Gribbin (1990: 192-3):

The flow of time in the everyday world is a statistical effect, largely caused by the expansion of the universe from a hotter to a cooler state. But even at that level the equations of relativity permit time travel, and the concept can be very easily understood in terms of space-time diagrams.

Motion in space can proceed in any direction and back again. Motion in time only proceeds in one direction in the everyday world, whatever seems to be going on at the particle level. … The technique for time travel allowed by relativity theory […] involves distorting the fabric of space-time so that in a local region of space-time the time axis points in a direction equivalent to one of the three space directions in the undistorted regions of space-time. One of the other space directions takes on the rôle of time, and by swapping space for time such a device would make true time travel, there and back again, possible.

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From the perspective of Systemic Functional Linguistic theory, the notion of a flow of time confuses the unfolding of a process (flow), with the dimension (time) along which the process unfolds.

Similarly, the notion of time travel misconstrues

Similarly, the notion of time travel misconstrues

**duration**in time as**motion**in time. It is because time is**endured**, rather than**moved**through, that we only experience time "in one direction". It is this category error that undermines the validity of treating the time axis as equivalent to any of the space axes.