About time

This is the first in a series of posts on connections between quantum physics and films. 

Ever dreamt of living life differently, if you were only given the chance? One of the major ways that fundamental physics crashes into films is with time travel.

Three major theories of time travel exist in the “movie-verse”—the fixed timeline, dynamic timeline, and multiverse.

The fixed timeline theory can be best summarised as “unchangeable history”; nothing can be done to rewrite history, in fact, the very attempt to might have resulted in things ending up the way they were. See Hitler's Time Travel Exemption Act.

Sounds hopeless as a plot device? But wait! Causal loops allow for some rather interesting phenomena, such as a person being both their own parent and child (as in the sci-film film Predestination (2014), and entities can have a past solely because they exist in the future.

In quantum physics, researchers are seriously considering the possibility that influences from the future travel back into the past. This could be one way to understand the results of measurements on entangled particles. Read more about this quantum concept of retrocausality here: http://nautil.us/issue/9/time/the-quantum-mechanics-of-fate

The dynamic timeline theory is similar to the fixed timeline theory, but with a twist. Here’s an example: suppose you travel back in time to prevent an apocalyptic war from breaking out. And hooray! You were successful! You travel back to the present and find that all is well in the world—but the alternate you grew up in a peaceful world and hence never had any reason to travel back in time to prevent the war from breaking out and—

You get the point.

This theory features famously in Back to the Future (1985) when Marty McFly accidentally almost erases his own existence by preventing his parents from meeting. You can see why this is also known as the Grandfather Paradox.

Physicists call the spacetime structures that can give rise to such paradoxes ‘closed timelike curves’. Most physicists dislike them because of the consistency problems. But they are still interesting to think about: send quantum particles down such curves and the paradoxes could disappear - and bring amped up computing power. That’s according to some theories at least. Learn more in this 2014 article from Scientific American.

The multiverse theory assumes the existence of multiple/infinite universes (hence the name); therefore whenever you time travel, a new alternate universe is created.

For example, suppose you decide to travel back in time to prevent an apocalyptic war from breaking out (we’ve been here before, haven’t we?). When you pushed the shiny red button, an alternate universe (B) was created; one in which you exist at the point in time you were supposed to travel to, but you have effectively ceased to exist in your “home” universe (A). Now in Universe B, you stop war from breaking out and get the girl—but wait! Your “real” friends and family are still trapped in the post-apocalyptic world you left behind! This happens to Spock in Star Trek (2009) when he travels back in time, although to an alternate timeline.

The Many Worlds interpretation of quantum physics puts us in a similar bind. In quantum physics, the outcome of an event can never be predicted with uncertainty, only with probabilities. Many Worlders believe that each of the possible outcomes happens in its own universe. You can read a lot about quantum many worlds in this Nature special, or get the quick take from renowned physicist Brian Greene (who also happens to be one of our judges!):

You might be having a better life in one of these other worlds, but have no way to get there. Could you travel back in time to choose another branch? The equations of quantum physics are symmetric with respect to time, which means they run as well backwards as forwards.

That pushes us to ask another big question. Where does the arrow of time come from? Physicists don't have a definite answer yet, but increasing entropy, or disorder, could be one culprit. On the quantum scale, we just might be able to reverse that...