Nivens Law Of Time Travel
A "law" proposed by science fiction author LarryNiven:
In any universe where [backwards] TimeTravel is possible, it will never have been invented.
Not unlike GoedelsIncompletenessTheorem or the GeneralHaltingProblem. The explanation is simply that TimeTravel permits events to be perturbed non-locally until, inevitably, except for limited scope self-defeating TimeLoops, TimeTravel is never invented. Anyone working on a time machine, therefore, is best ignored.
This doesn't take into account universes where forwards and backwards TimeTravel is only possible to points when the time machine is turned on.