Tachyons are theoretical particles that move faster than the speed of light. If they exist are they permanently out of our ability to reach them, or is there a conceivable way to find them?
If charged particles travel through a dielectric medium faster than the phase velocity of light, they emit Cherenkov radiation. So a tachyon might do this when traveling through a vacuum, and be detected that way.
This is a very interesting question. I hope Chronos and others will jump in with a reply!
In fact, for the reason OldGuy mentioned, if tachyons existed, we should have long observed them, at least if they were charged: the Cherenkov radiation emitted would lessen the energy of the tachyon, which, being a tachyon, consequently would accelerate; but this would lead to even more Cherenkov radiation, causing more acceleration, and so on, leading to a runaway effect associated with massive (universe-destroying) amounts of radiation. This isn’t limited to electrically charged particles, since there are analogues to Cherenkov radiation for the other forces (even gravitational, I believe); thus, if the tachyon interacted at all with normal matter, we should long know of their existence (most likely by having been fried to a crisp).
There’s also the issue that if it were possible to interact with tachyons, it would be possible to use them to communicate information, which would lead to violations of causality.
By the way, it’s important to distinguish between two distinct senses in which the name ‘tachyon’ is used: one is the notion of a particle that always travels faster than light as discussed above, while the other is that of a ‘tachyonic field’, a field with an imaginary mass (negative mass squared). The latter is known to exist since last year: the Higgs field, which in its unbroken phase does have such a mass term. The effect of such a mass term is essentially that it’s energetically beneficial to the system to spontaneously produce particles, which leads to a lowering of the total energy; basically, the vacuum for such a field is unstable, and decays on the slightest perturbation (picture a pencil standing on its tip). This provides a mechanism for the spontaneous breaking of a symmetry (analogously to how the pen standing on its tip is rotationally invariant—looks the same after a rotation through its central axis—but is no longer after it has fallen over).
The origin of the confusing terminology seems to be that originally, it was thought that tachyonic fields lead to tachyonic particles, i.e. that a field with a negative mass squared would have excitations propagating faster than light; but instead, what happens is the vacuum decay described above, now commonly called ‘tachyon condensation’.