In this article, I provide answers to a question that anyone in a current or future relationship who believes time travel is possible should have. Many couples these days make sure that their significant other and themselves are on the same page regarding romantic interest in others outside of the relationship. From open relationships, to polyamory, to monogamous relationships, the consensus about what is important is consent—and rightly so. While I am completely accepting of all of these kinds of arrangements between consenting adults, there is one question I think those in committed romantic relationships of all kinds are not concerned with, but should be. This is the question of whether a time traveller who sleeps with a past or future version of their significant other has crossed a moral boundary. Or, more simply, ‘is my time travelling romancer a cheater?’ In this article, I will gloss over some of the things one should consider when answering this question.

Picture the following scenario. You are in a committed relationship. You and your partner are completely monogamous and have both agreed that any romantic or physical involvement with another person would be cheating. Your partner meets a scientist interested in time travel who promises to have invented a time machine. Low and behold, the time machine works and your partner travels 1 year into the past. They are enamoured by you, 1 year younger, and enjoy a night of, well, let’s say, passionate romance. Have they cheated on current you?

The answer to this question depends on whether time travel is possible (i.e., if there’s somewhere to travel to), how objects persist across times in general, and whether you would survive such time travel. The first issue concerns one’s metaphysical views about time, the second, the persistence of ordinary objects across times, and the third, your personal identity across times.

Is there somewhere to which we can time travel?
As for the first category, one can broadly speaking be an A-theorist or a B-theorist. A-theorists think that the present is objectively real, and privileged. B-theorists think that all times exist and are equally real—times only differ from other times based on "earlier" and "later than" relations. Versions of the A-theory are supported by the theory’s apparent compatibility with the ordinary or folk idea of time, while the B-theory has in its favour its parsimoniousness with our best physics. The issues I will discuss in this article only matter for those who think that time travel is possible.

Time travel will only be possible, then, if there are times to which we can travel. The B-theorist accepts some form of time travel, then, because there are times to which one can travel. Things are a little more nuanced for the A-theorist, however. Suppose you are an A-theorist who believes that the past and present exist (so called growing block theorists), or that the past, present, and future exist (so called moving spotlight theorists). There is, according to such views, somewhere to time travel to. Just as I can travel in space to, say, India, I can travel in time to either some past or future time. However, presentist A-theorists think that the present is the only time that exists, so they will deny the possibility of the sort of time travel that is our current focus. The presentist has an advantage over other kinds of A-theorists at least insofar as they don’t need to consider tricky time travel scenarios. There are, however, many disadvantages to the view that will not be discussed in this article. Moreover, given that many A-theorists think that time travel is possible it is worth looking at how views which accept that time travel is possible can accommodate the scenario presented in this article. For the remainder of this article, then, I will ignore the previously mentioned presentist version of the A-theory, as there is nowhere to time travel on such a view.

Now, regardless of which of these views one signs up to, following David Lewis' article, most philosophers of time travel hold that we cannot change the past when we time travel. I cannot, for example, go back in time and kill my grandfather. Such a scenario would create a paradox whereby I both exist and do not exist—i.e., I would have to exist in order to time travel and kill my grandfather, but this move would ensure that my father never came into existence, and, by extension, me. So, let us assume that any action one performs while having time travelled was always the case. For example, when I travel back in time and see the first episode of Seinfeld air, it was always the case that there was a time traveller watching Seinfeld in 1989. If your time travelling romancer travels backwards and is romantically involved with someone, it will always have been the case that this happened.

How do objects in general persist?
As far as the second category, persistence, goes, there are roughly three types of positions. One could be an endurantist, a perdurantist, or an exdurantist. Endurantists think that objects persist through time by being numerically identical—the exact same object—at all the times at which they exist. Perdurantists think that each object persists through time by having different temporal parts at different times—the combination of each of these temporal parts is then the object itself. And, similarly, exdurantists think that each object persists through time by having different temporal parts at different times. However, the exdurantist holds that all that exists are these temporal parts and each of these parts is individually a complete object (i.e., there is no overall object). So, for example, take an apple. The endurantist will think that what makes the apple persist over time—i.e., what makes it the case that the apple I eat today is the same object I bought at the shops yesterday—is that the apple that I eat today is identical to the apple I bought yesterday.

