Credit: SPL

It makes regular appearances in science fiction — from the novels of Isaac Asimov, to TV's Dr Who and Babylon 5 — but the tachyon is less popular in science fact.

This hypothetical particle travels faster than light and the square of its rest mass is negative. So far, so unphysical. However, the proper treatment of tachyonic fields within quantum field theory does not allow any violation of causality, and writing in Physical Review D, John Ellis and co-authors make a plea to theorists to resist “unreasoning tachyophobia” (Phys. Rev. D 78, 075006; 2008). It could be that tachyons feature in an extended standard model.

The standard model is not the whole story for particle physics, and it is hoped that data from the Large Hadron Collider will direct us to the physics needed to extend, even if not complete, the picture. Supersymmetry — a broken symmetry that would endow each fundamental particle with a heavier supersymmetric partner — is a favoured addition to the model, typically in the form of the 'minimal supersymmetric standard model', or MSSM. Working with certain constraints to tie the model in with grand unified theory, and in the case that the lightest supersymmetric particle is the gravitino (partner of the graviton), the parameter space of the MSSM extends to negative mass-squared values.

This, say Ellis et al., should not be discounted. The point is that any tachyonic spin-zero fields that might exist do so at very high energy scales; in evolving down to the much lower energy scales of our present Universe — a trick that theorists perform using the mathematics of the 'renormalization group' — the sign of the tachyonic mass-squared can change, if the actual size of the mass-squared was sufficiently small up at the unification scale.

Models like this do, however, raise other issues: for example, whether our Universe exists in a true minimum of potential or only a local minimum, and whether there is a danger of introducing charge and colour-charge non-conservation. But these, the authors stress, are points that can be reasonably considered (or calculated) without wantonly discarding the tachyon.