A tachyon is any hypothetical particle that could travel faster than the speed of light. Curiously, the slower a tachyon moves, the more energy it has. Therefore, the more energy a tachyon has, the slower it would go, but never to be equal to or slower than the speed of light.
Scientists do not know if tachyons exist. Einstein's theory of special relativity says nothing can travel faster than the speed of light. If a tachyon did exist, it would have an imaginary number as its mass. Scientists do not know how something with imaginary mass would act, because a particle going this fast may not be affected by the normal "cause and effect" force that affects normal particles.
Many scientists believe that if one tachyon existed in the universe at any time, then the universe would be overrun by more and more tachyons. This is probably due to the fact that as they slow down, they increase energy. However, other scientists still believe that they could exist if they did not interact with normal matter.
Possible Sightings It was once thought that neutrinos in experiments at the ATLAS detector in CERN might have moved faster than the speed of light, which was a problem because neutrinos do have a positive, real number mass. This lead to further investigations into more unusual theories of, for example, how particles with normal mass may move faster than light speed, and some claimed that this result would disprove general relativity. However, it was later revealed that the abnormal results were almost certainly the consequence of instrument failure, leading a wide dismissal of tachyonic behaviour in this instance, although many have remained confident that the theories used to explain the faster-than-light case would still be found accurate. Claims that these observations would disprove general relativity, which was reported probably unhelpfully widely in the press, now also seem unlikely. However, some physicists still believe that general relativity will not be used anymore some day.