ABSTRACT
One of the central topics of philosophy is metaphysics, where we investigate the fundamental nature of reality and related questions. One goodway to begin to understand some of the central issues of metaphysics is to consider a topic which is interesting, wide-ranging and growing in popularity. So why time travel particularly? Well, there is an extensive (and fascinating) range of metaphysical issues centred on time: for example, questions concerning persistence and identity over time, the passage of time and even whether or not time actually exists. However, this chapter tackles a topic that might allow you to see how philosophers tackle an issue in metaphysics using logical analysis, and an issue that has plenty of potential for interaction with science too, namely the philosophy of time travel. It’s also the case that, odd as it sounds, where I might be hard put to it to offer a snappy, one-line definition of time itself, thanks to the work of American philosopher David Lewis (1941-2001), I can offer you a pretty snappy definition of time travel. (See below … ) Time travel, whether as a source of problems in logic, metaphysics or
physics, raises some serious, deep and seriously deep questions. The definition of time travel we’ll work with throughout comes from a philosopher (the aforesaid David Lewis), but it is a definition which is underwritten by physics just as much as by philosophy. Considering the topic of time travel might help guide our understanding of time, space, causation, identity and freedom, to say nothing of physical laws, computation, cosmology and the status of the past. So let me stress at the outset that our topic is travelling to other
times whether in the future or the past. Might such a thing be possible? If not, why not? If time travel were possible, what might that imply about time, freedom and even our selves? But before tackling those questions, let’s first consider this question:
The best and most famous philosophical paper about time travel to date is David Lewis’s ‘The Paradoxes of Time Travel’. David Lewis believed that it is logically possible to travel in time – forward or backward. What does that mean? Lewis argued that some time-travel journeys can be described without contradictions – that (to put it another way) time travel might take place in a possible world. (Maybe only a very strange world though – a world that differs from the world we think we live in in lots of ways.) Please note that claiming that time travel is logically possible is not
at all the same thing as claiming that time travel is physically possible – still less is it to claim that time travel is actually going on. This is an important point because Lewis’s argument does not carry any commitment at all to time travel being physically possible or technologically possible or actual. One might accept Lewis’s arguments and yet believe that time travel cannot occur. As Lewis emphasizes, a time-travel world, even if possible, might be very unlike our world. It’s still a remarkable thing if time travel is logically possible. Show-
ing that something is logically contradictory is a very powerful weapon in the philosopher’s armoury – it’s one thing to make a claim which is merely factually wrong but it’s quite another to fall into contradiction. Lewis offers this very helpful definition of time travel:
What is time travel? Inevitably, it involves discrepancy between time and time. Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival (positive, or perhaps zero) is the duration of the journey. But if he is a time traveler, the separation in time between departure and arrival does not equal the duration of the journey.
