What Is Time?

Presumably, ecologists are in agreement in assuming that time exists, that it flows, and that this flow has a definite and predictable direction. But perhaps we ecologists are also allied in wondering, at least on occasion, what time really is. It seems worthwhile, therefore, before proceeding under potentially false assumptions, that we address three questions related to the nature of time. First, does it in fact exist? Second, if so, does it flow or pass? And third, if it does flow, is it absolute and therefore gone when it passes, or does it recur? These questions will be essential in deciding whether time really can be considered a resource and a limited one at that.

PHILOSOPHICAL VIEWS OF TIME: IDEALISM, RELATIONISM, AND REALISM

The discipline of the philosophy of time offers insightful perspectives on the reality and nature of time, and this chapter will draw extensively on notable works in this field. Why should we review philosophical theories of time? Such treatments of time have been mostly anthropocentric, concerned with the reality and nature of time from human perspective. But if time is real, and represents a resource, then we must subsequently examine the nature of it from nonhuman perspective as well. Hence, examining what fields of study outside ecology have to say about the nature of time will aid us in developing a clearer understanding of the nature of time in ecology. As will be suggested in chapter 3, for instance, notions of the directionality of time may, in ecology, differ from the typical human experience.

Among philosophies of time, idealism denies the existence of time on the basis that change, an intuitive and apparently observable feature of time, is an illusion (Bardon 2013). A key feature of this perspective is the so-called paradox of movement: for an object to travel from one point to another, it must cross an infinite series of halfway points, which can never be achieved. Therefore, temporal idealism concludes that apparent movement, and change in general, is the misperception of an object occupying a space equal exactly to its own dimensions at any given instant. Furthermore, idealism contends that only the present is real. Any notion of past or future cannot be supported logically because neither is observable in the present, and if past and future do not exist, then the present cannot develop from the future or become the past, further refuting the reality of change. Relationism counters this by arguing that we should not conflate time and change because time is not a process but rather something independent of the process that merely allows us to measure it. In relationism, time consists fundamentally of events arranged according to their overlap, order, or rank with other events (figure 1.1) (Meyer 2013).

This view of time has relevance to the concept of the phenological community (chapter 3). It defines time according to subsets of events and their overlap or lack thereof, a notion that will be demonstrated in subsequent chapters as central to the interactions of individuals and species in time. Realism, last, simply represents the view that time is indeed real (Bardon 2013). Temporal realism has its most formal roots in Newtonian physics. Newton argued that the interdependence of time and space necessitates that time must be considered in absolute terms if we also consider motion in absolute terms (Newton 1687). This suggests that time exists independently of change, and that change occurs in time rather than as a result of the passage of time (Bardon 2013). This latter perspective is perhaps best represented by the cosmological or astrophysical view of time discussed later in this chapter.

A-SERIES, B-SERIES, AND THE FLOW OF TIME

If we accept that time does indeed exist, then we must next address whether time "flows" or "passes" and, if so, whether it does so unidirectionally and continuously (Callender 2011). This exercise is not simply esoteric or superfluous, but rather, for the purposes of developing an ecological framework for time, entirely necessary. The process of allocating time to biological maintenance, growth, and offspring production would, for instance, be very different if time were static or recurrent, and therefore unlimited, compared to such a process if time were in limited supply because of its unidirectional passage. Similarly, continuous passage of time might select for strategies relating to the allocation of time to maintenance, growth, and reproduction that could be expected to differ from strategies selected for under conditions of discontinuous passage of time. Ecology applies two different types of mathematical models in describing processes occurring in discrete time steps, such as population dynamics in species with nonoverlapping generations, and those occurring continuously, such as population dynamics in species with overlapping generations. However, ecology does not focus on the nature of time itself. Chapter 3will suggest that there are different forms of time of relevance in ecological systems, but in all of these the assumption is that time itself is continuous. Nonetheless, we might regard one of these forms of time, relative ecological time (about which more will be said in chapter 3) as more continuous in largely aseasonal environments such as the tropics than it is in highly seasonal environments such as the Arctic.

