Most movies that have time travel as a key element work from the foundation that time is plastic-think Marty McFly or Bill and Ted-that anything in any time line can be changed at will without there being any serious side effects. Although Prof Brown was often worried about there being an end of the universe event, that never seemed to happen. My favorite of the plastic universe movies is Groundhog Day, in which a heartless and self centered Bill Murray turns into a benevolent hero to virtually everyone in Punxsutawney.The other kind of time travel is the concrete determinism type…