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date: 06 June 2020

Introduction

Abstract and Keywords

The Introduction starts by discussing the mystery of time. While time as a parameter of the arena of the world seems remote, it states, time is also associated with many features that shape the way we live our lives. It is this juxtaposition between its remoteness and familiarity that makes time one of the great mysteries—and the study of time especially captivating. In philosophy, time has always been a challenging topic. It is hard to separate our egocentric representation of time from a more objective one. This book hopes to have a “prospective” rather than “retrospective” look at the field’s past. It looks as well as at work completed at new work and topics being investigated in the philosophy of time.

Keywords: time, philosophy, study of time, representation, prospective, objective

Time is one of the last great mysteries. Like space, it is part of the fundamental stage upon which the events of the world play out. We know it only indirectly through the movement of the actors upon it. For these reasons time seems deep and remote, just as space does. However, we also find in experience a number of important features that distinguish time from space and that are crucial to the way we live our lives. We remember the past but not the future, believe that our actions cause effects in the future but not the past, and prefer that our pains be past and our pleasures future. Indeed, life is ticking by, as our bodies are clocks set to expire in about eighty years whereas we have no such spatial “clocks” As a result of these features, time is often said to be flowing, the present special, the future open, and the past fixed. Whether any of this talk is correct is of course controversial, but the fact remains that it's almost irresistible to think this way about time. While time as a parameter of the arena of the world seems remote, time is also associated with many features that shape the way we live our lives. It is this juxtaposition between its remoteness and familiarity that makes time one of the great mysteries—and the study of time especially captivating.

Today there is little need to sell the subject of time. Topics connected to time seem more popular than ever, not just in rarefied academic circles but also in the popular imagination. Popular science magazines, television documentaries, bookshelves, and blogs are filled with discussion of the possible flow of time, whether time travel is possible, whether time has a beginning or ending, the perception of time, the direction of time, and even whether time fundamentally exists.

None of this interest should be surprising, for this reporting simply reflects what's going on in science and the humanities. The fields of time perception in cognitive science, time awareness in psychology, the direction of time in statistical physics and cosmology, the nature of time in quantum gravity, chronobiology in the life sciences, and time‐keeping in society, in addition to many other areas, are fields that have exploded in popularity in recent times.

(p. 2) Future sociologists can perhaps explain this surge of interest in time. I'll hazard no guesses here. However, no doubt as part cause and part effect of this surge, philosophy of time has also experienced an increase of activity and attention. Whereas it was once a respectable but small niche in metaphysics, it has of late grown almost into a major area in its own right. More and more articles, books, and dissertations are written on the subject. What's striking about this work is that while the central issues in metaphysics have probably seen the greatest surge in activity,1 analytic philosophers have also expanded their repertoire. Two of the more obvious areas of growth are in the phenomenology of time—how we experience time, how it's connected to the self, and more—and in the study of the science of time and its perception, from cognitive neuroscience to physics.

In philosophy, time has always been an especially challenging topic. At root, the problem is that quintessential difficulty that so often motivates philosophical discussion: the problem of disentangling the nature of the entity itself from features that we happen to attribute to it. Better put, it's hard to tease apart our egocentric representation of time from a more “objective” representation of time. Is time itself responsible for the causal asymmetry, the specialness of the present, and so on? This question, in many guises, has been considered for millennia, from the ancient pre‐Socratics and Aristotle to the medieval Augustine to contemporary scientists and philosophers. Making progress on this topic therefore requires work in philosophy and the sciences on both the egocentric side of things as well as the objective side of things. This Handbook makes substantial progress on both fronts.

