Just recently I watched a trailer for the upcoming Terminator film, Terminator 4: The Future Begins. The series is one of my favourite film franchises and I’m looking forward to the new film, even in spite of the disappointing Terminator 3. Part of the reason that the films are among my favourites are the ways in which they deal with time travel and the paradoxes that it could potentially cause.

The film tells a story where in the near future artificial intelligence has taken over the planet and sends cyborg assassins back in time in an attempt to kill the future leader of the human resistance. At the end of the first film the robot terminator is crushed and destroyed, the only remains being the forearm and part of the microprocessor ‘brain’. The irony here is that the only way that the machines are eventually created is by using the parts left behind from the destroyed one.

It creates an interesting paradox in that the terminators would never have been invented if they hadn’t come back in time, so in theory the machines came into being by simply appearing out of thin air. The film dealt with the fate of John Conner, leader of the human resistance and whether his actions were predetermined or the result of a choice. In the end nothing he did could stop the inevitable and the machines started the war. The series raises a large number of interesting questions about time travel and the effects it may have.

This leads me onto the subject of time travel. Is it possible? It is a subject that has captured the collective imagination and inspired countless works of science fiction. As well as inspiring works of fiction it is a subject that has been the topic of serious scientific investigation and thought. Many notable physicists including Albert Einstein and Steven Hawking have done significant research into the prospects of the possibility of time travel. In this article I will have a look at some of the more common theories of how time travel may be possible and the effects it may have on the universe.

In fiction time travel has been presented in a huge number of different ways. The most common use is that of a time machine that can propel people backwards and forwards in time, or in some cases only in one direction. Popular examples include the DeLorean from the Back to the Future trilogy and the Tardis from the Doctor Who television series. In reality such devices are impossible but provide a romantic vision of how one might travel through time.

However despite these ideas of time travel it has been proven that time travel backwards in time is for the most part impossible. Time travel into the future, however is seen as being arguably possible. There are considerable (ie virtually impossible) boundaries to overcome but the possibility within the laws of physics is there for time travel into the future.

Theoretical time travel into the future would work on the basis of time dilation. If one was to travel away from the Earth at close to the speed of light before turning round and returning at the same speed then considerably more time would have passed for the people on Earth than those on the spacecraft. This would allow a form of travel into the future where people could arrive back at Earth several thousand years after they left even though they’ve only felt like they’ve been flying for a few years.

This is explained in Einstein’s theory of special relativity. A common way to relate this to time travel is with the twin paradox. The twin paradox states that if someone travels away from Earth at light speed and returns several years later they will find that their identical twin has aged considerably more than they have. The maths and physics behind why this happens is extremely complex and most likely far beyond the scope of this article, but it has been proven to happen. Experiments have even taken place on Earth that have proved the theory to be correct. The National Physical Laboratory in the UK flew clocks to Washington DC and back to London and found the results to be as Einstein had predicted.

Another factor that is able to cause time dilation is extreme gravitational forces. It has been calculated under general relativity that if a person was able to live inside a sphere with a diameter of five metres and the mass of Jupiter that their time would move four times slower than of the outside world. It has been proven in scientific experiments that clocks will appear to tick slower the closer they are to a strong gravitational pull. A person will see that a clock at ground level will tick more slowly than one up a tower.

With the knowledge of how time dilation works is it just a matter of time until we’re able to build a craft that can travel sufficient distances at a high enough speed that we see some more dramatic effects than a few nanoseconds? Imagine what kind of effects a craft that travelled at near light speed to a black hole and used the gravitational pull to slingshot back home would see. The combined dilation from the journey and the gravity of a black hole should see them arrive back on Earth a very, very long time after they’ve left.

The only way that we will be able to experience any kind of time travel is through this method, or by travelling through a wormhole.

A wormhole is a theoretical object that can link two distant points in space via a tunnel. The possibility of wormholes is driven by Einstein’s theory that objects curve space and time. Imagine space as a bed sheet that is pulled tight, when you drop a ball onto it the sheet will curve and the ball will roll to the middle. It has been theorised that if two objects with sufficient mass were to depress space in this way enough that they might meet in the middle and form a wormhole between the two points. Travel through these points would allow us to cross vast distances of space in a fraction of the time it would normally take to get there.

However if we were able to move and manipulate the mouths of the wormholes then it has been thought of that they may offer a way to travel into the past. From inside a wormhole the two mouths will always remain synchronised. That is to say if a clock reads 3000 on one end, it will also read 3000 on the other end. If we were able to bring two mouths near each other and then accelerate one away at relativistic velocities before bringing it back then the mouth that had been on the journey would have aged less. Let’s say the clock on the stationary mouth reads 3050 and the mouth that has been accelerated reads 3000. If you were to enter the accelerated mouth then both ends of the wormhole would read 3000 from inside. In this respect if you’re in 3050 with the stationary mouth and you enter the accelerated mouth you will exit in the year 3000, a trip back in time.

While theoretically possible this relies on technology that is far beyond our current understanding. However it does answer one of Steven Hawking’s queries on travel into the past. He asked that if time travel were possible then why haven’t we seen any tourists from the future? Well by using wormholes it would be impossible to travel back to a point before they were discovered and accessed.

Travel back in time would also raise the well known question of the Grandfather Paradox. This is the name given to a basic cause and effect paradox that could be raised from time travel into the past. For example, imagine you were to travel back in time far enough to a point before your father was born and kill your grandfather. This would mean that you would never be conceived and would never exist. If you never exist then how do you go back in time to kill your grandfather? Logic would say that you can’t and so that means your grandfather would live and you would go back in time and kill him, which of course we know you can’t. It is this example that is often used when saying that time travel into the past is impossible.

There is a school of thought though that you would be simply unable to change anything if you went back in time. Say you went to try and kill your grandfather. You would encounter a continuous stream of bad luck and you wouldn’t be able to do it, your gun would jam, you would miss, your shot wouldn’t be fatal, you would die, etc.

The idea of time travel into the past certainly raises an enormous number of paradoxes and questions on cause and effect, none of which are answered easily. While time travel into the future remains an exciting extreme possibility I do not think we will ever see time travel into the past, and certainly not at 88mph in a DeLorean.