Tachyon

A tachyon (from the Greek ταχύς {takhús}, meaning "swift") is a hypothetical particle that travels at superluminal velocity. Many strange properties have been attributed to tachyons, which also play a role in some popular science fiction. In the language of the theory of special relativity, a tachyon is a particle with space-like four-momentum. If its energy and momentum are real, its rest mass is imaginary. It is difficult, for instance, to interpret exactly what a complex valued mass may physically mean. The proper time experienced by a tachyon is also imaginary. A tachyon is constrained to the space-like portion of the energy-momentum graph. Therefore, it can never slow to light speed or below. Curiously, as its energy decreases, its velocity increases.

Causality

The property of causality, a fundamental principle of theoretical particle physics, poses a problem for the physical existence of tachyons. If a tachyon were to exist and were allowed to interact with ordinary matter, causality could be violated: roughly, there would no longer be a way to tell the difference between the future and the past along the worldline of a given piece of ordinary matter. A particle could send energy or information into its own past, forming a so-called causal loop. This would lead to logical paradoxes such as the grandfather paradox, unless the theory was set up in such a way as to prevent them. At present such a fix is not known: for example, the Novikov self-consistency principle has not been obtained within a quantum field theory, but has to be imposed by hand. At the very least the principle of special relativity would have to be discarded. In the theory of general relativity, it is possible to construct spacetimes in which particles travel faster than the speed of light, relative to a distant observer. One example is the Alcubierre metric. However, these are not tachyons in the above sense, as they do not exceed the speed of light locally.

Field and string theories

In quantum field theory, a tachyon is a quantum of a field - usually a scalar field - whose squared mass is negative. The existence of such a particle implies the instability of the spacetime (vacuum) because the energy of the vacuum has a maximum rather than a minimum (at least with respect to the tachyonic direction). A very small impulse will lead the field to roll down with exponentially increasing amplitudes: it will induce tachyon condensation. The Higgs mechanism is an elementary example, but it is important to realize that once the tachyonic field reaches the minimum of the potential, its quanta are not tachyons anymore but rather Higgs bosons that have a positive mass. Tachyons arise in many versions of string theory. In general, string theory states that what we see as "particles" - electrons, photons, gravitons and so forth - are actually different vibrational states of the same underlying string. The mass of the particle can be deduced from the vibrations which the string exhibits; roughly speaking, the mass depends upon the "note" which the string sounds. Tachyons frequently appear in the spectrum of permissable string states, in the sense that some states have negative mass-squareds, and therefore imaginary masses.

Tachyons in fiction

In the Star Trek fictional universe, tachyons are frequently invoked to explain some aspect of the Romulan cloaking device. Cloaked ships have been detected by watching them pass through a tachyon beam, essentially creating a faster-than-light burgler alarm. (Ships using imperfect cloaking devices are also implied to produce residual tachyon emissions.) In general, tachyons are a standby mechanism upon which many science fiction authors rely to establish faster-than-light communication, with or without reference to causality issues. For example, in the Babylon 5 television series, tachyons are used for real-time communication over long distances. See also Superbradyon, X-waves.

External links

\n*The Faster Than Light (FTL) FAQ

• Superluminal