The Depth of Time

Immediacy, aliveness, and the temporal depth of field

This research note is part of the Clockless Clock book project and may turn into a chapter.

We often talk of “deep time,” especially in the context of time scales that are extremely long relative to the human lifespan, such as geological or astronomical time scales. But why deep?

Time, or at least Cartesian time, is famously supposed to be one-dimensional. It is usually understood via a spatial metaphor in terms of a point “now” position along a single forward/backward dimension.

What is this orthogonal “depth” dimension? And how does “deep time” relate to the other popular notion of the “long now”?¹

Are adjectives like deep and long merely about adding some poetry to a one-dimensional experience of time, or do they point to, and reify, meaningful aspects of the phenomenology of that experience?

An idea from optics called depth of field, suitably generalized, helps us think about this question. The depth of field of a lens is the distance between the nearest and furthest objects that can be in acceptably sharp focus together. We can think about deep time by constructing an analogous notion of a temporal depth of field, which will then allow us to construct a coherent notion of “length” for now.

To peek ahead, think of the temporal depth of field as the distance² between the slowest and fastest processes of change that you can attend to simultaneously.

If you could simultaneously attend to the flapping of a hummingbird’s wings and the drift of the tectonic plate under your feet (assuming there isn’t an earthquake going on right now), you’d have an extraordinarily high temporal depth of field.

This understanding allows us to characterize the “long now” as well — it is an experience of now-ness during which we attend to reality with an unusually high temporal depth of field. We can also define a corresponding notion of a “short now” that features a high temporal depth of field past the lower limits of our biological capacities, such as nanoseconds to microseconds. Matrix bullet-dodging time is an example (unfortunately fictional) of “deep time” at the ultrashort end of now.

Why should we care about deep time? Because cultivating a greater temporal depth of field, and being able to vary the length of “now” from ultrashort to ultralong, makes the experience of time richer and more vivid. You’ve probably already had accidental tastes of this — perhaps a sudden, momentary sense of ineffable precious depth while contemplating a historical artifact or astronomical image. Or an intense stab of delicious nostalgia upon being exposed to a particular smell, redolent of childhood memories. Or a moment of an empathy for an insect embarking on what to it must be a long journey across the living room. These are long-now and short-now glimpses of deep time, and it is possible to systematically cultivate our ability to sense it.