On Philosophy

December 14, 2006

The Philosophy of Time Travel

Filed under: Ethics,The Philosophy of — Peter @ 6:08 pm

No, not this philosophy of time travel.

It is my opinion that time travel, in which one has a real chance to change events after going backwards in time, is impossible. If time is a moving now then going backwards in time would mean changing the entire universe. If time exists all at once then changes are impossible. And if the multiple worlds interpretation of quantum physics is correct then the best you can do is simply end up in a different branch, since everything that can happen does happen, with or without time travel. And, in addition, there are worries about how an event in the future could even be said to cause some event in the past (the appearance of the time traveler).

But it is not my place to speculate about physics. Let us assume that meaningful time travel is possible, that we can go back into the past and make choices that change the future. Let us also assume that paradox isn’t an issue. But, even with the practical questions concerning time travel out of our way, there are still ethical considerations. We know that any change in the past, no matter how small, can have widespread effects in the future. And this means that instead of a world populated by people we know the world would be full of different people, no matter what changes we made (assuming we went far enough back in time). So is making any change immoral then, since doing so would erase a large number of people from time?

Although I do think time travel has ethical implications, I do not think that worries about erasing people from time are among them. Although killing someone is usually a bad thing, that is not what the time traveler is doing, he or she is preventing them from existing in the first place. And by making their changes they are granting a completely different set of people existence. Why should the original group of people be considered more worthy of existence than the new group? In any case our everyday decisions have this kind of impact already. Let us say that we are doing some activity, and plan on doing it for some time. As things stand there will be some group of people ten thousand years in the future. But if we decide to stop before we planned to then there will be a different future, and a different group of people will exist ten thousand years in the future. Even so we don’t consider the fact that one group of people will exist, and the other won’t, as a result of our decision to have any ethical relevance to what we decide to do. I have argued previously that the ethical choice is the one that results in the best outcome (by some standard of best), and this holds true not only for everyday decisions, but for those of time travelers as well. If the time traveler knowingly makes changes that result in the world being worse then they have acted unethically, and if they make changes that result in a better world then they have acted ethically.

What makes time travelers different than the rest of us, ethically, is the knowledge they can have about the results of their choices. A time traveler could visit the future, return, make a change, and then check the future again, thus giving them perfect knowledge about the results of their choices. And not only would they know the results of their choices with perfect accuracy they could also know the effects of their choices thousands and thousands of years into the future. As I mentioned previously a choice, made differently, will result in a vastly different future, as long as that future is sufficiently far away. But knowing what that difference will be is impossible, and so it usually doesn’t factor into our decisions. Ethically we aim for the best result, but we can only make reliable predications as to what that result would be over a relatively short time scale (at the most tens of years). But the time traveler is in a different position. They can know the long term effects of their choice to eat waffles for breakfast instead of pancakes. And thus because they can find out they have a responsibility to (just as we have a responsibility to be aware of the results of our actions). Additionally, the time traveler must make more difficult ethical choices than we do: does he or she favor a moderately well off civilization or one with a period of great suffering followed by a period of great prosperity? In light of these considerations I have to say that time travel is a technology that I am glad I don’t have access to. Sure it would be fun, but the ethical responsibility would be crushing.



  1. I think this just raises questions about the meaning of “to exist.” Let’s say world altering time travel is possible and after a fight, I go back in time and kill my best friend’s father before my friend was born. What happened to my friend? “He doesn’t exist, and (in this new time line at least) he never did.” But when my friend and I were friends, he existed. He had thoughts, feeling, consciousness, etc. when I knew him, but what happened to all that? Did his existence end when I went into the time machine? Or from his perspective was it that I went into the time machine but never came back and he lived on for the rest of his natural years? If my friend did “disappear” when I went into the time machine, I think that counts as murder, since he was living until the moment I went back in time. Thus, going back in time 100+ years would essentially kill everyone on the planet (since chaos theory dictates different people would be born if there were even a slight change in the deep enough past). On the other hand, if people don’t blink out, then time travel essentially clones everyone. Now there’s timeline A me (who knows my friend) and timeline B me (who did not), etc.

    Anyway, my theory it is that time travel might be possible, but effecting the past so that it comes out differently than it is now definitely is not possible. From a quantum point of view, you can say that the probability is 1 that the universe will be the way it is now, so if you try to make a time altering trip to the past, the odds are 0 that you will succeed.

    Comment by Carl — December 14, 2006 @ 9:30 pm

  2. You can’t really have a theory about time travel unless you have some math to back it up, since time travel is in the domain of physics, and relativity does allow it under certain circumstances. And physics cares very little about what makes sense to us. As I mentioned at the start I don’t think it is possible either, but there is no a way to know without a complete physical theory.

    Comment by Peter — December 14, 2006 @ 10:14 pm

  3. Carl,

    The paradox that you talk about is impossible. You say you will go back and kill your best friend’s father and thus make impossible the birth of your friend. Even if you did go back in time, you would be unable to kill him for some reason. This is because even though you thought about going back in time *now*, you had already thought about it in the future and already tried to kill his father in the past. You were unsuccessful and that is why he was born in the first place!

    Comment by venksster — December 14, 2006 @ 10:18 pm

  4. Makes sense to me Venksster. But this is all just speculation. We know so little about the possibility of time travel that none of this adds up to much.

    Comment by Carl — December 14, 2006 @ 10:39 pm

  5. I do not think you should feel any responsibility for whatever you do that alters the distant future, even if you can witness the long-term consequences, and the reason is called ‘sensitivity to initial conditions’ aka ‘chaos theory’ aka ‘butterfly effect’.

    Say you eat waffles for breakfast and take a long time travel to inspect all the possible consequences of that action – and you see all is sort of OK, like forever. You go back to your breakfast and choose pancakes instead, just in case it might make things any better (‘cos you want to be ethical in everything you do). You go check the consequences and this time, you find out that your country falls victim to a violent dictatorship in ten years from now. You go back to your breakfast panic-stricken about what you just did and revert to the waffles.

    But: it is impossible for you to have ‘saved’ your first action ‘to disk’ with infinite accuracy because it requires an infinite amount of information, therefore an infinite quantity of matter to store. Therefore the best you can do is try to eat your waffle in essentially the same way as you did the first time. Then you go check, out of conscience. And this time, it is even worse: a global nuclear war over water resources will kill most humans in 100 years from now.
    Either you spend the rest of you infinite life chewing the same bit of waffle over and over again as you would try to cast a bowl of dice to get back this one-in-a-billion combination that you once produced, or you just shed the guilt, because hell, why should you be accountable for the luck of the draw?

    In a nutshell: unless your actions have a linear effect on the future (i.e. chewing an extra molecule of waffle will make life a small bit better or worse for the generations to come), they cannot reasonably be considered as a cause: to hell with the guilt, and enjoy time travel while you can still afford it (‘cos of the rising oil prices).

    Comment by mandarine — December 15, 2006 @ 5:00 pm

  6. The way out is this: you make sure there are no warnings from the future before doing anything. If you act and then observe bad consequences you leave a warning before you acted to youself (say in a computer file with only two states, on and off), thus changeing things to a second possibility. And if that is worse you can remove the warning, going back to percisely the first state. I am assuming here one can alter the state of the file direcly, but if you can travel in time sending only an electic signal back shouldn’t be too hard either.

    Comment by Peter — December 15, 2006 @ 5:22 pm

  7. There is probably something to be done along those lines, but then my gut feeling is that basically, you’d see warnings everywhere and you’d probably stop living altogether. Food for thought though.

    Comment by mandarine — December 16, 2006 @ 12:43 am

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