WOODY ALLEN once said, “It is impossible to travel faster than light, and certainly not desirable, as one’s hat keeps blowing off.” But he forgot about a strange type of particle – the tachyon.
Named after the Greek word tachus, meaning swift, tachyons seem to break all the rules. If they exist – and no one has yet proved that they do – they would not only travel faster than light, but backwards in time too.
How does the tachyon manage to break these sacrosanct rules? After all, isn’t the speed of light – 300 million metres per second in a vacuum – the ultimate speed limit, imposed by Einstein’s special theory of relativity? In the 1960s a group of physicists decided not. They realised that Einstein’s speed limit did not apply to particles that were already moving faster than light. They called these particles tachyons.
Advertisement
The world of the tachyon is a weird place. Unlike normal particles, a tachyon gains speed when it loses energy, so a tachyon with zero energy would have infinite speed. In fact, it would require infinite energy to slow a tachyon to the speed of light, making it impossible for it to travel below the speed of light. It is as if tachyons live in a mirror world reflected by this invisible barrier.
How can we detect these strange particles? Even though we live on opposite sides of the mirror, it should be possible to see evidence of tachyons directly, if they exist. As they zip through the vacuum of space, they would produce a cone of light called Cerenkov radiation. But to date, this cone has never been seen, so tachyons remain a hypothetical possibility.
“They break all the rules. They travel backwards in time”
Real or not, tachyons make scientists uncomfortable because they violate the rules of cause and effect. From our perspective, these faster-than-light particles appear to be moving backwards in time. A tachyon from the present could interact with a particle in the past, leading to bizarre “back-to-the-future” scenarios.
Tachyons have been known about since 1967, when the physicist Gerald Feinberg coined the term. At first they were dismissed as non-real by-products of the relativity equations, but now tachyons are turning up in theories everywhere. It has been suggested that tachyons are produced in cosmic rays, and even that they could account for that mysterious dark matter. Tachyons also play a role in string theory, which has brought them back into the spotlight.
Murray Gell-Mann, winner of the 1969 Nobel prize in physics for his work on elementary particles, once said that in physics, “everything that is not forbidden is compulsory”. Tachyons might just be here to stay. Hold on to your hats!
