For decades, a vocal minority of credible particle physicists have hypothesized the existence of tachyons. These theoretical faster-than-light particles—defined by their speed (any particle that travels faster than light is, by definition, a tachyon)—were almost discovered in 2011 when scientists thought they found tau neutrinos capable of traveling faster-than-light, but later studies confirmed that this merely reflected a limitation of the measuring equipment. Some observers claimed that the early arrival of neutrinos from supernova 1987A demonstrated that neutrinos travel faster than light, but this claim, too, can be easily dismissed.
But all of this does raise an interesting question: if we were to encounter a particle that travels faster than light, how on Earth would we know it?
George Mason University particle physicist Robert Ehrlich may have an answer:
Ehrlich's new claim of faster-than-light neutrinos is based on a much more sensitive method than measuring their speed, namely by finding their mass. The result relies on tachyons having an imaginary mass, or a negative mass squared. Imaginary mass particles have the weird property that they speed up as they lose energy – the value of their imaginary mass being defined by the rate at which this occurs.
While Ehrlich’s paper has survived peer review, which suggests that his math holds up, his interpretation of that math is highly controversial and is unlikely to be accepted by a majority of particle physics unless it is corroborated by new experimental data. (For more on why, check out this awesome discussion thread.) But what it does provide is an indicator of both how difficult it might be to measure the speed of a faster-than-light particle, and a roundabout way we might go about doing it, if we’re feeling bold.