Tachyon

A tachyon is a hypothetical particle or entity that travels at superluminal velocities. Special relativity, as shown by Einstein, forbids tachyons from existing and indeed there is absolutely no evidence for their existence.

Many strange properties have been attributed to tachyons, mainly by science fiction authors.

A tachyon could have a negative mass. This is as absurd as it is strange when one considers the implications of a negative mass. Many physical laws would return zero, infinity or 'head in deep fryer'. Gravity, for instance, of such a mass would be positive. The gravity would actually repel objects rather than attracting them.

A tachyon could be a magnetic monopole. Many theoretical searches for magnetic monopoles have concluded they may not exist thanks to them breaking multiple physical laws (conservation of energy for one). It seems fashionable to attribute to tachyons anything proven to be impossible.

A tachyon would exist for a split second as it travels backwards in time. This is a great one! It was never proven that a superluminal object would travel backwards in time, only that time slows as you approach c. As you pass c, the laws of physics tend to curl up and die all around you.

Time may not even exist for a superluminal mass, making it's speed also non-existant (speed cannot exist without time) and so bringing up yet another paradox about superluminal motion. How can it be superluminal if it has no speed? Indeed, any mass travelling at light speed would occupy all points in the universe simultaneously and totally negate the reason for having velocity at all. Needless, a mass cannot travel at light speed and certainly not faster, not in this space-time continuum.

Like most other science fiction inventions, tachyons are very strange beasts, mostly used as plot devices.