In editing this collection, I sought as best as I could to give the Handbook, a “prospective” look at the field rather than a “retrospective” look at the field's past. Thus, while mindful of and desiring to represent the good work done in central traditional areas, I also tried to accentuate a lot of the new work and topics being investigated in philosophy of time. Thus I sought out chapters on time and ethics, time and probability, the fascinating CPT theorem, time and action, the possibility of time being discrete, and many others. Even where this Handbook covers traditional topics, often they are done with some novel slant. One chapter on the direction of time focuses on the electromagnetic arrow of time, a topic little discussed or understood. One chapter on presentism develops a new view of its relationship with relativity. The chapter on the twin paradox develops it within topology, promising something new to even those familiar with the paradox. The one on time travel focuses more on time machines than the typical “grandfather paradoxes.” And so on. Part of what helped me achieve this range of topics was the fact that I wanted to avoid redundancy with the chapters on time and metaphysics in the Handbook of Contemporary Philosophy and Handbook of Metaphysics, plus the one on time and tense in the Handbook of Philosophy of Language. In any case, only the future will tell whether some of my guesses about it are correct.

Let me now describe what is to come.

(p. 3) Time and Metaphysics

Is there more to the world than the present moment? This question is one of the most basic one can ask about time. Some metaphysicians (“eternalists”) believe that the past, present, and future are all real, others (“possibilists”), believe the past and present are real but the future is not, and yet a third group (“presentists”) hold that only the present is real. This dispute runs throughout many of the chapters in this Handbook (as well as the Handbook on Metaphysics), but the two chapters devoted specifically to this topic are ones by Dean Zimmerman and by Joshua Mozersky. Zimmerman seeks to defend a version of presentism. Among the challenges that presentism faces is a prima facie conflict with relativity theory in physics. Relativity theory affirms the relativity of simultaneity, a thesis that immediately threatens presentism. (See Savitt's and Luminet's contributions on this topic, discussed later.) Zimmerman develops a theory that he hopes will escape this difficulty. Mozersky reviews a quite different attack on presentism. Called the “truth‐maker” or “grounding” objection, the idea is that the presentist's temporally impoverished resources are insufficient to allow him or her to say that statements about the past or future are truth‐evaluable. The claim that “there were dinosaurs in the Mesozoic Age” is not made true or false by anything on the present time slice, or at least, not obviously so. So the presentist faces a quandary: the claim is true, yet there is nothing that makes it so. Mozersky, in contrast to Zimmerman, does not see a viable way out of this problem for the presentist.

The above problem arose from asserting that the past and future are unreal, but holding them to be real also presents puzzles. Focusing specifically on whether the future is real, a pair of natural questions arise. If the future is real, and our choices are “already” made, in what sense can we do otherwise? If I was “always” a philosopher in 2010, did I really have a choice about professions in 1990? Similarly, if the future is real, and the outcomes of our chancy processes are “already” occurrent, then in what sense is the chancy process genuinely chancy? If the coin “always” landed heads at a specific toss, in what sense did that roll have a 50 per cent chance of landing tails? The first topic is the notorious fatalism issue. Discussed by Aristotle more than two millennia ago, the question is whether various logical principles, when applied to propositions about future events, imply that the future is in some sense fixed—so that you couldn't have done otherwise. If the future is like the past, and the past is fixed, then are future events fixed in the same sense? Craig Bourne gives the latest on this traditional topic, carefully surveying various works and trying a new tack that steers us away from fatalism. The second question is not as venerable as the first but is just as interesting. Advocates of the propensity interpretation conceive of probabilities as dispositions or tendencies inherent in certain physical situations that cause a particular pattern of outcomes. In his contribution, Carl Hoefer argues that the propensity interpretation of probability is implicitly connected to the assumption of an unreal future. Thinking that an unreal future is untenable in light of relativity theory, Hoefer argues that the intuitive “boost” (p. 4) propensities get by this connection is illegitimate. Once sanitized of the connection, Hoefer claims that propensities lose much of their appeal.

Regardless of whether the future or past are real, there is also the question of what objects (like you) are. What is it for an object to persist in time? At one time you are short and at another time tall. Are you a four‐dimensional object with different temporal parts—a short part and a tall part? Or are you the wholly present three‐dimensional entity who changes properties? Or are there other options? In his essay, Yuri Balashov gives a clear and admirably rich survey of this problem, updating three positions on this problem to the relativistic context, and providing the reader with a solid base from which to later evaluate the positions.

