Models of time
The real physics behind time travel in the movies
Back to the Future, Planet of the Apes and The Terminator are all highly successful movies, but they have something else in common. Each of them involves speculation about the physics and philosophy of time. PhD researcher Guy Micklethwait explores the facts in science fiction about time travel.
Filmmakers have been using ideas about time as plot devices for years. I've found at least 170 movies that involve time travel or other temporal phenomena, and have reviewed over 100 of them including science fiction, psychological and fantasy films. How likely is it that these scenarios will one day become a reality? And if we do find ways of travelling through time, what consequences might we face? These are tricky questions, but first we need to consider the physics of time travel.
Einstein's Special Theory of Relativity argues for the existence of a four-dimensional ‘space-time continuum'. This is made up of the three dimensions of space: width, breadth and depth. We can see and touch all of these things. The fourth dimension is time, which we cannot see or touch, but whose passing we can experience. A physicist would say that we are experiencing ourselves passing through the dimension of time. In the 1960 film, The Time Machine, George Wells explains to his learned colleagues what the fourth dimension is and how his newly invented machine can travel backwards or forwards through it.
These four dimensions all relate to a constant. So for example, the faster a rocket moves through the three dimensions of space, the slower it moves through the dimension of time. This means that the rocket's onboard clock is ticking more slowly than a clock back on Earth - a concept known as ‘time dilation'. When the rocket returns, less time will have passed for its passengers, so they will have aged less than the people back on Earth. In the extreme case, they may return to the Earth's distant future, as in Planet of the Apes (1968). So forwards time travel is a reality: how far forwards you wish to go in time only depends on how fast your technology will allow you to travel through the three dimensions of space.
Backwards time travel is a little trickier. Scientists have speculative theories in place, but technology hasn't caught up yet to the point that they can be tested. Creating wormholes is one such theory. If space can be curved enough, and there is no theoretical reason why it can't, then maybe a wormhole could be constructed to shortcut from one side of the universe to the other. If one of the mouths of the wormhole was made to travel very fast compared to the other one, then there would be a time difference between the two. Passing through the hole one way would allow you to move forwards in time and passing back the other way would allow you to move backwards in time. The main limitation with this method is that you would never be able to travel back to any date prior to the construction of the wormhole. Such a wormhole occurs naturally in Donnie Darko (2001), where a jet engine falls off a plane and then passes through a space-time anomaly, which causes it to travel back 20 days before crash landing.
A jet engine is rather large, unlike sub-atomic particles. At that very tiny level, anti-particles moving forwards in time behave like particles moving backwards in time and vice versa. So backwards time travel is unproblematic on microscopic scales. As an aside, the physicist Stephen Hawking suggests that on the macroscopic scale (where we exist) time travel must be impossible. He argues that a ‘Chronology Protection Agency' would prevent ‘closed time-like curves' from appearing, thus making the universe safe for historians. But what is a closed time-like curve in the first place?
Causes and special effects
If you were able to curve space, then what would happen if you curved it right around to the point where you made a loop? This is theoretically possible using Einstein's equations. Indeed there may be parts of the universe where these loops naturally exist. A ‘closed time-like curve' is when you loop right back to a place in space and time that you had previously experienced. You could hypothetically traverse the loop just the once, get stuck in such a loop forever, or go around several times before exiting. This happened to the character Phil Connors in Groundhog Day (1993), who kept waking up each morning to find he had returned to the start of the previous day.
As a weatherman, Connors no doubt would have known the work of Edward Lorenz, who in 1961 created a simple weather forecasting model using a software program with 12 equations. Lorenz noticed that the slightest change in his input data would give very different long-term weather predictions. Such systems are known as ‘nonlinear systems' and are normally characterised by long-term unpredictability. Lorenz noted that the flap of a butterfly's wing could cause a tornado in some other part of the planet and thus the term the ‘Butterfly Effect' was born. Examples of such systems are seen in the stock market, evolution and even in psychology. In the film The Butterfly Effect (2004) a man goes back in time and makes a few changes at a critical point in his childhood, which causes his original timeline to disappear and a new timeline to form from that point onwards. This new timeline diverges away from where the original used to be, so that when he returns to his adult life, he and all of his friends are following a completely different destiny.
