Tachyons 

A purely speculative particle, which is presumed to travel faster than light. According to Einstein’s equations of special relativity, a particle with an imaginary rest mass and a velocity greater than c would have a real momentum and energy. Ironically, the greater the kinetic energy of a tachyon, the slower it travels, approaching c asymptotically (from above) as its energy approaches infinity. Alternatively, a tachyon losing kinetic energy travels faster and faster, until as the kinetic energy approaches zero, the speed of the tachyon approaches infinity; such a tachyon with zero energy and infinite speed is called transcendent.

Special relativity does not seem to specifically exclude tachyons, so long as they do not cross the lightspeed barrier and do not interact with other particles to cause causality violations. Quantum mechanical analyses of tachyons indicate that even though they travel faster than light they would not be able to carry information faster than light, thus failing to violate causality. But in this case, if tachyons are by their very nature indetectable, it brings into question how real they might be.