The issue with this view is that what is known as Leibniz’s Law. This law tells us that if two things are identical then they must be the very same thing. However, if one thinks that more than just the present time exists, as many A-theorists and B-theorists do, then it can’t be the case that the apple that exists yesterday and the apple that exists today is the same object. One kind of A-theorist, the presentist, holds that only one time exists, while all others used to exist or will exist. This avoids the problem with Leibniz’s Law. Next, both the perdurantist and the exdurantist hold that the apple I bought yesterday persists through time by having a certain temporal part located at a time yesterday, and a different temporal part located today. Each of these temporal parts is then connected to one another, such that the temporal part that exists at a time yesterday is not identical to a temporal part that exists today. The perdurantist, then, will say that each of these temporal parts put together is strictly speaking the apple, while the exdurantist will say that all that there is are these connected temporal parts or stages and no overall apple. But this is fine, so says the exdurantist, as what really matters for persistence is connection with other temporal stages, rather than identity

Are you the same person across time?
Finally, let us briefly look at the third category of views, views about personal identity—i.e., the identity of persons across times, rather than all objects. There are roughly three broad types of views about personal identity. One could be a psychological continuity theorist, a physical continuity theorist, or a desire-first theorist. Psychological continuity theorists think that what makes me, or anyone, the same person across time is having certain psychological facts true of each of the times at which I exist. So, for example, I am the same person as I was when I was four years old because the psychological features true of me today are connected to the psychological features true of 4-year-old me. On the other hand, the physical continuity theorist thinks that it’s a chain of physical continuity between my current stage—or temporal part—that makes me the same person as 4-year-old me. So, even though my cells aren’t the same now as 4-year-old me, they are connected by all the stages in between—including the stages just when each change in cells first appears.

To see the difference between the two views, let’s consider a case first discussed by Derek Parfit in Reasons and Persons. Imagine that you can be transported to another place and time using a new invention—a teletransportation machine. Your psychological features will be exactly the same on the other side of the teletransporter. The catch, however, is that your body will be destroyed in the process and rebuilt on the other side of the machine. Everything will appear to be exactly the same—however, in reality, the chain of continuity between your body before the teletransportation occurred, and your body afterwards, will have been broken. Supposing you wish to travel to another time and place, should you get into the machine? The thought goes that the physical continuity theorist will say yes, and the psychological continuity theorist will say no.

What to make of such vastly different intuitions about this case? Enter: the desire-first theorist. Desire-first theorists hold that whether or not one survives in such cases and should, therefore, enter the teletransporter, depends on one’s individual desires. If I believe in psychological continuity theory and so believe that I will survive such a machine, then it is true for me that I would survive teletransportation. On the other hand, if I am a physical continuity theorist then I would not survive teletransportation. Even if my friend, a strict psychological continuity theorist, thinks that I have survived teletransportation, they would in fact be wrong according to this view. What I believed, after all, was that I would die going through the teletransportation machine. My friend, then, has begun a new friendship with a neat substitute in this scenario. But it isn’t me.

Is your time travelling romancer a cheater after all?
So, then, let us turn to look again at the question with which we started this article. Is my time travelling romancer a cheater? In other words, if one time travels and sleeps with a temporal part of their partner, have they cheated on them?

Endurantism: According to all endurantists, if your partner were to go backwards or forwards in time and sleep with a different temporal part of you, they would not be a cheater. This is because endurantists hold that each temporal part of you is numerically identical to one another. So, if your time travelling romancer sleeps with a temporal part of you from one year ago—when you weren’t together—they have not cheated. That temporal part is as much you, as you are today. This is true whether or not one is a psychological continuity theorist, physical continuity theorist, or a desire first theorist. That being said, if it is the case that, in the process of time travelling, your partner does not survive—i.e., because physical continuity theory is true or because desire-first theory is true (and they desire physical continuity)—then it may be the case that the time travelling romancer is not your partner. In such a case, nobody has cheated, you’ve merely had a romantic dalliance in the past or future.

