Tachyon condensation is a process in particle physics in which the system can lower its energy by spontaneously producing particles. The end result is a "condensate" of particles that fills the volume of the system. 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.
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