An object that constantly exists in space has left and right parts, so if it constantly exists in time, does it also have past and future parts? Minkowski’s interpretation of Einstein’s relativity suggests that it does, because objects exist in a four-dimensional space-time matrix. If an object exists at an (x, y, z, t) point, where t is time, it travels along a four-dimensional world line in space and time, so past, present, and future could all exist now.
This interpretation allows a block theory of time where past, present and future events exist in a timeless time (Barbour, 1999) p31, where a time capsule can be browsed like the pages of a book. If spacetime is the landscape within which physical objects endure, then time travel is indeed possible. The equations of relativity allow this, but equations aren’t theories, as assuming that all the mass of an object exists at its center of gravity lets us calculate its trajectory, but it isn’t actually so. When physicists say that time travel is based on general relativity, they mean it is based on Minkowski’s mathematical interpretation of it, which is a model not a theory.
Actually, no physical evidence at all supports time travel, and assuming that it occurs creates paradoxes. For example, Minkowski’s interpretation allows closed time-like curves, where an object world line returns to its start point, just as an object in space can return to where it began, giving the paradox that it collides with itself. There are two basic paradoxes implied by a block theory of time, as follows:
- The grandfather paradox: A man travels back in time to kill his grandfather but then couldn’t be born, and 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 and then return, but next morning decide not to, so I didn’t go forward in time. Forward time travel lets me see a future that isn’t chosen, and so denies future choices. If life is a movie already made, the future is predetermined, 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 backwards 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 choices, it can’t allow randomness, but it does. Newton’s idea of space as the canvas upon which reality is painted was rejected, and making that canvas spacetime doesn’t alter that. After all, if we ever learned to travel in time, surely our first job would be to go back to fix past errors! Like the multiverse fantasy, time travel is great science fiction but poor science.