Perdurantism: Things are a little different if perdurantism is true, though not much. Recall that according to perdurantism you are all of your temporal parts combined. So, if your time travelling romancer sleeps with a temporal part of you from one year ago—when you weren’t together—they have not cheated. That temporal part is as much you, as you are today, as all of your temporal parts combined is you. Like the case of endurantism, this is true whether or not one is a psychological continuity theorist, physical continuity theorist, or a desire-first theorist. Moreover, again, if it is the case that, in the process of time travelling, assuming some teletransportation machine is used, your partner does not survive—i.e., because physical continuity theory is true or because desire-first theory is true (and they desire physical continuity)—then it may be the case that the time travelling romancer is not your partner. However, if one thinks the chain of continuity isn’t broken—even if it is weakened by teletransportation—then because all of the temporal parts of your partner together make up them, then it won’t be the case that they have cheated.

Exdurantism: Exdurantism, remember, is the view according to which all that exists is temporal stages. There is no ‘object’ overall—things merely persist through time by there being different temporal stages. These temporal stages are all there is to things and beings. The temporal stage of me that exists at this time is a whole object in its own right, rather than a mere temporal part of an object as perdurantism would have it. Depending on exactly how one cashes out exdurantism, then, we can see how the view leaves open that your time travelling romancer is a cheater after all. If your partner sleeps with some stage of you from one year ago, say, that stage is a wholly complete person. It isn’t the same person as the you of today is. Sure, you might be more strongly connected to that stage than other person stages, but the fact remains that it’s not you. All you are is the current stage. This means that if the view is correct your time travelling romancer may well be a cheater. However, on the plus side, the stage of your partner that you’re next in contact with won’t be the same stage who cheated.

Now of course, in some ways the views considered above need not directly affect a couple’s decision about what exactly counts as cheating. After all, it is as always consent that matters. For example, suppose you and your partner are both exdurantists. You don’t think that there is any single thing that is “you”. But even exdurantists will agree that ordinary cheating, that doesn’t involve time travel, occurs. They will hold that later stages in some sense “inherit” your relationship from previous temporal stages. So, as long as this inheriting of a relationship from other temporal stages continues, of course the exdurantist can cheat on their partner. However, when time travel comes into the picture things get more complicated. It’s less clear that, when travelling to a time at which you and your partner were not yet together, the temporal stage that time travels is bound by the decisions made in the objective future.

For example, suppose that you and your partner are together in the year 2023, when your partner time travels to 2024 and they sleep with a temporal stage in 2024. You are an exdurantist, but you are troubled by this turn of events. After all, the temporal stage in 2024, while connected to the current temporal stage, has, for all you know, nothing in common with your current temporal stage. But do you have a right to be troubled by this? Suppose that in objective time—the events that exist objectively at each time—the stages continuous with you and your partner in 2024 are no longer bound by this agreement. You break up, say, somewhere between 2023 and 2024. Your partner sleeps with some temporal stage in 2024, a time at which the two of you are not together. But perhaps this doesn’t matter. Perhaps it is enough that the fact that it is a temporal stage of your partner from 2023 who engaged in the 2024 dalliance for the scenario to count as cheating. Similar scenarios can be constructed for the other views on the table also. One can imagine a person feeling as though they have been cheated on in a time travel scenario, even if they are an endurantist and, so, seemingly their partner has slept with them, just at a different time.

There will be no one size fits all answer to the questions that arise in such scenarios. In other words, it is not as though these metaphysical views can tell you what you need to accept in any given relationship. That the endurantist, perdurantist, or exdurantist feels comfortable or uncomfortable with what their time travelling partner does is enough. However, given these considerations, it is important that you and your partner discuss what matters in such cases. Is it the agreement that exists in objective time that is important, or is it the stage of the time travelling romancer that is important? It’ll be up to the two of you, then, how best to move forward given the different views the two of you likely have about these scenarios.

To sum up, in this article I have provided a brief overview of the different views one would need to consider as a person who wishes to be in a relationship in the age of time travel. While I haven’t given any independent reasons to sign up to one view over any other, I hope I have at least provided an additional point one should consider once they side with some of these philosophical positions. At the very least, I hope I have illustrated that those in either current or future (and, given what we’ve discussed, maybe past?) relationships have more to consider and discuss if they believe in time travel. Otherwise, they may be left wondering whether or not their time travelling partner is a cheater.

I am curious to see what others think about such considerations—so please let me know in the chat below!

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