Tachyons and Time Travel

Special Relativity states that nothing that is moving initially slower than the speed of light can be accelerated to exceed the speed of light. This is not strictly speaking the same as the statement that nothing can travel faster than light. It is in principle possible that particles exist which move faster than light from the moment they are created. If such particles did exist, however, Special Relativity implies that they could never be slowed down to velocities below the speed of light. The existence of such particles, called tachyons, has not been totally ruled out, but several experiments have tried, without luck to detect them. If they did exist, and they interacted with ordinary matter, it would give us the means to communicate with the past. Tachyons could literally be sent outwards, bounce off a tachyonic mirror, and return before they were sent. This in turn would give rise to a great many logical problems. For example, if you sent a message back in time that caused your grandfather to be killed before your father was conceived, then you would not be around to send the message that prevented your birth, so that you would be around to send the message, so that you wouldn't be around....etc.