Tachyons are hypothetical sub-atomic particles that always move faster than light. These were hypothesised by Arnold Sommerfeld and advanced by other scientists. Tachyons have always held the fancy of scientists and science fiction authors. However, till date the concept has been considered only as theory and scientists have held that tachyon particles are too unstable to be treated as existent. Scientists have also held that faster-than-light information transmission and causality vibration with tachyons are impossible.
According to Gerald Feinberg, one of the proponents of tachyon theories, even if these are conventional localised particles they would still preserve the basic tenets of causality in special relativity and not allow transmission of information faster than light. So we are yet to see whether the neutrinos observed by OPERA are to be considered as tachyons, and whether they will continue to stick to the theories of special relativity or unveil new truths hitherto unknown!