Some physicists have argued that time is unreal, but philosopher Brian Kierland argues this is a mistake. Not only can we imagine time passing without anything changing in the physical world, the claim that time flows independently of physical change may be verifiable, at least in principle. This makes time more real than matter itself. The growing push among some physicists to eliminate time from fundamental physics deserves more skepticism than it gets.

“Can there be time without change?” is an ancient question. Many, from Aristotle and Leibniz to McTaggart and Mach, have thought that time (in the sense of time passing) requires change. It’s undeniably a natural thought, as all direct measurements of a duration of time use or involve change: change in shadow positions (sundials), change in clock-hand positions (old-fashioned clocks), change in energy levels of electrons in cesium-133 atoms (atomic clocks), etc. Relatedly, our experiential awareness of the passage of time surely involves some kind of change in mental states.

Besides being a fascinating metaphysical issue in its own right, whether time requires change connects with several other issues in the metaphysics of time:

(1) McTaggart famously argued that time is unreal, and a crucial premise in his argument is that time requires change (although McTaggart had a somewhat special conception of change, one on which genuine change specifically involves change in temporal features like being present and being past).

(2) Paralleling the famous debate between Newton and Leibniz about whether space is absolute, there is the debate between absolutism about time (time is an entity in its own right, independent of the objects and events in time) and relationism about time (time is nothing more than certain relations between, or some kind of ordering of, the objects and events “in” time); absolutism implies the possibility of time without change (so that, if time requires change, absolutism must be false), whereas whether relationism does so depends on the particular version in question.

(3) If time requires change because time just is change (i.e., time is nothing more than an ordering of objects and events in terms of change), then certain views in physics and metaphysics become live options, such as (a) certain versions of the block-universe conception (reality is a “static” four-dimensional spacetime block), a conception that some take to be supported by Einstein’s Theory of General Relativity, and (b) physicist Julian Barbour’s interpretation of quantum mechanics, on which Schrodinger’s time-independent wave equation in effect describes the independent creation of physically unconnected “instants of time” (although he himself appeals to change, or at least something in the near neighborhood, not to give an account of time but, instead, the illusory appearance of time).

In recent decades, one of the most-discussed arguments in philosophy bearing on the issue of whether time requires change centers on an exceptionally creative thought experiment given by the American philosopher Sydney Shoemaker in 1969. In what follows, I present and critically assess Shoemaker’s argument.

Rather than directly arguing that there could be time without change, Shoemaker argues that there could be empirical evidence that a period of time has passed without any change. This claim is obviously important: if there couldn’t be evidence of time without change, then even if time can exist without change, we could never know about it!

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No one ever observes a time when everyone is asleep. Should we, for this reason alone, infer that, at all times, at least one person is awake?

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