In the language of Einstein's theory of special relativity, a tachyon is a particle with space-like four-momentum?. If its energy and momentum are real, its rest mass is imaginary. It is doubtful if an imaginary mass is physically meaningful.
The [proper time]? experienced by a tachyon is imaginary.
If tachyons exist and are allowed to interact with ordinary, causality? could in principle be violated.
In the theory of general relativity, it is possible to construct a spacetime in which particles propagate faster than the speed of light, relative to a distant observer. One example is the [Alcubierre metric]?. However, such particles would not be considered tachyons in the above sense, as they travel do not exceed light speed locally.
![[Home]](wikipedia.png)