San Diego, contemporary

Craig Callender

Physics treats time differently from space, and so do we. Explain why physics does, and much of why we do follows. The flowing present of experience is not in the world, but it is no accident either.

The view

Callender is a philosopher of physics at the University of California, San Diego. He edited The Oxford Handbook of Philosophy of Time (2011), and his book What Makes Time Special? (2017) is a sustained attempt to connect time as physics describes it with time as we experience it.

He starts from a puzzle. In relativity, time and space are woven into one spacetime. Yet time keeps a special role: it has one dimension against space's three, a different sign in the geometry, and above all a different role in the laws. Explain that role, Callender argues, and you have the key to the rest.

The argument

The laws determine along time. The equations of physics let us take the state of the world across space at one moment and work out its state at other moments. They do not, in general, let us take what happens at one place throughout time and work out what happens at other places. Time is the direction in which the world is most simply and powerfully predictable.

Time is what the best laws make it. Callender is a Humean about laws: the laws are the simplest, most informative summary of everything that happens. On his view, the direction we call time is singled out because describing the world as evolving along it gives the best summary.

Time is special because it is the direction the laws work best in. after Callender, What Makes Time Special? (2017)

The manifest image follows. Creatures like us exploit that predictability to survive. We gather information about the past and act on the future. Our perceptual systems bind events into a shared present, which works because, for nearby creatures, the delay in signals is small compared with the time it takes to process them. Hence our sense of a common now, of flow, and of a fixed past and open future.

But the manifest image is not the world. None of these features, the shared present or the flow, is a fundamental feature of physical time. They are features of the way agents like us are built to use it.

Where he meets Deutsch

Deutsch's chapter makes Callender's starting point vivid. In spacetime physics, he says, given everything that happens at one moment, the laws fix what happens at every other: determinism along time is the glue that holds the snapshots together. That is exactly the property that Callender takes to make time special.

Both also explain the experienced now and its flow as features of observers rather than of reality, and both point to memory and records as the source of the asymmetry between past and future.

Where they part

What are laws? Callender's laws summarise what happens. Deutsch treats the laws of physics as real explanatory structure: they determine which snapshots exist and how they relate. For Deutsch, a law is not a description of the pattern but what makes it.

Time beyond determinism. In Deutsch's multiverse, a single snapshot no longer fixes the next one. Groups of snapshots determine groups, and the classical sequence of moments is an approximation. Callender's account is built around single-world determinism along time, and it is not clear how much survives when that breaks down.

Causation. Like other Humeans, Callender treats causation as derived from the pattern of events. Deutsch insists that causation is objective because the variants it depends on are physically real.

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Further reading

• Craig Callender, What Makes Time Special? (Oxford, 2017).
• Craig Callender (ed.), The Oxford Handbook of Philosophy of Time (Oxford, 2011).
• Craig Callender and Ralph Edney, Introducing Time (Cambridge, 2001): a graphic introduction.
• Time, Stanford Encyclopedia of Philosophy.