Furthermore, if time does pass and if it does so unidirectionally, then in which direction does it flow? Our innate perception may be that the future lies ahead of us while the past lies behind us, but does this mean we move forward through time and thus that time washes backward over us? Causality, for instance, appears temporally asymmetrical: any action or decision in the present influences at least to some extent actions in the future but not those in the past, imbuing time with a sense of directionality (Callender 2011). Philosophers of time offer insights into such questions through the opposing theories of dynamic, or A-series, and static, or Bseries, time. It may be tempting to dismiss philosophical theories of time as irrelevant to the role of time in ecology because the former appear concerned mainly with human perception of time while the latter should operate universally and independently of human awareness. But astrophysics gives consideration to such questions as well, and does so from a perspective that is decidedly nonanthropocentric. Hence, there is no inherent reason for ecology to avoid such questions.

The A-series, or dynamic, theory of time assigns nonstationary temporal values to events. According to this theory, events move from being future events to present events, and then to past events (Hoefer 2011). Moreover, the designation of events as future or past can itself be variable and fluid in the A-series model. For instance, events can be far in the future or far in the past. And the same event can change from being far in the future to being in the near future, and then, after it has occurred, similarly move from being in the near past and eventually in the distant past (Bardon 2013). Within the A-series or dynamic theory of time, the present may be seen as moving toward a determined future (figure 1.2a) or an open future with many possible realizations or consequences of the present (figure 1.2b; Hoefer 2011). In astrophysical theories of time, summarized briefly in the next section, this latter view is consistent with the notion that the universe is asymmetrical with respect to time and information (Hawking 1996c; Penrose 1996a). Chapter 5 will suggest that in ecology, we can adopt such notions in our thinking of the manner in which the timing of events in, for example, a phenological sequence, influences the timing of subsequent events.

In contrast, the B-series, or static, theory of time views events as having fixed relations to one another, without notions of past, present, or future (Hoefer 2011). In two dimensions, we can visualize this as a number or event line (figure 1.2c), while in three dimensions this comprises the so-called block universe (Hoefer 2011; Bardon 2013). According to the static theory of time, the relations of events to one another in time do not change: an event is either before, simultaneous with, or after another event. Hence, the B-series theory of time conceptualizes events as existing in an unchanging order. According to this theory, events can be viewed in association with one another in time in much the same way that locations are viewed with respect to one another in space: hence, notions of now or then, and of here or there, are equally subjective (Bardon 2013). However, whereas relations among objects in space necessitate coexistence in time, relations among events in time do not necessitate coexistence in space. For instance, to observe that one object is beside, in front of, or behind another requires that they exist at the same time; in contrast, to observe that one event preceded, occurred simultaneously with, or followed another event does not necessitate that they did so in the same location (Meyer 2013). The static theory of time derives from the view that perceived change in the temporal relation of events is actually just an illusion and that there is no objective past, present, or future (McTaggart 1908; Price 2011).

SPACE-TIME AND ASTROPHYSICAL THEORIES OF TIME

The theory of space-time argues that space and time are inextricably interwoven, and as a consequence one cannot discuss processes unfolding, or dynamics occurring, in space without also considering such processes or dynamics in time, and vice versa (Minkowski 1909). Owing to the interdependent nature of space and time and their interaction in the continuum of space-time, the rate of passage of time, and perhaps even the order in which events unfold, depends upon relative spatial location or movement. Hence, the perceived rate of passage of time may depend upon the observer's state of motion or rest. This is perhaps best illustrated by the famous twin paradox, in which a twin sent into space on a near light-speed journey returns years later and is observed to have aged less than the identical twin who remained on the Earth. Moreover, whether an event is simultaneous with others, precedes others, or follows others depends upon the orientation of the space-time manifold (figure 1.3).

