We all experience the passage of time. This is not an illusion. We agree on the serial delivery of moments, and it is very hard to dislocate us from this agreement. However, the core of this experience can be located as a feature of our physical theories. These theories do not replace our notion of passage. Rather, physics provides a reliable account of the regularities underlying our common experience of time passing. It does this by providing us with a concrete relationship between any given now, together with its future and past.
1. We Experience Time Passing
We can apprehend the passage of time through the succession of three mental states, in this order:
- the expectation of an event;
- the experience of the event; and
- the memory of the event.
We prepare for lunch, we eat lunch, and we remember how tasty it was afterward. Moreover, we all agree that this is the way it works — the order doesn’t change.
This is an account of passage that John Norton has proposed. As John suggests, this agreement shows all the signs of not being an illusion. There’s no known procedure that eradicates the experience of passage. There’s no obvious underlying mechanism that reduces passage to an artifact of our perception. The best explanation of the reliable succession of these three mental states is that they reflect an objective regularity in the world. Passage is real. It produces the serial delivery of moments to our minds. We become aware of it through the succession of these three identifiable mental states.
2. Physical Theories Describe Passage
In a physical theory, passage appears as a relationship between matter at any given moment (the ‘now’) and its future and past developments. This relationship is determined by Cauchy evolution. The regularities appearing in Cauchy evolution correspond precisely to the regularities we experience with passage. This gives us good reason to suspect they reflect the same objective feature of reality.
We have said that the core feature of passage is the serial delivery of moments: a future event becomes present, and then recedes into the past. Another way to describe this situation is to say that, once a now has been specified, we are guaranteed a definite future and a definite past. These categories never get mixed up: the past cannot arrive after now; the future cannot arrive before now. There is some real regularity that guarantees future, present, and past will be delivered to us in the right order.
The regularity underlying our experience of passage reflects the same reality as the regularity underlying the way physics describes change. In physical theories, this change is called evolution. Here is how the regularity appears.
We can input a configuration of matter representing ‘now’ into a physical theory. These are called (local) initial conditions. Once now has been specified, the theory determines (in the most common physical situations1.) a definite future and past. The equations of motion of our theory determine the moments in the past from which our matter evolved, as well as the moments in the future to which it will evolve soon. This separation into past, present, and future states of matter matches the key feature of passage: the serial delivery of moments is fixed once we specify our ‘now.’
For example, I might input my ‘now’ to be 10am this morning. My matter configuration at 10am is determined (according to some hypothetical theory) to have evolved out of my bed at around 7am, and is determined to evolve to eat lunch at noon. When I set my ‘now’ to be noon, then everything changes in a regular way. I am determined to have evolved from my 10am configuration, and determined to wash my dishes soon. And so on.
This is a striking match of our experience of the serial delivery of moments. At 10am, I am in a state of remembering my bed, and a state of expecting to eat lunch. This is followed by my noon state of remembering my 10am state, enjoying my lunch, and expecting to do the dishes.
Craig Callender (2000) argues that Cauchy evolution is what makes time informative. Here, we have reached a simpler conclusion: Cauchy evolution is how physics reflects time’s passage.
3. Experience and Theory Reflect the Same Thing
The human mind apprehends the passage of time through a change of state: we expect, we experience, and then we remember. This is a fact about our human experience. The steadfastness of this experience suggests it reflects an objective feature of the world.
Physical theory describes passage by describing a state’s evolution: given an initial state, we determine the past states as well as the future ones. This is a fact about our physical theories. The empirical confirmation of these theories suggests that they too reflect something objective about the world.
This does not mean that our apprehension of passage can be reduced to the physical description of passage, or vice versa. Rather, our conclusion should be that both the experience and the physics are very likely reflective of the same phenomenon. Physics is not devoid of passage. It is even more evidence for its existence.
1. This is not to ignore the difficult problem of specifying, and justifying, the conditions under which the Cauchy problem is well-posed. There are many well-known cases in which initial conditions fail to settle future/past evolution. However, it is sufficient for us to note that in a great many cases, and in almost all common applications, initial conditions can be chosen so as to guarantee a well-posed Cauchy problem.
Soul Physics is authored by Bryan W. Roberts. Thanks for subscribing.
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