3.8 Other Theories
- Chronology protection conjecture: It is a conjecture by Stephen Hawking that the laws of physics are such as to prevent time travel on all but sub-microscopic scales. Mathematically, the permissibility of time travel is represented by the existence of closed timelike curves. Many attempts to generate scenarios for closed timelike curves have been suggested, and the theory of general relativity does allow them in certain circumstances (for example, it would allow for a time machine constructed from a wormhole). But attempts to incorporate quantum effects into general relativity using semiclassical gravity seem to make it plausible that vacuum fluctuations would drive the energy density on the boundary of the time machine to infinity, destroying the time machine at the instant it was created or at least preventing anyone outside it from entering it.

- Retrodiction: It is the act of making a "prediction" about the past. This is especially useful when one wishes to test a theory whose actual predictions are too long-term to be of immediate use. One speculates about uncertain events in the more distant past so that the theory would have predicted a known event in the less distant past. This is useful in, for example, the fields of archaeology and cosmology.