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.
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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, 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.
Beyond light speed
Such strange routes exist in Gödel’s universe because it is so odd. While our universe can be imagined as a rubber sheet that curves around massive objects – think bowling balls on a trampoline – Gödel’s universe consists of an infinitely wide rotating cylinder of evenly spread dust. It has previously been investigated mathematically, but a team led by theoretical physicist at the University of Ulm in Germany is the first to visualise what it might be like to exist there.
The first of their movies places the camera floating at the centre of the cylindrical universe and uses ray tracing to simulate what an Earth-like object some distance away would look like. Ordinarily, ray tracing involves drawing a straight line from a virtual camera to an object in 3D space, building up an image one light ray at a time. In Gödel’s universe, there are two differences. First, because the universe is rotating, light rays move in spirals. Second, the outer parts of a rotating cylinder move faster than the inner ones, so there is a radius beyond which an infinitely wide cylinder starts to rotate faster than light. Light rays cannot cross this horizon, so it acts like a mirror, reflecting rays back towards the centre.
These optical gymnastics have some weird effects. Light from the planet’s front gets squashed as it curves towards the camera, while light from the back gets reflected off the cylindrical horizon. And as the light from the back face takes longer to reach you, it is a vision of the past. The striking thing about the team’s visualisation is that you see two views of the same object from different points in time, simultaneously.
Yet more weirdness arises when the team move their planet along a circular orbit. “While the main Earth moves, images of many others are arriving at different times,” says Schleich. This plethora of squashed planets, all at slightly different points in time (pictured above), is pretty weird, but does not involve any actual time travel – it just recreates how this universe would look at given points in time.
To film actual time travel, the team went further and sent a sphere along a CTC and thus back in time. Watch this brain-bending movie above (and see diagram). For simplicity, they used a solid-coloured sphere, not an Earth, that changed colour as it aged. We watch as a young sphere gets clobbered by an older version of itself which has suddenly emerged from beyond the universe’s horizon. This collision sends the young sphere off along the CTC. It flies fast into the future before looping around into the past, ageing all the while. Next thing it knows, the sphere finds itself back at the time and place of the collision, but this time, it is the one mounting an attack on its younger self. After knocking its younger self onto the CTC, the older sphere continues on its way ().
OK, it’s hardly Back to the Future, but it provides a unique outlook on Einstein’s theories that equations cannot. “I find ray-tracing visualisations to be a powerful tool for deepening our understanding of general relativity,” says at the Massachusetts Institute of Technology.
“It’s hardly Back to the Future, but it provides a unique outlook on Einstein’s theories”
“Visualising these strange space-times is a great way to begin to get your head around them,” agrees of the University of Colorado at Boulder. That is important because we do not actually know why time seems only to move forward in our universe. According to physical laws, there is no reason why things cannot move backwards and forwards in time, just as they can in space. “Why causality holds is a deep mystery of the universe and it could be that thinking about space-times where causality fails, such as Gödel’s universe, might give insights,” Hamilton says.
We know that Gödel’s universe is not a model of our own as it is “static”, with no cosmic expansion following the big bang, but there could be pockets of our universe that are Gödel-like: a rotating black hole may drag the space-time surrounding it, forming a rotating sphere, say. “It is possible that we can expect similar effects in other space-time regions which might exist in our universe,” says Schleich’s colleague Michael Buser.
A toy microcosm of the “time travel universe” could also be built, perhaps with metamaterials, which manipulate the path taken by light in extreme ways – though they could not produce a CTC.
Developing new techniques for ray tracing in curved spaces could be useful for space telescopes, says at the Weizmann Institute of Science in Rehovot, Israel, as light beams bend slightly when passing massive stars or galaxies. “If people make something cool and interesting then applications will come in unexpected ways,” he says.
How to cheat time
Not as iconic as Back to the Future‘s DeLorean, perhaps, but a rotating universe is the star of the first scientific time-travel movie. In fact, there are many ways to cheat time.
Speeding spaceship
To travel to the future, go on a space voyage, then return to Earth. The faster you travelled, the more time will have passed at home than on your ship, courtesy of Einstein’s theory of special relativity.
Cosmonaut Sergei Krikalev holds the record for this kind of time travel – he is 20 milliseconds younger than he should be after two years on the space station Mir.
Holes in time
Stretch space-time enough and you’ll form a wormhole, a tunnel through the fabric of the universe linking two distant points. Now take one end and send it hurtling on a circular trip at near light speed, causing it to age more slowly than the other end. Pass through the wormhole’s younger end and, voila, you’ll travel back in time.
String theory
Cosmic strings the width of a proton but light years long may have been formed in the big bang. Sending two flying past each other at near light speed would warp space-time enough to let an object travel into the past.
This article was updated to reflect the fact that although cosmonaut Sergei Avdeyev once held the time-travel record, he lost the title to Sergei Krikalev in 2005, as Krikalev spent a couple of months more in space. The time dilation is still around 20 milliseconds, though.