On the eternalist view of time, other non‐present times are like other spatial locations that aren't here. Times are like locations. But a very different picture emerges if one thinks instead of other times as analogous to other possible worlds in modal logic. Modal logic is the logic of possibility and necessity. Even if I don't help you move out of your house, you might still insist that it was possible for me to have done so. In other words, a possible world exists wherein I do help you move. Modal logic is the set of rules that seem to govern our inferences about such possibilities. From it we can also construct a tense logic, whereby “possibly” is interpreted as “sometimes”, “necessary” as “always”, and so on. This logic can be developed in various ways and put to various uses. It might be thought to provide a linguistic theory of time—describing the way we ordinarily talk about time—or underpin a new metaphysical picture of times as like possible worlds. In his chapter, Ulrich Meyer argues against the first use and develops the second.

Time is usually assumed to be continuous. No doubt the reason for this is that science generally takes time to be continuous. But the reasons it does so are not directly empirical. That is, no experiment has been done that directly tests whether time is continuous, dense, or discrete. Rather, time is considered continuous because our best theories of the universe say it is, but they do so mostly because it is much easier to write a theory using notions from calculus this way. Could time instead be discrete? What reasons might there be for thinking so, and what puzzles appear when studying this possiblility? In a very original essay, van Bendegem tackles these questions and others.

The Direction of Time

The future seems very different from the past. These differences are manifested in various ways.

One way is that we, in some sense, know more about the past than the future. With the simple act of looking in yesterday's paper I can now discover yesterday's stock report. But short of a crystal ball, there is no way I can today determine tomorrow's stock reports. Today's newspaper may contain some information about the future— tomorrow's tides, a street fair next Saturday—but this information is very limited. By (p. 5) contrast, going in the past direction, I have access to a much richer kind of information, such as the nearly exact tide levels, how many people attended the street fair, and so on.

Another temporal asymmetry is what we might call the causation or mutability asymmetry. Actions right now can cause me later to eat in a different restaurant than I otherwise would have; but nothing I do now can cause me to have eaten at a different restaurant than I did last week. I can change where I will die, but I can't change where I was born. Causes typically have their effects in the future, not the past. Another way to think about this is via counterfactual dependence. The future depends counterfactually on the present in a way the past seems not to. This asymmetry leads us to believe that the future is open in a way the past is not.

Our valuations of events and objects also manifest a temporal asymmetry. Given a choice in the matter, I would rather have had a bad headache than be about to have a bad headache. I want my pains in the past and pleasures in the future.

There are other temporal asymmetries, but the above list is sufficient to motivate a set of questions. What is the nature of these asymmetries? How are these asymmetries related, if at all? How are they related to various temporal asymmetries found in physics? Are these asymmetries in time reflecting some kind of asymmetry of time?

Douglas Kutach, in his contribution, considers the nature of the causal asymmetry, or even more generally, the asymmetry of influence. Putting aside explanations that would appeal to an asymmetry in time as explaining this asymmetry, Kutach hopes to show, using current physical theory and no ad hoc time asymmetric assumptions, why it is that future‐directed influence sometimes advances our goals, but backward‐directed influence does not. The article claims that our agency is crucial to the explanation of the influence asymmetry.

Might the explanation of some temporal asymmetries simply be that time itself is asymmetric? Some people believe that time flows, and others that it is intrinsically directed. But what do such claims mean, precisely? In his chapter, Huw Price considers three ways of understanding flow—through a distinguished present, an objective temporal direction, and a flux‐like character—and finds them all wanting. He spends considerable time evaluating, in particular, the idea that the world possesses a time orientation, critically scrutinizing the ideas of John Earman and Tim Mauldin on temporal orientation and time's arrow.

It's often claimed, or hoped, that some of the above temporal asymmetries are explained by the thermodynamic asymmetry in time. Thermodynamics, the macroscopic physics of pressure, temperature, volume, and so on, describes many temporally asymmetric processes. Heat flows spontaneously from hot objects to cold objects (in closed systems), never the reverse. More generally, systems spontaneously move from non‐equilibrium states to equilibrium states, never the reverse. What explains the thermodynamic temporal asymmetries? To some it will come as a surprise that this is itself highly controversial. Delving into the foundations of statistical mechanics, Jill North reviews the many open questions in that field as they relate to temporal asymmetry. Taking a stand on many of them, she tackles questions about the nature (p. 6) of probabilities, the role of boundary conditions, and even the nature and scope of statistical mechanics.

