I reckon time-travel – the sort we see in science-fiction stories and movies – is simply not possible. It is not possible – no matter how advanced your technology – to invent a machine that can transport a human being back in time. The article below is the transcript of a presentation on Time-Travel I gave on Wednesday, June 1st 2005 at the Philorum Philosophy forum.
Synopsis Is time-travel possible? In both directions? Is it just a matter of “time” before we fulfil the sci-fi writers’ predictions and invent some kind of time-machine? Or is it some sort of philosophical or scientific impossibility, no matter how advanced our technology? What’s the difference between a paradox and a causal loop? Does time-travel violate free-will?
Is Time-Travel Possible?? Before I discuss that, have a think about what you believe to the answers to the following questions:
- Is it theoretically possible to travel into the future?
- Is it possible to travel into the past?
- If so, is it possible to alter the past? And what does it mean to say that?
- If time-travel is possible, and simply remains to be invented, then where are all the time-tourists from the future?
- If time-travel is possible, wouldn’t someone from the future already have come back in time and told us how to do it?!?
As I describe each of the following four scenarios, have a think about each one, and decide whether you think it’s possible or not…
- A man gets into a spaceship and heads off to the Andromeda galaxy at 99.999999% of the speed of light, and then returns at the same speed. Due to the effects of Relativity, he ages only 20 years during the trip, but when he returns, five million years have elapsed, and all his plants have died. (Or alternatively he simply heads off to the nearest black hole, parks his spaceship in the tightest orbit possible without being sucked into the hole, and then returns to earth to find the same amount of time has elapsed.)
- A man decides he’s bored with the 21st Century, and decides to fast-forward his life. He adopts a habit of living one week of every ten years for the rest of his life, by jumping into a time-machine and skipping over the rest. Assuming he has 60 real years left to live, that gives him roughly a 30,000-year perspective – 15 times longer than modern human history (since the birth of Christ).
- A teenager is visited by a man from the future. The man describes to him the words and music to a song. The teenager thinks it’s a pretty cool song, gets a band together and records it. The song is a hit and makes the teenager a star. Later in his life, the teenager gets into a time-machine and travels back 20 years in time, and describes the words and music to his younger self.
- A man gets into a time-machine and travels back 40 years. He locates himself as a baby and suffocates his younger self.
What do you think? Which ones are possible, if any? I have described each of those situations in what I believe to be increasing order of impossibility. In other words, the first scenario is the most likely, and the last is the “most impossible”.
The first scenario is not only possible, but it happens every day. Well, clearly we don’t have people travelling to Andromeda every day, but we do have people travelling every day. And every time you travel, your experience of time differs from someone who is not travelling. In other words, if you travel, you experience events of the universe more quickly than someone who remains stationary. The closer you approach the speed of light, the more significance this difference becomes. This is in accordance with Einstein’s Theory of Relativity, and has been scientifically verified on many instances. The same applies to your proximity to massive (gravitational) objects. The closer you are to a large object, the faster the Universe appears to run. This means that you will age more rapidly at the top of a tall building than at the botton (although the difference is barely measureable, even with atomic clocks).
The second scenario is really just a more hi-tech version of the first. It makes the assumption that someone has found a way to speed up one’s experience of the universe arbitrarily – perhaps without accelerating to vast speeds or being close to massive objects. The philosophical implications are the same. So whilst we may never find a way to make such a device, or the construction of such a device may violate some physical law, there is no theoretical reason that we know of at this time that says it can’t be done.
The third scenario is where it starts to get interesting. The really interesting question about the teenager scenario is: Where did the song come from? Or, Who wrote the song? Clearly no-one wrote it, so where did it come from? Well, it spontaneously came into existence. This violates our present understanding of the laws of cause and effect, but that’s not to say that it’s impossible. It may well be impossible, but not necessarily because it violates the laws of cause and effect. Our understanding of cause and effect may be quite limited, and it may indeed be possible for self-contained and self-consistent phenomena such as these to spontaneously come into existence. These phenomena have been given a name, by the way. They are called “Causal Loops”. No-one has ever observed one, but that’s no proof that they don’t exist. They may be occuring every day, and we’re simply unaware of them. Or they may be impossible.
