Faster-than-light particles

Tachyons
Tracking the elusive faster-than-light particle

In 1962, a group of physicists made the provocative observation that Einstein’s Special Theory of Relativity does not actually prohibit matter from traveling faster than light, only from being accelerated to faster-than-light speeds. This may seem like an irrelevant distinction—and perhaps it is. But suppose there were a particle that came into existence already traveling faster than light. Because it did not have to be accelerated in order to reach that speed, it does not violate Special Relativity. Physicist Gerald Feinberg gave this hypothetical particle the name tachyon in 1967, from a Greek word meaning “speedy.” Later, the term tardyon was coined in order to identify ordinary, slower-than-light particles; these are also sometimes known as bradyons.

The tachyon, if it existed, would have a number of fascinating properties. Unlike ordinary particles, it would have to decrease in mass as it went faster, meaning that the speed of light—at which its mass would be infinite—would be just below its slowest possible speed. Likewise, adding energy to the tachyon would slow it down, rather than speed it up; to slow it all the way down to the speed of light would require infinite energy. For a long time, physicists believed that a tachyon’s mass would have to be an imaginary number—a number with a factor that’s the square root of –1—though more recent formulations of tachyon theory suggest that such a particle could have a real mass. Most intriguingly, a tachyon, if it is to adhere to the principle of relativity, would actually be able to travel backward in time—seemingly making all sorts of trouble for the notion of causality.


—Joe Kissell
Tachyons
Tracking the elusive faster-than-light particle