Time, metaphysics of

2. Time and space

The difference between the A-theory and the B-theory can be illustrated by considering how each of them compares time with space. According to the A-theory, time is fundamentally different from space. Time, unlike space, is essentially dynamic. Furthermore, the temporal distinction between past, present and future marks (for many, though not all A-theorists) an ontological distinction, such that what is present is more real than what is past or future. But there is no spatial distinction that is also an ontological distinction; all places are equally real, no matter where they are.

According to the B-theory time is much more like space than we ordinarily think. For one thing, all times are equally real, no matter when they are located, just as all places are equally real no matter where they are located. Furthermore, time is not dynamic, but static, as we ordinarily think space is.

When we use spatially indexical terms, such as ‘here’ and ‘5km to the west’, we take their reference, on any occasion of utterance, to be determined by the place at which they are uttered. According to the B-theory, we should treat temporal indexicals in an analogous way. The term ‘now’ refers, on any occasion of utterance, to the time at which it is uttered. By contrast, the A-theorist treats this term as referring to the time that is the ontologically privileged present moment. For the B-theorist the term ‘three days ago’ refers, on any occasion of utterance, to the day three days prior to the day on which it is uttered. For the A-theorist it refers to the day that is, objectively, three days past.

This is not to say that time and space are analogous in all respects according to the B-theory. There is no privileged direction in space, but there is in time. The A-theory accounts for time’s direction by appealing to the passage of time itself. B-theorists account for it by the existence in time of asymmetric processes, such as, for example, the fact that causes always precede their effects, or that entropy is always increasing. This has implications for the possibility of time travel (see Time travel). A-theorists are, arguably, committed to the view that time travel is, in principle, impossible. If the past and the future do not exist, a view adhered to by presentists (see Presentism), then there is nowhere (nowhen) for the time traveller to visit. B-theorists typically think it impossible only because we are not presently able to subvert the aforementioned asymmetric processes.