The last scenario is the most famous example of a time-travel paradox. These paradoxes are hypothetical scenarios that contain logical contradictions. Many people believe them to be impossible – for exactly this reason. However, there are certain circumstances where such an event may indeed be possible. In other words, it may be possible to kill your younger self. I discuss this further below.
As a slight digression for a moment, here’s another question for you… Imagine a time-machine that was precisely that – a machine that simply moved you in time, while your spatial location remained unchanged. What would happen if you travelled exactly six-months into the future – or past? Where, exactly, would you end up? Well, if you think about it a bit, you’d realise that you end up in empty space. In six months’ time, the Earth will be around the other side of the Sun. If you don’t move in space (only in time), then the Earth will continue it’s path through space without you. Let’s assume that you can devise your time-machine so cleverly that you can account for this. In other words, your machine can move you through space as well, and you can program it to compensate for continental drift, the rotation of the Earth, the movement of the Earth around the Sun, the revolution of the Sun around the centre of the galaxy, and the movement of the galaxy with respect to the rest of the Universe. So then a further question: Let’s pretend that you want to travel exactly 12 hours into the future. Is there some other problem that might arise from this? If you think about it, you’ll realise that at the time you step into the time-machine, not only do you have a location, both in time and in space, but you also have a momentum. In other words, if your laboratory was located in Sydney, you have (at the very least) a momentum associated with the speed with which you’re rotating around the Earth’s core – a speed of about 700 kilometres per hour. If you send yourself to your laboratory 12 hours in the future, that laboratory will be on the other side of the Earth (with respect to the Sun), and thus will be travelling at 700 kilometres per hour in the opposite direction. In other words, at the moment you arrive in your laboratory in the future, you’ll be travelling 700km/h in one direction, and your laboratory will be travelling at 700km/h in the other direction. Something to think about if you’re about to test your time-machine.
But these matters, while interesting and even annoying, are simply technical and don’t have much to do with the philosophical issues of time-travel in general. Let’s return to the core issue…
I do not wish to discuss the possible technologies that we might develop to help us travel either direction in time (or how such a machine might work), but rather to talk about whether it’s philosophically or theoretically possible. Claiming that something is impossible is a very dangerous thing for any scientist or philosopher to do, but nevertheless I’m going to claim that time-travel forwards in time is fine (we’re already doing it, every time we move about), but time-travel backwards is simply not possibleat least, not in the physical universe (and I’m going to restrict myself to a discussion about the physical universe). When you think about it, we’re all travelling forwards in time, at the rate of one second per second. [Aside: This seems something of a coincidence to me, that we have exactly one second of experience for every second that goes by. This leads me to believe that we actually have no idea what the nature of time is – but that’s another discussion. End of aside]. So time-travel into the future would simply be a matter of speeding up our experience of time (which Einstein has shown to be possible), or slowing down the particles and energies of our bodies and brains. It appears to me that this doesn’t violate any metaphysical principles, so I’ll simply assume it’s possible and move on…
So why isn’t it possible to travel backwards in time? Well, before I answer that question, let’s define exactly what I mean by that. I will define travelling back in time to mean: Leaving your time of origin, physically existing for some duration in a period of history (recent or ancient), and then returning to the present – the same present that you left from! If any of these points are not held, then I don’t define that to be true time-travel backwards, and so may indeed be possible. To be precise:
- Physically existing – able to interact with the past, not just an ethereal, disembodied observer
- Your own history – not the history of some other universe. For example, you don’t go back 100 years and find dinosaurs walking around
- You have the capacity to return again – you’re not stranded back in time
- You can return to the same present (or some point after your original time but on the same timeline) – In other words, not an alternate timeline.
I believe this is the image that most of us picture in our heads when we envisage time-travel. I don’t believe it’s a particularly restrictive definition of backwards time-travel. However, it may be exactly that – too restrictive – so I may adjust it as we go along…
Let’s assume, for argument’s sake, that this flavour of time-travel is possible. So I’ll posit another scenario for you: You go out and buy yourself a Mitsubishi time-machine from Bing Lee, bring it home and switch it on. As soon as you do, a future version of yourself steps out of the machine and says to you, “Hi, I’m you from ten minutes in the future. You must destroy this time-machine immediately, this minute, before it’s too late!” And then he disappears back into the time-machine and vanishes back into the future. So you have a little think to yourself… I obviously don’t destroy the time-machine, because if I had, then I wouldn’t be able to come back to now and tell myself to destroy it. If you think about it a little more, you realise that you actually can’t destroy the time-machine in the next ten minutes, even if you wanted to, for exactly the same reason. And if you think about it just a little bit more, it starts to dawn on you that in ten minutes you absolutely have to travel back in time ten minutes and instruct yourself to destroy the time machine, even though such an instruction is futile (because you know full well you don’t destroy it). You have to do it – you have no choice. It’s already happened that way!
