The Possibility of Time Travel

Preliminaries:

Metaphysics is the branch of philosophy that I have the least interest in. However, certain metaphysical topics are quite interesting and thought-provoking. One such topic is the possibility of time travel. I'm sure that we, at some point or the other, have read fictions or watched movies that talk about people engaging in time travelling. For example, it is mentioned, from time to time, in Doraemon - a popular Japanese manga. Some philosophers have discussed the possibility of time travel and have suggested certain paradoxes entailed by it. In one of my philosophy courses, called Epistemology & Metaphysics, I was given a list of essay topics and asked to write a paper on one of those topics. I've chosen the topic The Possibility of Time Travel, which is described below:

Here is my essay, copied and pasted below, with some parts modified and some links and titles added:

Outline:

1. General introduction and definitions
2. The Grandfather Paradox
           2.1. The argument (in explicit form)
           2.2. Lewis' views on the argument
3. Prior's A-theory vs Lewis' B-theory
           3.1. Prior's A-theory
           3.2. Lewis' B-theory
4. My argument
           4.1. Logical possibilities of time travel and the killing of one's grandfather
           4.2. Lewis' view is more plausible than Prior's
5. Conclusion

1. General introduction and definitions:

Time travel is the idea that we can travel through time, that is, travel into the past or into the future. Lewis maintained that time travel is a logical possibility, meaning that it is logical relative to the laws of logic. His view is a four-dimensionalist view, which is the view that persisting things (objects or people) are to be reduced to sequences of momentary things call “time-slices of objects” or “person-stages”.

In this paper I will discuss the main argument against time travel using the Grandfather Paradox, as well as Lewis’ assessment of it using the concept of causal possibility – possibility relative to the laws of nature, and logical possibility. Next, I will explain Prior’s view on the notion of time and how anyone holding his view will disagree with Lewis. I will also mention how Lewis is a B-theorist, a theorist who maintains that time is defined in terms of dates and that four-dimensionalism is true, whereas Prior is an A-theorist, a theorist who maintains that time is defined in terms of past, present and future (that is, relative to now), and that four-dimensionalism does not hold. Next, I will provide my argument, which discusses how it is logically impossible for anyone to kill their grandfather while time travel remains a logical possibility, and that Lewis’ B-theory is a more plausible theory than Prior’s A-theory. This paper will end with a short summary of all the above along with some final comments.

2. The Grandfather Paradox:

2.1. The argument (in explicit form)

1. Suppose that you travel into the past, which is the time inhabited by your grandfather, and his enemy Y.
2. If you have really gone back to the past then you can act there in much the same way as the people who were in the past can.
Therefore,
3. Y can kill your grandfather, and similarly, you can kill your grandfather. (from 1, 2)
4. But if you killed your grandfather, you would not be born and you would become non-existent.
Therefore,
5. You cannot kill your grandfather. (from 3, 4)
So, the argument generated a contradiction since it deduced that,
6. You can and cannot kill your grandfather. (from 3, 5, reductio ad absurdum)

2.2. Lewis' view on the argument

Lewis argued that there are two senses of “can” in the Grandfather Paradox: The “can” in the second sub-conclusion means capability relative to causal laws. That is, assuming that you are back in the past, you have the ability, for example, the necessary strength, aggressiveness and weapons to kill your grandfather. The “can” in the conclusion, on the other hand, is the “can” relative to the laws of logic. That is, something will always prevent you from killing your grandfather, since doing so will result in your non-existence. And if you are non-existent, then you cannot kill your grandfather, and this violates the laws of logic.

Lewis also argued that the paradoxes of time travel are oddities but not impossibilities. By “oddities” he meant that a world in which time travel is possible is a strange one; it is contrary to what usual people think. However, it is not an impossibility: Here he meant that there is no reason to think that time travelling is a logical impossibility and so does not violate the laws of logic.

3. Prior's A-theory vs Lewis' B-theory:

3.1. Prior's A-theory

Prior maintained that the past is fixed and the future is open. As time progresses, more and more facts are fixed, and more and more (future) possibilities get closed off (232 Lecture Number 22). He said that “we can lay it down as a law that whatever now is the case will always have been the case; but we can’t interchange the past and future here and lay it down that whatever now is the case has always been going to be the case.” (“Some Free Thinking About Time” p. 48) and “Past and future are in fact not to be defined in terms of earlier or later, but the other way round – ‘X is earlier than Y’ means ‘At some time X was past and Y was present’, and ‘X is later than Y’ means the opposite of this” (“Some Free Thinking about Time” p. 50).

