A tachyon () or tachyonic particle is a hypothetical particle that always travels faster than light. Most physicists believe that faster-than-light particles cannot exist because they are not consistent with the known laws of physics. If such particles did exist, and could send signals faster than light, then according to the theory of relativity they would violate causality, leading to logical paradoxes of the "kill your own grandfather" type. Tachyons would also exhibit the unusual property of increasing in speed as their energy decreases, and would require infinite energy to slow down to the speed of light. No experimental evidence for the existence of such particles has been found.