@TheKingOfRhye #77406
Well, they are a real scientific concept at least.
Basically: your relativic mass is restMass/squareroot(1-v/c), where v is your velocity and c is speed of light. Obviously if both are the same, you get division by zero, so that obviously doesn’t make sense. Unless you’re a photon and your rest mass is 0, too.
But if v > c, you "only” get square root of a negative number, which can maybe still make sense, since imaginary numbers are used in other contexts in physics, too. So it has been proposed that such particles could exist.
Of course, this could just as well be a meaningless artifact of mathematics and no tachyons were ever observed. But such details have never stopped woo peddlers.