Prior is presenting an A-theorist view here since he defines time only in terms of past, present and future (that is, relative to now) and nothing else. He also sees time as passing, and that the events in the past become unchangeable while those in the future contains many (unknown) possible events. Hence, according to Prior, it is not the case that all the events, including past, present and future, are fixed. There is no such thing as a tapestry of time, where all the events are already lay out.

3.2. Lewis' B-theory

On the other hand, Lewis holds a B-theorist view. First, he advocates four-dimensionalism, and argues that there is a tapestry of time. He holds that time travel is possible since travelling into the past or into the future simply means that there are two person-stages of you in the tapestry of time and there is nothing absurd about it, and this view assumes four-dimensionalism. In other words, Lewis’ view is merely the opposite of Prior’s – Lewis sees time and all the events (past, present, future) as fixed, not passing. And in the case of future events, they are not unknown possibilities, but fixed events. Lewis then, as a B-theorist, would say, for example, “there is no time later than the time of this utterance at which X is stronger than he now is” instead of “X will be stronger” because an event that X will become stronger is actually fixed, while Prior would say the complete opposite.

Therefore, anyone who supports Prior’s view will disagree with Lewis because he will say that four-dimensionalism is false and that time and all the events are not all lay out at once.

4. My argument:

4.1. Logical possibilities of time travel and the killing of one's grandfather

First, I will show that there is still a paradox in the Grandfather argument even though there are two different senses of “can”.

First, if I detest my grandfather and think about killing him, and suppose that I can travel back to his time and perform the killing. Assume that I successfully killed him. This will, in effect, prevent my father from being born, which will prevent me from being born as well. Therefore, I will become non-existent, meaning that there is no way that I can even detest my grandfather or think about killing him, much less being able to do it. But I did have the detestation and the thought of killing him in the first place. So I end up with at least two contradictions: I detest and I do not detest my grandfather, and I think about killing him and do not think about killing him. In other words, the entire chain of events, from the hatred for my grandfather to the desire to kill, to the eventual action, both exists and do not exist. This is a contradiction, and contradictions are logical impossibilities. It then follows that it is logically impossible to kill one’s grandfather.

However, this does not imply that the time travelling itself is logically impossible as well, for I might be able to travel into the past but fail to kill my grandfather. It is possible that, for example, lighting struck all of a sudden which distracted me from killing him, or I might shot him with a rifle successfully but he somehow recovered, etc. In other words, having what it takes to kill him does not mean that I will do so with success, for it is reasonable to think that there might be some unexpected events that prevent me from doing the killing, and this has nothing to do with my abilities.

4.2. Lewis' view is more plausible than Prior's

I will now discuss how Lewis’ view is more plausible than Prior’s.

First, there is no questioning that the past must be fixed, and that the present is also fixed when it is happening. The question here is whether the future is fixed as well. Now, out of the many possible events in the future, some must happen and some mustn’t, and it all depends on the present events. I first argue that the present events determine the future ones, for example, if I punch someone (present), then it leaves two possibilities: He will either punch me back or he won’t (future). Suppose that he does punch me afterwards. In my perspective, there are two open possibilities only because I do not know how he will react beforehand. But in reality, there is only one possibility - being punched by him and this is fixed. In general, future possibilities are not fixed in our perspective only because we are not aware of them, but it does not imply that the possibilities themselves are. In other words, certain future events must happen, as fixed (i.e. there are no other possibilities), it’s just that we don’t know what they are yet.

I have, in effect, argued that there is a tapestry of time since I have argued how the past, present and future events are all fixed. Since I have argued in support of Lewis’ B-theorist view, therefore I have argued that his view is more plausible than Prior’s A-theorist view, that B-theorist view holds while the A-theorist view doesn’t.

5. Conclusion:

The Grandfather Paradox attempted to show that time travel is a logical impossibility by deducing that one can and cannot kill one's grandfather. The different senses of "can": causal "can" and logical "can" were introduced by Lewis in order to avoid the paradox. Prior's A-theory and Lewis' B-theory, and the contrast between them, were also introduced in the discussion of time travel. In my argument I have pointed out that travelling into the past in order to kill one's grandfather is logically impossible while time travel remains logically possible (more precisely, the former does not entail the negation of the latter). Furthermore, there is such thing as a tapestry of time because all events are fixed in reality even though they are not the case in our (humans') perspective, hence Lewis's view is more plausible than Prior's.

So, I have discussed the different senses of possibilities relating to the Grandfather Paradox, Lewis’ and Prior’s views, and my argument showing that I agreed with Lewis. Although the idea of time travel seems strange, it still remains an interesting subject to be pursued by many, including myself.

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