Tachyons Finally, a diversion on a possibly related topic. If tachyons exist as physical objects, causality is no longer invariant. Different observers will see different causal sequences. This is illustrated by the pair of spacetime diagrams below. C occurs first.
C occurs first.
The unusual situation here arises because conventional causality assumes no superluminal motion. This tachyon example is presented to demonstrate that our intuitive notion of causality may be flawed, so one must be careful when appealing to common sense. See the FAQ article on tachyons , for more about these weird hypothetical particles. The possible existence of time machines remains an open question. None of the papers criticizing the two proposals are willing to categorically rule out the possibility.