A gust of air whirled round me as I opened the door, and from within came the sound of broken glass falling on the floor. The Time Traveller was not there. I seemed to see a ghostly, indistinct figure sitting in a whirling mass of black and brass for a moment–a figure so transparent that the bench behind with its sheets of drawings was absolutely distinct; but this phantasm vanished as I rubbed my eyes. The Time Machine had gone. Save for a subsiding stir of dust, the further end of the laboratory was empty.
–The Time Machine, by H. G. Wells, 1898.
Travel, of any kind, involves journeying from one point to another point. In the case of spatial travel, this involves journeying from one location in space to another location. This sort of travelling happens all the time. For example, millions of people make the trip from the fridge to the couch each day. Time travel, being a form of travel, also involves journeying from one point to another. However, in the case of time travel, the journey is made from one point in time to another point in time. While it might seem an odd sort of thing to say, time travel is happening all the time. In fact, you are doing it right now. Even as you read this sentence you are travelling towards the future at the rate of sixty minutes per hour. Of course, that sort of time travel is not what most people find interesting. One of the more interesting types of time travel involves moving from the present time to the past. Another interesting type of time travel is going into some future time, without all that time consuming mucking about in all the time between now and then.
While it has been claimed that if a person travels far enough, she will end up back where she started, no one claims that if you travel far enough you will meet yourself. However, if time travel is possible, a person should be able to travel back in time and, in theory, meet herself in the past. If a person takes care not to travel too far ahead in time, he should be able to meet himself in the future.
Naturally, there are all sorts of problems and paradoxes involved with people traveling about in time to meet themselves. For example, suppose Bill, who is thirty four now, decides to go back in time and kill himself at age twenty. Obviously, if Bill succeeds in killing himself, he would not exist at age thirty four. Hence, he could hardly go back to kill himself. Yet, if Bill is able to travel through time, he should be able to go back and kill himself. These sorts of problems probably help fuel the sale of aspirin.
Fortunately, this article is not about those brain teasing paradoxes. For example, I will not argue whether Bill would be committing murder or suicide if he went back in time to kill himself. Instead, I will focus on the problem of simply going back in time and meeting yourself. Or your past self. Or however one would word it.
As has been noted above, if you can travel backwards in time, then you should be able to meet yourself. This creates an interesting metaphysical problem, that of explaining how the very same thing, namely you, can be in two places at exactly the same time.
In order for the same person to be in two different places at the same time, the components that make up the person would, of course, have to be capable of existing in more than one place at the same time. In other words, the components would have to be capable of multiple location.
Another philosophic problem, namely the problem of universals, also involves the issue of the same thing existing in different places at the same time. Very, very briefly, one part of the problem of universals is determining what it is for two tokens to be of the same type. To give a concrete example, part of the problem would be determining what it is for six different green objects to all be the same in respect to their colour. Two popular solutions to the problem of universals, as it relates to the possibility of entities existing in multiple locations at the same time, are as follows.
David Armstrong, a well known Australian philosopher, argues that there are instantiated universals. Briefly, an instantiated universal is a property (such as being green) that can exist in multiple locations at the same time. Going back to the problem of universals, for six different objects to all be green would be for each object to instantiate the universal green. The very same, identical universal green would be wholly located at each green object. To be even more specific, if a frog and a leaf are the same shade of green, the green of the frog and the green of the leaf are one and the same entity which happens to be multiply located.
Now, suppose that any person is essentially a unique set of instantiated universals. In this case it would seem that going back in time to meet yourself would be possible. What would make this possible? First, it has been assumed that what makes a person who he is, say Bill, is made up of universals. Second, a universal, as has been established, is capable of existing in distinct locations at the same time. Hence, the universals that make up the person Bill happens to be can exist in different places at the same time. So, it would be possible to have a person identical to Bill standing five feet from Bill. This identical person could be the Bill from the future. Since Bill and Bill from the future would be identical, then it would seem they would be the same person. Hence, if a person is composed of universals, then he could travel back in time to meet himself. It would simply be a case of the same person existing in different locations at the same time.
Keith Campbell and I, among others, reject instantiated universals in favour of tropes. Briefly, a trope is a property (such as being green) that can only exist in one location at one time. Trope theorists explain what it is for two tokens to be of the same type in terms of resemblance. As an example, for six different objects to all be green would be for each object to have its own distinct green trope. Each green trope would be a different entity from the other green tropes, but they would resemble each other and would all be taken to be green because of their resemblance.
Now, suppose that any person is essentially a unique set of tropes. In this case it would seem that going back in time to meet yourself would be impossible. What would make this impossible? First, it has been assumed that what makes a person who he is, say Sally, is made up of tropes. Second, a trope, as has been established, is incapable of existing in distinct locations at the same time. Hence, the tropes that make up Sally cannot exist in different places at the same time. So, it would not be possible to have a person identical to Sally, say the Sally from the future, standing five feet from her. Thus, if Keith Campbell and I are correct, it would seem that a person could not travel back in time and meet herself. This would also entail that time travel is not possible.
Since I am committed to tropes, but find the notion of time travel fascinating, it would be nice if there was a way to reconcile trope theory with time travel. Perhaps there is a way of doing this.
According to modern physics, which is based on Einstein’s special theory of relativity, there is no such thing as absolute and universal time. Instead, time is seen as being relative to each thing. In this context, time is relative in the sense that each thing carries around its own personal time scale which does not, in general, agree with the time scale of other entities. The relativity of time is subject to empirical proof. For example, if one precision atomic clock is left on earth and another is placed into the American space shuttle, the clock in the shuttle orbiting the earth will lag behind the clock left on earth. The difference in time is due to the speed of the shuttle and its location in earth’s gravity well. Given the fact that this experiment has been conducted, it is hard to deny the relativity of time.
Once relativity is established, the notion of same time goes out the window. Each thing has it own time scale which varies with its location and speed, so there simply is no objective basis upon which sameness of time can be grounded. In this case, nothing can be in two different locations at the same time. Roughly put, being in a different location would put it in a different time.
One effect of the relativity of time would seem to be the end of instantiated universals. This is because an instantiated universal has to exist in different places at the same time. Since there is no such thing as sameness of time, instantiated universals simply cannot exist as defined.
A second effect of the relativity of time is that it enables time travel to be reconciled with tropes. The way this happens is as follows.
It is contended that people can be made of tropes, yet still be able to travel back in time to meet themselves . For example, imagine that Bill has travelled back in time to ask himself where he left his keys. Bill tells future Bill where they are in return for a bit of advice on how to play the ponies later on. Both Bill and future Bill can be composed of identical tropes, yet still meet. This is possible because Bill and future Bill, like any other entities, will actually exist at different times. Hence, there is no need for Bill and future Bill to exist at the same time. They simply have to have their times “close enough” to allow them to interact. Thus, I can have my tropes and time travel, too.