So as I sit at my desk, I realise it’s only 2 weeks until my exams start. That’s pretty scary.
If I’m honest, every year it’s the same. Summer exams seem a mile off, something that can be thought about later, until suddenly there’s 2 weeks to go and the panic sets in. Tempus fugit, as Virgil said, and I’ve a mind to agree. Time is a funny old concept, an arbitrary value invented to organise the day into set sections; yet it is inextricably linked to not only our own lives, but that of the universe too – for without time, the universe would never change, stuck as a snapshot of the very moment it was brought into existence.
Imagine that – a universe without time; a universe where everything is constant. Of course, in many ways it is silly to think about such a thing, seeing as without the progression of time we could not be here to think about it. Yet, we do. Most of us have come to understand it as the ticking of a clock, as the invisible force that makes the weekday last forever, and the weekend finish as soon as it’s begun. To a physicist, however, time is not some imaginary concept. Time is very real indeed, and could be harnessed for useful applications with the right technology.
The world as most people know it has 3 dimensions – forwards and backwards; left and right; up and down. We can move through all 3 of these dimensions with relative ease; for example, if anyone reading has travelled up a spiral staircase, then you have travelled in all 3 dimensions at once. You walk up the staircase (1 dimension), but at the same time move yourself forwards (2 dimensions) and from side-to-side (3). But, if you would care to stick with me here, imagine a 4th dimension; the dimension of time. Imagine if we could find some way to travel through this dimension as we walk up the spiral staircase.
Unfortunately, to achieve this we must do one of 2 things; travel really, really fast, or find some object with a lot of mass, and orbit around it. Time slows down around massive objects, resulting in the possibility of time travel; if we could send a spaceship to orbit around a supermassive black hole, for example, the people on board could return to Earth having aged more slowly than the rest of us, for time will have passed more slowly for them. They would still be older, of course; they just wouldn’t be as old as anyone who was born in the same year.
So that leaves the possibility of travelling incredibly quickly, of ‘brute-forcing’ our way into the future. The speed involved would be immense; the speed of light is 2.998 x 108 metres per second, or around 670 million miles per hour. That’s a tad more than current technology can manage; some incredibly advanced mode of transport would need to be used. Of course, we’d also need a lot of space – shall we use a circle then, essentially giving us infinite room in the direction of travel? Using this method, passengers could travel for (from their point of view) a week, yet find themselves stepping off a full century into the future.
There are good reasons why I only talk about travel into the future. Time travel into the past is (unfortunately) impossible, so the general consensus says. But whilst I say it’s unfortunate for those wishing just to stay for a day in the past, it could be incredibly dangerous if the time traveller had a more malicious purpose, or even caused some timeline-altering event without ever intending to do so. Paradoxes are what happen when such an event occurs, and paradoxes are bad news.
As a slight diversion, who enjoys Back to the Future? A film involving time travel, where Marty McFly is transported to the year 1955 by his DeLorean and his totally-legit-and-not-at-all-fictional flux capacitor. It is one of my favourite movies, and one of my earliest experiences of the concept of a paradox. In the film, Marty (as I said) ends up in the year 1955, when his parents were around his age. Finding his father in a tree, Marty watches as his father falls onto the road, right in front of a moving car. Being the good soul he is, Marty rushes out to save his father; as a result of this action, he ends up in his mother’s house. She of course does not know who he is, and ends up having a teeny tiny crush on him.
The reason this is a paradox is because she was meant to have these feelings for Marty’s father, George. As a direct result of Marty’s actions, she is right on course to never know George, and thus never have Marty. He very nearly causes his own demise, only being saved through the combined forces of Doc Brown and Chuck Berry. But if he was never born, then who travelled back in time to interfere with events?
Take another example. In the 2002 film adaptation of H.G Wells’ novel, ‘The Time Machine’, Dr Alexander Hartdegen is with his fiancée, Emma, when she is killed by a mugger. In his grief, Dr Hartdegen invents a time machine (and this is crucial) with the intention of going back and saving her. Eventually, he completes his machine and saves Emma, only for her to instead be killed in a road accident moments later. Further on in the film it is explained to him that he cannot possibly save Emma; if she was alive and well, then Dr Hartdegen would never have built the machine, so would never have travelled back to save her from the mugger. But then, who did save Emma? In the action of travelling back in time and saving his fiancée, Dr Hartdegen has removed his only reason for inventing the machine, so has not travelled – and yet, evidently, he has.
This loop could be gone through indefinitely, but the crux of the matter is this: no matter what he does, Dr Hartdegen cannot save Emma, for doing so would create a paradox. Paradoxes are highly confusing things, so it is perhaps beneficial that they do not happen. In Stephen Hawking’s series, Into the Universe, he uses the example of a ‘mad’ scientist to demonstrate a paradox, one in which the scientist in question shoots himself through a time portal (or wormhole, as they are more commonly known) and thus brings up the question of who actually shot him; it couldn’t be the scientist as he was dead by this point (and that’s before taking into account that the gun hasn’t been assembled). Hawking believes that, should such a wormhole be opened, it would destroy itself through feedback in much the same way as amplifiers do, preventing the scientist from being able to create the paradox. Thankfully, nobody has had cause to find out.
Quite frankly I could carry on for a lot longer on the subject of time, as it is a very extensive topic (and I love talking about paradoxes). But as I write, it is getting late, and I have lectures in the morning. Time, I think, to stop.
“Great Scott! Jennifer could conceivably encounter her future self! The consequences of that could be disastrous!”
“Doc, what do you mean?”
“I foresee two possibilities. One, coming face to face with herself 30 years older would put her into shock and she’d simply pass out. Or two, the encounter could create a time paradox, the results of which could cause a chain reaction that would unravel the very fabric of the space time continuum, and destroy the entire universe! Granted, that’s a worse case scenario. The destruction might in fact be very localised, limited to merely our own galaxy.”
“Well, that’s a relief.”
– Doc Brown and Marty McFly, Back to the Future 2
All pictures from Flickr, used under the Creative Commons license found at http://creativecommons.org/licenses/by/4.0/legalcode