Physical realism assumes real objects that constantly exist in time and space, so if an object has left and right parts in the dimension of space, perhaps it has past and future “parts” in time. Minkowski interpreted Einstein’s theory of relativity using a four-dimensional space-time matrix, so instead of existing at an (x, y, z) point in space, objects now exist at an (x, y, z, t) point in space-time, where t refers to time. An object now exists on a world line that extends in space and time, so one can talk of its “location” in time as one does for space and this enhanced idea of how objects self-exist has been generally accepted.
The Minkowski interpretation allows a block theory of time, where all past, present and future states exist in a “timeless time” (Barbour, 1999) p31 and a “time capsule” can be browsed like the pages of a book. That spacetime is the landscape that physical objects endure within implies that time travel is indeed possible. The equations of relativity work but equations are not reliable indicators of reality, as assuming all the mass of a body exists at its center of gravity works to calculate trajectories but no-one believes it is actually so. When physicists say that time travel is “based on General Relativity” they actually mean it is based on Minkowski’s interpretation of relativity, which is just a mathematical model.
Actually, no physical evidence at all supports time travel and assuming it creates impossible paradoxes.For example, Minkowski’s interpretation predicts closed time-like curves, where an object’s spacetime world line returns to its starting point, just as an object in space can curve back to where it began. This implies that a physical object can collide with itself, which is impossible. Other paradoxes include:
1. The grandfather paradox: A man who travels back in time to kill his grandfather couldn’t be born and so he couldn’t kill him. Reverse time travel allows an entity to interfere with its own cause, so that causality breaks down. It follows that one can have going back in time or causality but not both.
2. The Marmite paradox: I see forward in time to me having Marmite on toast for breakfast but next morning I decide not to, so I didn’t see forward in time. If reality is a sequence of pre-existing states run forward, as block theory suggests, then life is a movie already made so there is no choice. It follows that one can have going forward in time or choice, but not both.
Such paradoxes suggest that spacetime is a mathematical artifact and so time travel is a fantasy. Physics turned Newton’s canvas of space upon which objects were painted into a spacetime canvas, but it still doesn’t work. After all, if we ever do travel in time, surely our first job would be to go back in time to stop the stupid things we are doing now! Like the multiverse fantasy, time travel is great science fiction but poor science because it denies both causality and choice.