In figure 1.3, the order of events A, B, and C varies according to the observer's perspective in relation to the orientation of the axes of time and space: the events are either simultaneous or they occur in the order ABC or CBA without the events themselves having changed (Bardon 2013). This conclusion has relevance to phenological dynamics in time and space that will become clear in the discussions of relative ecological time and the phenological community in chapters 3 and 5. For instance, whether the timing of expression of a given life history event is early or late from the perspective of the individual organism may depend not only on its absolute timing but also on its timing relative to that of the same trait by other individuals elsewhere in space or by the timing of expression of a trait by a member of another species with which that individual interacts.

Cosmological or astrophysical theories of space-time and the flow of time are inextricably linked to gravity and the second law of thermodynamics. Classical general relativity suggests that the perceived flow or directional passage of time may be a product of the action of gravity on the space-time manifold — in other words, the action of gravity on "the arena in which it acts" may have given time a beginning (Hawking 1996a). Sufficiently strong gravitational action may bend space-time and, thereby, alter the direction or flow of time. To illustrate this, Hawking (1996) used null geodesics to represent successive future-directed events in space-time, so-called event cones (figure 1.4). The points labeled p in figure 1.4 represent simultaneous events on a normal space-time surface resulting from all possible events in the past (backward-directed light or event cones) and from which all possible future events emanate (forward-directed light or event cones).

In such a case, the outgoing rays of the paired event cones are divergent, while the ingoing rays are convergent (figure 1.4, top panel). However, on a closed trapped surface, such as during the collapse of a star, the gravitational field becomes sufficiently strong to pull the event cones inward (figure 1.4, bottom panel). In this case, both the outgoing and the ingoing rays become convergent, altering the perceived relation of past and future (Penrose 1996b). This suggests that, at least under conditions of sufficiently strong gravitational action, the directionality of time is not absolute (Hawking 1996b). Furthermore, in continuous Minkowski space-time, all points can be described as lying in the past of some future null infinity; however, near the singularity characteristic of a black hole, there can be said to be points that do not lie in the past of any future null infinity (Hawking 1996b). In chapters 3 and 5, the concepts of recurrent and relative ecological time will be developed, in which notions of phenological past and future become somewhat fluid.

The second law of thermodynamics dictates that entropy increases in the absence of an input of, or expenditure of, energy. Hence, the universe can be said to be asymmetrical with respect to entropy and order. Similarly, the universe can be described as asymmetrical with respect to time (Penrose 1996a), and as containing more information in its future than in its past (Hawking 1996b). This appears to relate to the possibility that the boundary conditions near the universe's past are smooth and regular, whereas those near its future are rough and irregular or chaotic (Hawking 1996c). It is this difference in the so-called Weyl tensor structure at the space-time topological boundaries of the universe that lends perceived unidirectionality to the cosmological arrow of time (Hawking 1996c). From the perspective of living systems, biological maintenance, growth, and offspring production represent the maintenance and creation of order, or reduction of entropy, which requires energy. As chapter 3 will argue, the allocation of time is the necessary element governing the use of, and transformation of, energy into biomass and offspring production.

Where, then, does the foregoing discussion leave us on the questions of whether time exists and, if so, what it really is? From a philosophical perspective, it is tempting to conclude that time is an experience, a concept applied to understand or describe change, rather than a thing in and of itself, just as speed or rate or direction all describe and allow us to understand motion, or just as distance allows us to describe and understand space (Kant 1781). This view may, at least in part, explain why ecology has struggled to recognize and formalize the notion of time as a resource because we, as ecologists, intuit that time is not necessarily a "thing" but rather a perception. However, cosmology and astrophysics appear to provide the most suitable interpretation of time for the purpose of understanding its role in ecology. A deceptively simple, yet profound, definition of time deriving from quantum theory suggests that time may be the amount of energy required to separate mass instances from a coincidence of two or more superposed alternative events (Penrose 1996a). This conceptualization is appealing because of its parallel to space as the separation of two or more potentially superposed positions. There are useful analogues in the treatment of space in ecology that can be applied in our thinking about time in ecology (chapter 3). And after the brief review of space-time theory in this chapter, it would seem grossly negligent to accept that space is a resource without simultaneously accepting that time is one as well.