An object that constantly exists in space has left and right parts so if it constantly exists in time, it could also have past and future parts. Minkowski interpreted Einstein’s relativity to be that objects exist in a spacetime matrix at (x, y, z, t) points where t is time, and so travel along four-dimensional world lines in space and time. This allows a block theory of time, where the present is just a slice of a larger block of past and future events, giving a time capsule that could be browsed like the pages of a book (Barbour, 1999), p31. If matter exists on the landscape of time, then time travel is indeed possible because the past and the future exist right now.
The equations of relativity allow time travel, but that doesn’t make it so, because equations aren’t theories. For example, the equations of movement assume that all of an object’s mass exists at its center of gravity, but it isn’t actually so. It follows that when physicists say time travel is based on general relativity, they mean it is based on Minkowski’s interpretation of it, which is a mathematical model not a theory.
Actually, no physical evidence at all supports time travel, and assuming it is so creates unsolvable paradoxes. For example, Minkowski’s interpretation allows closed time-like curves, where an object’s world line returns it to its start point just as an object can return to where it was in space, but this means it can collide with itself! A block theory of time allows the following paradoxes:
- The grandfather paradox: A man travels back in time to kill his grandfather, but then couldn’t be born, so he couldn’t kill him. Backward time travel lets an entity prevent its own cause, so causality breaks down. It follows that there can be going back in time or causality, but not both.
- The toast paradox: I go forward in time to see myself having toast for breakfast, then return, but next morning I decide not to, so I didn’t go forward in time. Forward time travel assumes a fixed future so it denies future choices. If life is a movie already made, the future is predefined, so random events can’t occur, but they do. It follows that there can be going forward in time or choice, but not both.
Going back in time denies causality, and going forward in time denies choice, but physics requires both. Without causality, it must allow magic, but it doesn’t, and without choice, it must deny randomness, but it doesn’t. Physics rejected Newton’s idea that reality is painted upon the canvas of space, and making that canvas spacetime doesn’t improve this. After all, if we ever learn to travel in time, wouldn’t we immediately go back in time to fix past errors? Like the multiverse fantasy, time travel is great science fiction but poor science. It follows that if physics accepts causality and choice, it must deny time travel.