system
Given the following post, respond as if you're commenting on Reddit:
human
What would be the implications of finding the hypothetical tachyon particle/ tachyon field?

I've read somewhere that if such a particle/field existed where the particle would travel faster than light, then causality would be broken. If this is the case, why does that matter and what would the implications on physics be if such a phenomenon was discovered? Thanks!
gpt
Forget causality, tachyons in a quantum theory break everything.

If tachyons exist, then considering one tachyon there is at least one frame where its energy is negative (this comes from a bit of relativistic geometry). Now, consider the lowest energy state there is (the ground, or real vacuum) and then create such a negative-energy tachyon: this lowers the energy to below that of the ground, which means there actually isn't a lower bound to energy and no ground.

The energy not being bounded from below is uncomfortable in a classical theory but completely unworkable in quantum mechanics, since it means that the probability of any process is irreparably divergent. Basically any process includes uncontrollable contributions from the production of an arbitrarily large number of virtual tachyons and everything is infinite.

Lower-boundedness of energy is therefore usually taken as an axiom of quantum mechanics and this excludes tachyons as physical particles (and partly saves causality as a bonus).