however, tells a different story. Because Special Relativity requires that time and space be shifted based on the velocity when shifting reference frames, Observer B’s space-time diagram, with respect to Observer A’s can be seen in the picture above.

The tachyon moves backwards in time!