Time, Ethics, and Experience

Whatever the true metaphysics of time is, it's clear that time plays a complicated, fascinating role in the way we experience the world and in the way we live our lives. Our temporal experience is richly structured psychologically. Not only do we have memories of the past and anticipations of the future, but it also seems to be the case that the contents of our experiences at a given time represent temporally extended events. Moreover, the timing of events matters to us ethically, although whether it should is another question. The chapters in this Part focus on these aspects of temporal experience, among others.

Four chapters focus directly on our experience of time, and three bounce off from William James' so‐called “specious present.” The specious present is the claimed temporal breadth in the content of an experience at a particular time. The content of my experience—right now—of a musical note has temporal breadth. And the same for the one “after” that, and so on. Our experience has a stream‐like aspect to it.

But how can this be, asks Barry Dainton, if the world itself does not pass? That is, why does my experience have this stream‐like quality to it when (as he assumes) time is itself not flowing? While making the eternalist world safe for the specious present, Dainton carves out and evaluates two contrasting understandings of the specious present.

Jenann Ismael begins her analysis with a careful look at the specious present, but this is only the beginning of her survey of many of the psychological temporal structures that arise in creatures like us. She also examines memory, anticipation, and the building up of our experience through time, focusing especially on the contrast between time from an “embedded” perspective and time from an external perspective. She ends with some exciting suggestions for how this work may link to our conception of the self and also the metaphysics of time. In particular, she claims that the apparent fixity of the past emerges from the adoption of the “embedded” perspective she describes.

Shaughn Gallagher's work links phenomenology with cognitive science. But here he is concerned with what he calls the “intrinsic temporality” in both bodily movement and action, some of which is experienced, but some of which happens at the subpersonal levels of analysis. Like Dainton, Gallagher begins with Husserl's dynamic model of retention and protention; but unlike Dainton, he extends this model to unconscious motor processes too. Bringing empirical studies to support his claims throughout, he then focuses on various timescales in an effort to show how the concept of free will becomes important.

The above papers assume that we already possess a rich set of temporal concepts because we are talking about normal adult human beings and their experiences. But (p. 7) what about young children? What temporal concepts do they have, and when do they acquire them? There are many interesting questions here, and a growing body of empirical work upon which to search for answers. When, for instance, do children master the use of past, present, and future? Are kids presentists? Hoerl and McCormack, in their chapter, pay special attention to the acquisition of a concept of a linear time series. Are children fitting experiences into a linear time series, or are they just doing particular actions that happen to make up such a series? When do children grasp the concepts of ‘before’ and ‘after’? Can work on children and the causal priority principle (that causes precede effects) shed light on this question? Hoerl and McCormack defend the idea that children first conceive of events without genuinely employing tenses.

Finally, the temporal locations of benefits and harms matter to us. We prefer past pain to future pain, even when this choice includes more total pain. We regret our death but not our pre‐natal non‐existence. But should the location of benefits and harms matter to us, all else being equal? This question is an ethical one and the subject of David Brink's chapter. Brink is concerned with defending temporal neutrality, the thesis that agents should attach no normative significance to the temporal location of benefits and harms, all else being equal. A powerful argument for temporal neutrality comes from prudence. However, prudence also assigns normative significance only to benefits and harms that occur to you, not other agents. Given the symmetries of the case, can this hybrid position be defended? Brink thinks so, arguing, contra Parfit, that the fact that you are later compensated for present sacrifice is crucial to assigning equal importance to all parts of an agent's life, but not equally to all agents.

