Minkowski interpreted Einstein’s theory of relativity using a four-dimensional space-time matrix. Instead of existing at a Cartesian (x, y, z) point in space, objects now exist at a Cartesian (x, y, z, t) point in space-time, where t refers to time. An object sitting at a point in space now has a world line that extends it in time. One can allocate “locations” in time as for space and talk of the “temporal parts” of an object as one does its left and right parts. This enhanced idea of how objects self-exist has been generally accepted.
The Minkowski interpretation presents a block theory of time where “time capsule” states can be browsed like the pages of a book, as all past, present and future states exist in a “timeless time” (Barbour, 1999) p31. That the future and past already exist in space-time implies that time travel is indeed possible. It also predicts closed time-like curves, where a material object follows a spacetime world line that loops back to its starting point, just as an object moving in space can curve back to where it began. Spacetime is the landscape that physicists assume objects to endure within. The evidence is that the mathematics works but mathematical models are notoriously unreliable indicators of reality, e.g. assuming all the mass of a body exists at its center of gravity works to calculate trajectories but no-one believes it is so. When physicists say that time travel is “based on General Relativity” they really mean it is based on Minkowski’s interpretation of relativity, i.e. a mathematical model.
Since no physical evidence whatsoever supports time travel, quantum realism sees the idea of a spacetime on which things exist as just an upgrade of Newton’s idea of space as a “canvas”. It is a mathematical convenience not a reality description. After all, if we ever do achieve time travel, surely the first job would be to go back in time to stop the stupid things we are doing now! There are practical reasons why time travel isn’t possible in our world:
1. The grandfather paradox: A man travels back in time to kill his grandfather, so he could not be borne and so he could not kill him. In the same way, closed time-like curves deny causality as they give an event that is “simultaneous” with its cause. 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.
Quantum realism accepts that time dilates but denies that this prevents causality or free choice, i.e. it denies time travel. Like the multiverse myth, time travel is great science fiction but not great science.