One of the most famous temporal paradoxes thrown up by backwards time travel is the ‘Grandfather Paradox'. If you went back in time and killed your grandfather when he was a child, this would mean that you would never have been born and would cease to exist. An example of this paradox occurs in Back to the Future (1980), when Marty McFly goes back in a time machine and meets his parents before they were married. His presence causes his mother to fall in love with him and reject his father. Marty fears this will prevent his birth, so spends the rest of the film trying to get his parents back together again.
If you were to travel back in time and send your grandfather to an untimely grave, there is a possibility your own timeline would continue unharmed. The physicist Hugh Everett III proposed his Many Worlds Interpretation of Quantum Physics in 1955. He suggested that when we measure quantum objects the world actually splits into two parallel realities. In this theory there are no paradoxes, but the idea of an infinite number of timelines scares away many scientists. How does this relate to time travel? Well, if you were to travel back in time, another timeline would be created in which you would begin to interact. You could kill your grandfather, if you wished to do so, and you would continue to live in a world where your grandmother did not marry your dead grandfather, so you would never be born in this timeline, and would not get to meet yourself. In parallel to this, your grandfather would still be alive on the original timeline and you would be born and live to become a time traveller. An example of parallel worlds is seen in the movie, Sliding Doors (1998), which follows two different parallel lives that branch out from each other after a brief time reversal.
Fate of a time traveller
What if you go back in time and your actions cause an event that later becomes the reason why you originally went back in time? This would mean that you would be predestined to go back in time in order to create a self-consistent timeline. This is known as the ‘Predestination Paradox'. Any changes you thought you were making in the past would be not be changes at all, as they would be helping to make history just as it was originally written. This happens in The Terminator (1984) when a man is sent back from the future to protect Sarah Conner, the mother of his comrade, John. He gets her pregnant and thus becomes John's father. Therefore John is predestined to send him back in time so that he can be born and keep the timeline self-consistent.
But perhaps we wouldn't have to go to so much trouble to keep our timelines tidy. In the mid-1980s, Dr Igor Novikov developed the Novikov Self-Consistency Conjecture, which stated that if time travel were possible, there must be a law of physics that would prevent time travellers from doing any action that would cause an inconsistency and hence a paradox. This is exactly what happens in Terminator 3 - Rise of the Machines (2003). No matter what they do, or whom they kill, the protagonists can't stop the inevitable rise of the machines. The war has to take place for the robots to be sent back in time and for Sarah Conner to give birth to her son.
So it can be seen that although forwards time travel is a reality and its effects are being used in modern engineering, the technology required for backwards time travel is speculative. In fact, arguments between physicists are ongoing about how likely it is that a wormhole would actually work even if built. That hasn't stopped filmmakers from using wormholes and other time travel techniques as plot devices for their films to inspire audiences to hope that one day they will be able to venture back and change their past, just as people once dreamed of a rocket reaching the moon. As we move into the era of space tourism, who can say if and when backwards time travel will be possible.
Guy Micklethwait picks five time-bending movies worth watching.
• Twelve Monkeys (1995) Time travelling backwards inadvertently causes a closed causal loop.
• Slaughterhouse Five (1972) Experiencing life along a fixed timeline, but in a random order.
• Déjà Vu (2006) Viewing and trying to change the past using wormhole technology.
• Frequency (2000) Changing the present by communicating with people in the past.
• The Jacket (2005) Involuntarily travelling to the future brings justice to the present.
Guy Micklethwait is doing his PhD, Models of Time: a comparative study using film, jointly at Centre for the Public Awareness of Science and the Research School of Physics and Engineering.