Well not to be too factual, but saying tachyons are actual particles is overstating the case. More correctly, tachyons are *actual theoretical* particles ;) In other words, nobody knows if they exist or not. Just from the viewpoint of causality, I'd say they probably don't. (And I'm pretty loose about causality :) I always like Feynman's half-advanced/half-retarded waves explanation of electron self force)