Time in Classical and Relativistic Physics

Two parts of this Handbook are devoted to time in the physical sciences. Why so much physics, and not time in other sciences such as economics, biology, or chemistry? Although one can certainly imagine and even find very good work on the intersection of time and these sciences, the answer to this question is, I think, very simple. Apart from the sociological explanation—physics is where many philosophers interested in time work, and hence is where a lot of good work exists—there is also an intellectual answer: physics, and physics alone, is the science that actually considers time itself to be a target of study. Economics, biology, and chemistry of course implicitly model time, and as such, implicitly hold that time possesses various features, for example discreteness. Although confirmation theory is tricky, very few people would hold that the success of an economic model supports the particular conception of time adopted in the model. Time itself isn't a target of what economists, etc, study, either in theory or experiment. By contrast, time itself is very important to what physicists study. Experiments in physics define the second, which in turn defines the meter and (p. 8) many other quantities. Experiments in general relativity also reveal that it's crucially important that spacetime is curved in the so‐called timelike directions of spacetime. The violation of time reversal invariance, something central to particle physics, is also tested experimentally. And, of course, in theory many specific features attributed to time are of paramount importance, for example, in the “signature” of the spacetime metric.

Given its centrality in the subject, one might be forgiven for thinking that everything to do with time in non‐quantum physics has been worked out already. Fortunately for philosophers, that is hardly the case.

Time in even classical mechanics has yet to be fully appreciated by philosophers. Lawrence Sklar's chapter begins with time as it's presented to us in Newton's famous Scholium. He shows how and why Newton developed a notion that has various specific features, namely, those needed for time to play the role it does in classical dynamics. Along the way, several useful distinctions are made that will help the reader in later chapters.

Time in electromagnetism shares many features with time in other physical theories. But there is one aspect of electromagnetism's relationship with time that has always been controversial, yet hasn't always attracted the limelight it deserves, the electromagnetic arrow of time. While philosophers and physicists have expended much effort on the thermodynamic arrow, the electromagnetic arrow has by comparison been relegated to cameo role. This neglect is especially odd because a good argument can be made that the arrow of radiation is at least as important to the direction of time as the thermodynamic arrow. Beginning his chapter with a re‐analysis of a famous argument between Ritz and Einstein over the origins of the radiation arrow, John Earman frames the debate between modern Einsteinians and neo‐Ritzians. Earman tries to find a clean statement of what the arrow is—a surprisingly difficult problem—and then explains how it relates to the cosmological and thermodynamic arrows. This chapter represents the most developed and sophisticated attack yet, in either the physics or philosophy literature, on the electromagnetic arrow of time.

No theory has offered more shocking lessons with respect to time than relativity theory. Three chapters in this Part consider various ways in which relativity impacts our understanding of time.

One striking feature of relativity is the fact that clocks moving with respect to you “tick” slower. Combined with the relativity of motion, one arrives at the notorious twin “paradox.” Your twin leaves the Earth in a rocket and then returns much later. To her you are moving, and hence you should age more slowly; to you she is moving and hence she should age more slowly. However, when she returns she is younger than you—why? Jean‐Paul Luminet is an observational cosmotopologist, a physicist who searches the skies for evidence that spacetime has non‐trivial topology, and he discusses this paradox in his chapter. Of course, the fact that this paradox is not really paradoxical is a well‐worn fact. Yet I daresay that even the most sophisticated reader will learn something new about the twin paradox from this chapter. Looking at the above scenario played out on a hypersphere and on multiply connected finite spaces, (p. 9) Luminet shows how this complicates the paradox. More importantly, he shows how its resolution in those cases yields new insights into the nature of spacetime and the equivalence between inertial reference frames.

Restricted to special relativity, the most significant change in our concept of time is certainly the relativity of simultaneity. What events are simultaneous with some event for one observer are different from those that are simultaneous with respect to an object travelling in a different inertial (“force‐free”) frame. If you and I are both inertial observers moving apart, then the events that comprise my “now” are not the ones that comprise your “now.” Many believe that this relativity can play a role in an argument for eternalism. In his chapter Steven Savitt critically surveys these arguments before developing his own take on the implications of relativity for the metaphysics of time. First, however, he tackles another topic related to simultaneity, namely the conventionality of simultaneity. Many philosophers of science, especially during the early days of relativity, felt that simultaneity is not only relative but also conventional—that is, that there is a crucial element of choice in deciding what events are simultaneous for any other in a given inertial reference frame, that there is no fact of the matter about what is simultaneous. Savitt gives us the latest arguments on this debate, too.