If you’re finding this difficult to beleive, then remember our definition of time-travel. There is only one timeline – you know what’s going to happen in the future (you are going to get into a time-machine in ten minutes and travel back ten minutes in time) because you’ve seen it! Furthermore, you have to use the exact same words that your ten-minute-older self used when he came back and spoke to you. What if you don’t want to? What if you change your mind? What if you forget to, or forget your lines? Tough buddy – it’s happening anyway, and you’re along for the ride! What if a version of you arrives from tomorrow and they’ve had an awful haircut? Guess what? You have to have that same haircut, and you have to have it within the next 24 hours, even though you know it’s shit! There are dozens more examples I could give you about this. Even our original scenario – the teenager who learns the song from his future self – is an example of this (the older version of the teenager has to go back in time to give the song to the younger version. He has no choice, because it’s already happened that way).
So clearly, if the time-travel that I’m suggesting is possible, we end up with no free will. So let’s loosen our definition of time-travel a bit – let’s relax rule 4 – and allow that you don’t have to travel forwards to the same future that you started from. This means that you can effectively do whatever you want – free will is never violated. This even means that you can go back and suffocate yourself as a baby and get away with it, because you’d then return to the future and find that you never grew up (obviously) because someone suffocated you at birth. So the last 40 years of your life as you know it doesn’t exist. But you exist! It’s a bit like you’re a visitor from a parallel universe, now stepping into a universe where you had been suffocated at birth. Back in your universe – your original universe, your life exists, but you’re no longer in it. You disappeared one day by stepping into a time-machine, and were never seen again. What’s more, you’ve got no way of getting back to that life! Every time you travel back in time, when you go forward again you end up in yet another parallel universe. This is the sort of time-travel most often found in science fiction. You’ve all seen it: The hero goes back in time to try to right some wrong, so that the present day is somehow “fixed”. Then Marty McFly returns “Back to the Future” (the present) and lives happily ever after in his new “fixed” life. Naturally, it can’t possibly work like this, because there’d be two Marty McFlys in the fixed universe, and none in the original one! Think about that for a bit, and you’ll see it’s true.
So reverse time-travel seems fraught with philosophical dilemmas – sometimes even paradoxes. In my “perfect” form of time-travel, free will is violated. To have any sort of reverse time-travel, you probably need to introduce the notion of parallel universes, and even then you still can’t get back to where you started from. And let’s just talk for a moment about parallel universes. The multiple-universes idea is a pretty well-known one, and is invoked as a possible explanation for several problems that theoretical physicists have with the universe (the apparent random behaviour of sub-atomic particles in Quantum Mechanics being another one). Personally I don’t like it too much, because it violates Occam’s Razor, the scientific principle that suggests that the simplest theory that fits all the facts is the correct one. The notion that the only way we can have backwards time-travel is by assuming the existence of a virtually infinite number of universes is vaguely disgusting to me…
It’s probably fairly clear what my conclusions are: There’s nothing inherently problematic about travelling into the future, but, if you remain in this physical universe (i.e. remain in physical form and remain in the same universe), then it’s not possible to travel back in time. You’re going to violate free will or set up a logical paradox. A looser form of time-travel relies on the existence of parallel universes, something that I don’t personally believe in.
Here’s a little list of time-travel books that I’ve enjoyed and recommend strongly..? Non-Fiction
- How to Build a Time Machine, by Paul Davies. Very accessible, very easy to understand. Deals with both the practical (scientific) and theoretical (philosophical) issues surrounding time-travel.
- The Time Machine, by H. G. Wells. The original time-travel story, and still one of the finest.
- The Man Who Folded Himself, by David Gerrold. Nothing profound, but a fun, lively and interesting read.
- The Time Traveler’s Wife, by Audrey Niffenegger. The finest time-travel book I’ve ever read. In fact, one of the finest novels I’ve read – of any genre.