What takes time?


To what extent can science have a rationally consistent basis, given that its concepts are grounded in the everyday experience of a biological creature? Biologically based experience need not be rational or internally consistent, only consistent with survival. Many of the most basic concepts of physics are derived from common sensory experience, including space and time, force and causality. Some conceptual difficulties of physics may arise inevitably because of human thought patterns, rather than inconsistency in the physical world. The wave-particle duality, for example, is rooted in ancient unresolved conundrums—of the void and the plenum, or the discrete and the continuous, or the one and the many.

The Greek atom was by definition a discrete indivisible unity, without parts or internal structure, though separated by physical space from other atoms. Space itself was also uniform. But natural intuition, based on everyday experience, tells us that any material thing can have parts—and so can the parts have parts, indefinitely. Conceptually, at least, anything with extension can be divided. And, if something has properties as a whole, these may be explained in terms of the properties and interactions of its parts. This was the advantage of atomism, which bore fruit in the ability of the modern atomic theory to explain chemical properties of various substances. However, of course, it was discovered that the atom is not an indivisible unity, but itself composed of parts, which in turn can explain its properties. Natural intuition suggests that electrons and protons must have parts that can explain their properties. Do quarks too have parts—and their parts have parts?

While there is no logical end to the decomposition of things as constituted by parts, there could be a physical limit. On the other hand, logic itself is based on intuitions derived from ordinary experience. For example, the tautology A=A may be based on the empirical observation of continuity over time, that things tend to remain themselves. Similarly, the principles of set theory depend on a spatial metaphor derived from common experience: the containment of elements within sets. It would be circular thinking to imagine that physical reality must obey a logic that is derived from observing physical reality in the first place!

Similarly, ordinary experience tells us that everything has a cause, which in turn has a cause. But does common experience justify thinking that logically all events must have a cause? Whether there is a physical end to decomposition or to reduction is not necessarily dictated by logic. In fact, if there is a bottom to the complexity of nature, that may imply that the fundamental level does not consist of things or events in the everyday sense. For, objects are decomposable; if something is not decomposable, then it is not an object in that sense. And if there is an end to the analysis of causation, either it is impossible for some epistemic (i.e., physical) reason to establish the cause or else some processes are self-causing.

In the classical view, at least, particles are miniature objects, subject to determinism. Though idealized as point locations for mathematical treatment, to have material reality they must have spatial extension, be individually identifiable, and be potentially decomposable into other things. Such entities, interacting either at a distance or through direct contact, provide the basis for the particle paradigm.

Now, elastic collisions between ideally rigid spheres should ideally be instantaneous. If they are not, there must be some compression within the particle, which takes time on some basis involving transmission of internal forces over a finite distance. That process could involve interactions among internal components composing the particle. These, in turn, could either be instantaneous; or else involve internal forces among parts a level down—ad infinitum.

The other paradigm for processes that take time is the wave or field. Waves do not have individual identity or clear location in space. Unlike particles, they interpenetrate. An alternative picture is thus the field, or the wave in some medium. The internal forces responsible for elasticity could be conceived as wave-like actions within the particle, which for some reason take time to be transmitted. But again, the field—or wave medium—could be conceived as consisting of discrete parts, like the molecules of water in the ocean. (The classical mechanics of waves is often treated this way.) Alternatively, it could be conceived as monolithic, as ideal in having no parts insofar as it is only a mathematical description. (Before being reified as a physical entity, the field was originally conceived to be no more than a mathematical device.)

In the particle case, there is no reason given why forces should take time to act over distance, either between or within parts. In the wave case, even with no interacting parts, there is still no physical explanation for why the transmission of a force or wave in a field should take time rather than be instantaneous. Some property (parameter) of the field is simply postulated to require a particular rate of transmission of a disturbance within it. While that property may bring to mind the viscosity of a material fluid, such literal viscosity on the macroscopic scale would be explainable in terms of molecular forces on the micro scale—that is, on the basis of parts which are material particles. Again, we are implicitly caught in circular reasoning. Forces do take time to move through space. We can accept that axiomatically, as brute fact without explanation. Yet it remains unclear what exactly takes time, or even what sort of explanation we could seek for why forces take time to act over distance. Neither paradigm provides a plausible rationale. Wave-particle duality is not only an observed physical phenomenon but the symptom of a logical dilemma.

