Is the passage of time an illusion? This has meant many different things to different people. As rightly noted by Gruber, Smith, and Block (2018), both āpassage of timeā and āillusionā invite a multitude of interpretations. Moreover, itās a question that cries out for interdisciplinary efforts (such as this volume), since numerous disciplinesāincluding physics, cognitive science , developmental psychology, evolutionary biology , and philosophyāare potentially relevant. Itās also a very timely question (no pun intended). For example, time has recently taken center stage at the frontiers of physics. Physicists working toward a theory of quantum gravity have had to re-examine some very basic assumptions about time, such as whether it exists, and if it does, whether it is fundamental or in some sense emergent (Callender, 2010).
The title of this chapter might lead one to expect a theory of what temporal passage is, and an argument for the view that itās not an illusion . In fact, my aims are a bit more narrowly circumscribed. First, Iāll describe what āpassageā stands for in current philosophical usage. For there to be an illusion of passage, one would have to answer āYesā and āNoā, respectively, to the following two questions: ā(a) Does time seem to pass?ā, and ā(b) Does it pass?ā. Assuming for present purposes that the answer to (b) is indeed āNoā, Iāll suggest that the answer to (a) is also āNoā. The upshot will be that there is no illusion of passage: All we perceive is one thing after another.
1 āThe Passage of Timeā
The phrase āthe passage of timeā (or āthe passing of timeā, ātemporal passageā, or ātemporal flowā) has a particular meaning in philosophical debates about the nature of time. Passage is a (putative) feature that time is supposed to be able to have or lack, while in any case existing. That is, weāre not here concerned with the possibility that time may not exist, nor with the possibility that it may not be fundamental. (So this is not directly about those basic questions arising from within current research in physics.) Rather, the question is whether or not time, a real feature of the world, itself has the feature of being such that it passes. This question may well sound a little strange at first. Ordinary usage may not allow much leeway between time existing and time passing. But philosophical reflection has produced a distinction between two ways that time could in principle be, namely dynamic (such that it passes) or non-dynamic. To illustrate, consider a few ideas for what timeās passing might consist in, according to different dynamic views of time: Time would pass in this sense if only one time was (ever) real, so that times came into and went out of existence constantly; or, if only the past and the present were real, so that reality as a whole grew constantly; or, if future events constantly became less and less future, until they became present and then more and more past. All these views have in common that they take the metaphors about time being like a river, exhibiting some kind of sui generis movement of its own, very seriously. On these views, time is not much like space, at least not in this respect. Space doesnāt pass, after all.
All these dynamic views (versions of the A-theory ) are opposed by the non-dynamic, or block universe view (also called the B-theory ). The block universe view denies that time does anything like passing in this sense. At least in this respect, time is no different from space.
Though itās the product of philosophical reflection, the distinction between dynamic and non-dynamic views of time is very intuitive at root. After all, the metaphors that drive the distinction are commonplace in many languages (though the details reveal many fascinating differences as well, see Sect. 4). We commonly speak as if we were moving through time, or as if time and events were moving past us. Is time really like this, or does it lack the dynamic features the metaphors point to? Itās not surprising that something like this question has left traces throughout the history of philosophy. The British idealist philosopher J. M. E. McTaggart brought it into sharp focus (arguing that for time to exist it would have to be dynamic, but it canāt be, so it doesnāt exist), but its history stretches back at least to Heraclitus and Parmenides .
Its history is also intertwined with the history of time in physics. The block universe view has often been associated with time in relativity theory. One not so good reason for this is the four-dimensional nature of relativistic spacetimes. On a spacetime diagram, past, present, and future are seemingly already there. There is no dynamism , just the block universe. This isnāt a good reason to associate the B-theory with time in relativity theory, mainly because there are spacetime formulations of non-relativistic physics too. But there are other reasons. In particular, all the dynamic views seem to rely on there being a ānowā that is the same everywhere in space. But relativity teaches us that there is no objective notion of distant simultaneity āno notion of the same time at different spatial locations. Whatās simultaneous with what depends on oneās state of motion . So this makes trouble for any dynamic view that says that whatās dynamic is some global present, ever moving on, or constituting the edge of what exists, or the only time that exists. By elimination, the block universe view emerges looking vindicated. Philosophers and physicists alike have expressed the sense that relativity theory shows time to be very much unlike what the metaphors suggest. Hermann Weyl famously remarked that ā[t]he objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upwards along the life line of my body, does a section of this world come to life as a fleeting image in space which continuously changes in timeā (Weyl, 1949). Admittedly, this almost sounds like a dynamic view of time again, but only almost: Time itself is non-dynamic; āonly to the gaze of my consciousnessā does temporal reality present itself as dynamic. Similarly, Arthur Eddington said that ā[e]vents do not happen; they are just there and we come across themā (Eddington, 1920). Albert Einstein himself went so far as to claim spiritual significance for this view of time. Shortly after the death of his friend Michele Besso, he wrote: āNow he has also gone ahead of me a little in departing from this peculiar world. This means nothing. For us believing physicists, the division between past, present and future has only the significance of a stubbornly persistent illusionā (Einstein, 1972).
2 āIllusionā
This brings us to the term āillusionā. Roughly, Iāll take this to mean a mismatch between how things seem, and how they are. In this case, the mismatch would be between time ās being non-dynamic and yet seeming dynamic. Thus, the question at hand is whether two things are the case: Time doesnāt pass (in the sense of the A versus B debate), and yet it seems to (in that same sense). (From now on āpassageā will denote passage in the sense of the A versus B debate.) Letās call the corresponding questions ā(a) Does time seem to pass?ā and ā(b) Does time pass?ā. And letās assume for present purposes that time doesnāt pass, and concentrate on (a): Does it seem to? that is, do we have passage phenomenology ?
Is passage something one should even expect to show up in experience? Some are skeptical about this. Suppose that the passage of time turns out to be the ābirthingā of new elements of a causal set, i.e. discrete spacetime points. This is a suggestion recently made by some of the physicists working on causal set theory , which is intended to be a stepping stone toward a theory of quantum gravity . Isnāt the growth of causal sets the last thing one should expect ordinary experience to be responsive to? Since when does our phenomenology track the most fundamental layer of reality as revealed by our best physical theories? How likely is it that when I glance at my watch, I somehow intimate the stochastic growth process constituting spacetime itself? On the other hand, those physicists writing about this seem to be motivated precisely by the link to experience. For example, Rafael Sorkin writes, ā[S]equential growth [ā¦] provides an objective correlate of our subjective perception of ātime passingā in the unceasing cascade of birth-events that build up the causal setā (Sorkin, 2007). More generally, as the Weyl and Einstein quotes already suggest, itās somewhat natural to think that a non-dynamic temporal reality is at odds with how things seem. Thereās something striking about the claim that time doesnāt pass, and itās natural to suppose the reason is that it seems to.
Forget passage for a moment and just consider our experience of time as such. That is, just consider our experiences as of temporal duration , of events as occurring in a particular order, and as following on from or succeeding one another. (āAs of xā is meant to leave it open whether the experience is veridical or illusory , that is, one can have an experience as of x without there being any x.) In order to understand the question at hand, namely ā(a) Does time seem to pass?ā, one has to keep in mind that both sides to this dispute can and typically do allow that we have temporal experiences. Whether we have experiences as of time passin...