Take that, Hollywood. You may have grappled with time travel for decades, but now that physicists are in on the action, they have the first movies of what travelling to the past actually looks like.
The visualisations are surreal, the result of the shape of a hypothetical universe in which time travel is possible. They might help us understand the origins of causality, which is still shrouded in mystery, and pave the way to physical, table-top models of time travel. Made via ray tracing, a computer graphics technique, the movies may have practical applications, such as aiding the interpretation of light rays from ancient galaxies that are only now reaching our telescopes.
From Einstein's equations of general relativity, which can be used to deduce the curvature of a given universe's space-time from its shape and other properties, we know there are plenty of hypothetical universes in which time travel would be possible.
In 1949, Kurt Gödel homed in on the first, and simplest, of these: a universe that, when run through Einstein's equations, produces what is known as a closed time-like curve (CTC), a physical path through space that loops back on itself in time. CTCs are not time machines that can take you back to any date in the past, from anywhere. However, follow one of these curious paths and you will travel first into the future, and then back to the point, in both space and time, where the CTC began. It is a bit like taking a left turn and finding yourself in last week.
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