Such an impasse may be inevitable when focus remains exclusively on the external world. That focus, carried to the extreme, results in some non-intuitive concepts in the micro-realm, such as entanglement, non-locality, and indeterminism, which defy our ordinary notions of causality, space, time, and how “objects” should behave. Just as space (between separable things) is required for there to be more than one thing at all, so time is required for anything at all to happen—that is, for there to be more than one event or moment. These are fundamental aspects of experienced reality for us as finite embodied observers—meaning that we could not exist if we did not perceive and conceive the world thus.

Whatever the nature of the Big Bang as a physical event, it is a logical condition for a world of things that change—therefore for a world in which life (that is, ourselves) could exist. We can say that space and time originated in the Big Bang. Yet, we could also say (with Kant) that they originate in our own being, as cognitive categories necessary to experience the world at all. Similarly, we could recognize (with Hume and Piaget) that causality is a human concept, originating in bodily experience during early childhood. The discovery that limbs can be moved by intention is projected onto interactions among inert external objects. The psychological ground of the notion of causality is our own intentionality as agents—which appears ironically uncaused (or, rather, self-caused)!

Basic physical concepts, if not innate, are formed from ordinary experience on the scale to which our senses are attuned. They are products of that specific experience and well adapted to it. Because we possess imagination—which can extend the familiar into unfamiliar territory—it is natural (though not logical) for us to transfer ideas, gleaned from the macroscopic realm, to the microscopic realm beyond our senses, and to the cosmic realm also beyond our unaided senses. The universe is not obliged to follow our lead, however. It is not obliged to be uniformly conceivable in the same ways, and in the same terms, on vastly differing scales. Humans inhabit a scale roughly midway between the smallest and largest known things. The observable universe is roughly 1035 times larger than the smallest detectable thing. We live somewhere between, within a very narrow range of conditions to which our ideas are adapted. There is no inherent (i.e., “logical”) justification for transferring our local mesoscopic notions to the extremely small or to the extremely large and distant. To do so may be literally natural, but it is little more than a convenient habit.

If it has any sense at all, the question of what takes time cannot be separated from our parochial assumptions about space, time, and causality. The speed of transmission of forces cannot be separated from the speed of transmission of information. For us, the vehicle of the latter is light, whose speed has a definite value. We take this also to be the maximal speed for the transmission of physical causation, or the rate at which things can occur. Strictly speaking, however, that is a non-sequitor. It results from confounding events in the world with our knowledge of them. Yet, it gives rise to quite specific ways of viewing reality, such as the 4-dimensional continuum in which light is built into the very definition of space and time.

We have sense modalities responsible for our intuitive notions of space and of time, but there is no sense modality for the perception or measure of spacetime, which is purely an abstract construct. We have sense modalities behind our notions of mass and energy, but no sense modality to perceive or measure phase space. (If it happened—as a hypothetical future discovery, say, or in an alternative universe—that a supraluminal signal could take the current place of light, physics would have to be revised, with new values for c and h.)

Abstractions such as the light cone in relativity theory and the wave equation in quantum theory extend our natural expectations, as embodied creatures, about the external world. Length contraction and time dilation are as counterintuitive as entanglement and non-locality. Such phenomena are apparent mysteries about the world. Yet, they point to the need for a re-examination of the origins of our intuitive expectations: the embodied origins of our basic notions of time, space, cause, object, force, etc. The fault may not be only in the stars (or the atoms), but in us. The ancient formula was ‘As above, so below’. We have yet to explore our mediating role between them: As within, so above and below.