Tachyon condensation

In particle physics theoretical processes which eliminate or resolve particles or fields into better understood phenomenon are called, by extension and metaphor with the macroscopic process, "condensation". In particular, tachyon condensation is such a process which eliminates the tachyon which many physicists believe does not exist.

Technical Overview

Tachyon condensation is a process in which a tachyonic field—usually a scalar field—with a complex mass acquires a vacuum expectation value and reaches the minimum of the potential energy. While the field is tachyonic (and unstable) near the original point—the local maximum of the potential—it gets a non-negative squared mass (and becomes stable) near the minimum.

The appearance of tachyons is a potentially lethal problem for any theory; examples of tachyonic fields amenable to condensation are all cases of spontaneous symmetry breaking. In condensed matter physics a notable example is Ferromagnetism; in particle physics the best known example is the Higgs mechanism in the standard model that breaks the electroweak symmetry.

Condensation evolution

Although the notion of a tachyonic imaginary mass is troubling, what is really being quantized here is the scalar field; even for tachyonic quantum fields, the field operators at spacelike separated points still commute (or anticommute), thus preserving causality. Therefore information still does not propagate faster than light. Also the "imaginary mass" really means that the system is unstable and that solutions will grow exponentially, but not superluminally (there is no violation of causality). Tachyon condensation drives the physical system to a stable state where no physical tachyons exist. The zero value field is at a local maximum rather than a local minimum of its potential energy, much like a ball at the top of a hill. A very small impulse (which will always happen due to quantum fluctuations) will lead the field to roll down with exponentially increasing amplitudes toward the local minimum. Once the tachyonic field reaches the minimum of the potential, its quanta are not tachyons any more but rather have a positive mass-squared, such as the Higgs boson.

Tachyon condensation in string theory

In the late 1990s, the Indian string theorist Ashoke Sen conjectured that the tachyons carried by open strings attached to D-branes in string theory reflect the instability of the D-branes with respect to their complete annihilation. The total energy carried by these tachyons has been calculated in string field theory; it agrees with the total energy of the D-branes, and all other tests have confirmed Sen's conjecture as well. Tachyons therefore experienced a comeback in the early 2000s.

The character of closed-string tachyon condensation is more subtle, though the first steps towards our understanding of their fate have been made by Adams, Polchinski, and Silverstein, in the case of twisted closed string tachyons, and by Simeon Hellerman and Ian Swanson, in a wider array of cases. The fate of the closed string tachyon in the 26-dimensional bosonic string theory remains unknown, though recent progress has revealed interesting new developments.

See also

Bose-Einstein condensation a condensation process that was experimentally observed 70 years after it was theoretically proposed.

External links

• Tachyon condensation on arxiv.org