The physics of tachyons, a particle faster than light

Imagine sending a friend a text and getting the reply before you have even sent it. It sounds like a glitch on your mobile or a joke, yet this is exactly the picture physicists reach for when they explain what a particle faster than light would mean. It is called the tachyon, and its story in physics is full of bold ideas and ends with one famous mistake.

A tachyon, or tachyonic particle, is a hypothetical particle that always moves faster than light. Hypothetical means that theory allows for it, but nobody has yet measured one reliably. Most physicists think that particles like this cannot exist, since they would not fit the laws we know about motion and time.

The antitelephone and a message from the future

If tachyons existed, physicists say you could build a tachyonic antitelephone out of them. It is a hypothetical device that would let you communicate with the past, that is, with events that have already happened. The term was introduced in 1970 by the physicists Gregory Benford, David Book and William Newcomb, although the idea rests on a paradox that Richard Tolman had described as early as 1917.

The usual example goes like this. One person is on Earth, the other in a spaceship moving away from Earth. The astronaut sends a tachyon message to the person on Earth. That person replies straight away, and the reply reaches the ship before the astronaut sent the original message. It is as if a letter with the answer landed on your doormat before you had written the question.

Who came up with the tachyon

The first thoughts about particles faster than light are linked to the German physicist Arnold Sommerfeld, who worked on them at the beginning of the 20th century. In 1962 Olexa Bilaniuk, Vijay Deshpande and George Sudarshan published a paper showing that the theory of relativity does not completely rule such particles out, as long as they never travel more slowly than light. In 1967 Gerald Feinberg gave them a name in his paper "Possibility of Faster-Than-Light Particles". The word tachyon comes from the Greek tachys, meaning fast or swift.

Strange properties

In theory, tachyons have properties that run against everything we are used to. When they lose energy, they speed up. When they gain energy, they slow down. With ordinary things it is the other way round: a car needs more petrol to go faster. The lowest speed a tachyon can have is the speed of light, so it never drops below that line.

Tachyons also seem to break causality, the relationship between two events where one is the cause and the other the effect. The antitelephone from the previous section is exactly such a case. If tachyons existed, the special theory of relativity in its present form would appear to be wrong. We can give tachyons the same properties as ordinary matter, for example spin, an inner property of a particle that resembles turning on its own axis, and also an antiparticle, which physicists call the antitachyon.

Searching in cosmic ray showers

One idea was that tachyons might be created when particles collide at very high energies. So physicists looked for them in cosmic radiation. Cosmic rays hit the Earth's atmosphere with enormous energy, some of them at almost the speed of light, up to 99.99 per cent of it. They crash into air molecules, and the particles produced in the collision go on to hit more air. The result is what we know as a cosmic ray shower. It works a little like snooker, when one fast ball breaks the whole triangle and the balls scatter in every direction.

Some physicists worked on the concept of tachyons from the early 1960s until the mid-1970s, and only a handful tried to actually catch them in cosmic ray showers. Their starting point was that high-energy particles from cosmic radiation strike air molecules in the upper atmosphere. This produces secondary showers made up of billions of particles, and some of them can be picked up by instruments on the ground. It is quite possible that tachyons could be produced in collisions more than 20 kilometres above the ground.

In 1973 a pair of Australian scientists, Roger Clay and Philip Crouch, announced that they had managed to detect a tachyon. They described signals that arrived about 60 microseconds before the main secondary shower, which was made up of particles slower than light. Nobody else, however, has repeated their result.

The neutrinos of 2011

The best-known chapter came in 2011. The OPERA experiment, which measured neutrinos fired from CERN near Geneva to the Gran Sasso laboratory in Italy, found that they had arrived slightly earlier than light would have. The news went round the world. The scientists behind the project themselves asked colleagues to check the measurement, and within a few months it turned out that the result came from a loose fibre-optic cable connector and an inaccuracy in the timing. In the end, the neutrinos were not faster than light.

So for now the tachyon lives on in equations and in the questions physicists ask about time and about what counts as cause and what as effect. Future discoveries may shed more light on the subject. How the word tachyon travelled from physics journals into the books of David Wagner is something we describe in the articles Have you heard of tachyons? and Tachyon energy. The products that grew out of Wagner's work are in the product overview.

Sources

• Eric W. Weisstein: Tachyon, Eric Weisstein's World of Physics, scienceworld.wolfram.com.
• G. Feinberg: Possibility of Faster-Than-Light Particles, Physical Review 159, 1967.
• O. M. P. Bilaniuk, V. K. Deshpande, E. C. G. Sudarshan: "Meta" Relativity, American Journal of Physics 30, 1962.
• G. A. Benford, D. L. Book, W. A. Newcomb: The Tachyonic Antitelephone, Physical Review D 2, 1970.
• R. W. Clay, P. C. Crouch: Possible observation of tachyons associated with extensive air showers, Nature 248, 1974.