/* I wrote this a while ago for the Coursera course “Introduction to Philosophy“.
* More @ https://www.coursera.org/course/introphil
* See the sources at the end for more information on the topic.
Since time travel was first considered they’re have been many who objected it, not only saying that will be impossible, which it may be, but that it’s very nature is non-logical, as it generates logically contradictory outcomes. In this essay I will argue why time travel is logically possible, by showing that there are no contradictions or paradoxes that will happen if time travel is physically plausible. I will try exploring this question philosophically, rather than scientifically.
It’s often thought that backward time travel would enable one to change the past in countless ways. But since it’s impossible to change the past, make what did happen not to have happened (or vice versa), backward time travel is impossible. It’s a mistake, however, to think that backward time travel implies the ability to change the past. Stories, that involve time travellers allegedly changing the past, may be entertaining, but they’re logically incoherent. It’s impossible to change the past.
For backward time travel to be possible, there need to be only one instance of an event. There needn’t be a second one, with a possibly different outcome the second time around. Of course it’s undeniable that one could legitimately speak of times or events being repeated, but it would still be a mistake to think that this would enable a time traveller to change the past. If a time traveller is going to travel back to some past time, they have already been there. And if they are going to do something when they get there, they have already done it. Also, to elaborate, some time travel stories rely on the idea of parallel or branching universes. While this may provide a sense in which times or events can occur more than once, it won’t give us a case in which a time traveller can change the past.
Moving on to the classic objection to backwards time travel, which is usually referred to as the grandfather paradox. Well, the classic argument goes something like this. Suppose backward time travel were possible, then there would be nothing to stop a person traveling back in time and killing his grandfather before his grandfather had a chance to produce offspring. But then the time traveller himself would not be born. So the problem is, that if you could travel back in time and eliminate the causes of your own existence, you could make yourself both exist and not exist at the same time, which is surely logically impossible.
The reason that the grandfather paradox argument have such an appeal, it’s because, there’s an ambiguity or in the argument’s very first premise, being that if it was possible to travel backward in time, it would be possible to generate paradoxes. In Lewis’ analysis, as we learned in the lectures and I wrote above, each moment, each time, happens only once. If Lewis is correct, each moment happens once and once only, and the course of history is consistent over time. Because it’s a feature of possibility and compossibility, that what’s possible relative to one set of facts, may not be possible relative to another set of facts, Lewis says, backward time travel can be logically consistent provided you bear in mind that what’s possible relative to one of set of facts may not be possible relative to another set of facts.
Besides the self-undercutting “grandfather paradox” cases, the other great bone of contention is the causal loop; the event whose antecedent causal conditions prove to contain that very event itself. We have no complete explanation for the existence of any causal sequence (closed or linear) and we may just have to accept spontaneous creation of information in other cases. Explaining the existence of the whole loop may be a very different matter from explaining the existence of any loop-component.
Finally, if time-travellers travel within their own lifetimes and meet other temporal stages of themselves, there arise cases of multi-location where the same person exists twice at the same time. Lewis’s answer to multi-location is that our self-meeting time-traveller is really two different stages of the same four-dimensionally extended person. These stages can have quite different co-ordinates in their personal time despite their similar co-ordinates in global time. Having in mind that our world is four-dimensional we see that no paradox occurs.
Concluding, we find that none of the arguments considered here, being the most popular and obvious, shows that time travel is impossible. We can now see that time travel is logically possible.
Sources: [http://www.iep.utm.edu/timetrav/], [http://plato.stanford.edu/entries/time-travel-phys/], ‘The Paradoxes of Time Travel’ D.K. Lewis, and ‘The Paradoxes of Time Travel’ K. Perszyk, N. J.J. Smith