  This article proves that elementary particles can violate the second
postulate of the Special Theory of Relativity. According to Heisenberg's
principle, superluminal particles are allowed in the theoretical framework of
Quantum Mechanics. In this paper the tachyons, are identified with the unitary
irreducible representations of the "full" Poincare group (Poincare group plus
space-time reflections.) on the past time-like cone.