When we turn from special to general relativity, many features commonly attributed to time once again change. To name a few: typically there are no global inertial frames, so clocks once synchronized and then freely falling will become unsynchronized; the timelike aspects of spacetime enjoy significant curvature; some general relativistic worlds do not even permit a global carving up of the world into global moments of time.

Of all of these features, perhaps the oddest is the possibility in the theory of non‐trivial time travel. This is the subject of the chapter by Christopher Smeenk and Christian Wüthrich. After dispelling the logical and metaphysical arguments against the possibility of time travel, they turn to time travel in general relativity. They pay special attention to the possibility of being able to build a time machine, a device that would create a path suitable for time travel where none otherwise would have existed. Then they end their analysis by briefly reviewing if the quantum nature of matter alters the sense in which time travel is possible.

This last question reminds us that the world is governed by quantum theory, too, so an investigation into time should also turn to the strange world of the quantum.

Time in a Quantum World

Quantum mechanics presents us with many mysteries. Most readers will have stumbled across all kinds of perplexing topics associated with quantum mechanics, from Schrödinger's cat, to quantum non‐locality, to many worlds theory. But quantum mechanics and time? Sometimes one hears that time in quantum mechanics is more (p. 10) or less the same as time in classical mechanics. While there is some truth to this, this claim obscures the fact that quantum mechanics does present those interested in time with several new and distinctively quantum topics. We finish the volume with three different ways in which quantum mechanics bears on time.

First, quantum mechanics seems to care about the direction of time, unlike classical mechanics. In 1964 K‐long mesons were shown to violate time reversal invariance. But this violation was indirect, inferred from a remarkable theorem known as the CPT theorem. Derived by Gerhart Lüders and Wolfgang Pauli, and independently by John Bell, the CPT theorem states that local quantum field theories that are Lorentz invariant (i.e. relativistic) must also be invariant under the combined operations of charge reversal (replacing matter with anti‐matter), parity (replacing “right‐handedness” with “left‐handedness”), and time reversal (changing the sign of the momenta). Since CPT holds, the 1964 violation of CP meant that T was violated.

The CPT theorem is quite strange. Why should a quantum field theory be invariant under the combination of two spatiotemporal discrete transformations, and then a quite different type of transformation (matter‐anti‐matter transformation)? What do these transformations really mean, and what does CPT symmetry imply? In one of the first attacks on these and related questions by a philosopher, Frank Arntzenius argues that CPT symmetry is better understood as PT symmetry. If he is right, CPT symmetry is really saying that quantum field theory doesn't care about temporal orientation or spatial handedness.

Second, unlike classical mechanics, quantum mechanics assumes the famous Heisenberg uncertainty relations. One of these concerns time: the energy‐time uncertainty relation. However, there is something fishy about this uncertainty relation. Unlike the canonical position‐momentum uncertainty relation, the energy‐time relation is not reflected in the operator formalism of quantum theory. Indeed, it's often said and taken as problematic that there isn't a so‐called “time operator” in quantum theory. Physicists Jan Hilgevoord and David Atkinson shed much‐needed light on these questions and others, including the absorbing matter of whether quantum mechanics allows for the existence of ideal clocks (yes, they conclude).

Finally, science does not end with quantum theory and relativity. Quantum theory and relativity conflictinvarious ways, and anew theory—dubbed “quantum gravity”— is needed. It is appropriate to end the volume with a chapter reminding us of the challenges the study of time will face in the future. Just as reconceiving our classical notions of time was key for Einstein, in his discovery of special relativity, so too many believe that time will again hold the clue for theoretical advancement, but this time with quantum gravity. Claus Kiefer, a noted expert in the subject, details the challenge of reconciling quantum theory with relativity, concentrating especially on why time in particular causes trouble. He describes a result in canonical quantum gravity that is possibly of signal importance, namely, that fundamentally there is no time at all. In the timeless world he describes he shows us one way that time may emerge in particular physical regimes. This breath‐taking possibility is, I hope, an excellent way to leave the reader wanting to study time further.

Notes:

(1) A JSTOR search for the keyword ‘presentism’ across 63 philosophy journals yields 20 matches for 1980–1990 and 146 for 2000–2010.