1. Time and Passage: a Hidden Problem
It is commonly thought that what is taking place in the present is all that exists. This present moment - as we perceive it - has a privileged position over both the past and the future. The past no longer exists, and the future is yet to exist: it is merely an abstract set of possibilities. The present is the moment of which we are currently aware, but it is not fixed: every moment of the past was once the present, and at some time each moment of the future will be too. The Universe is steadily unfolding, with each moment coming into the spotlight of existence just once before inevitably being lost in time forever. Moreover, if we know a future event by description, such as the death of the last veteran of the First World War, or, more concretely, midnight on May 5th 2059, then if we live long enough we should observe this future event appearing closer and closer (although as in the former case we may not realise it), eventually becoming the present itself, before passing out of all existence into the recent and finally distant past. This view is known as ‘tensed’ time.
However, although the perception of physical change is natural, there is something deeply problematic about this tensed description of time, with the present trundling along through history, churning the future into the past. Indeed, our usual internal picture of the present progressing through the time-line of history should strike us as incoherent, were we not so set in our ways. When we represent change mentally it is always with respect to time, which is viewed as its absolute currency. A careful study of the internal images representing the progression of time will reveal that it always plays a dual function: it is represented by metaphor as some physical object - a river, or moving spotlight, say - as well as appearing in its usual temporal form, as the mechanism by which this object is set in motion. Clearly this pervasive temporal representation of change is not valid when it is the progression of time itself under consideration.1
This inconsistency was inevitable: there can be no satisfactory physical model of a tensed Universe because the notion of absolute change has no analogue in mathematics - the surest manifestation of pure reason. Mathematics is thoroughly tenseless.2 A change in the temporal status of a particular event - that is, the property of being past, present or future - would be intelligible only with respect to some other temporal quantity. But this new variable could only be set in motion with respect to yet another still, and so on ad infinitum. To make the tensed view tenable we are required to postulate an infinite hierarchy of temporal dimensions. And now when we look back over our description of tensed time we notice all sorts of misuse of language.
Another powerful argument against the tensed view of time comes from Special Relativity, the physical theory proposed by Albert Einstein in 1905. The theory predicts many situations where our observations can run contrary to normal, everyday experience. For instance, we rarely disagree with our friends about the ordering of a particular sequence of events when they happen at relatively slow speeds and across short distances - but under special relativity such agreement is not always possible.
According to the theory, observers moving at different velocities can actually disagree about which events are simultaneous with a given event through which they are both passing. And if the present is all that exists, then they must also disagree about what exists too. It seems unthinkable that existence itself should be observer relative, and dependent upon the speed at which one is traveling.3 This argument is then further strengthened the by non-deterministic interpretations of quantum mechanics: where for one observer an event is in the future and not yet fixed, for another it is in the past and hence already determined.
2. The Four-dimensionalism Solution
Noting that the classical picture is self-contradictory, some philosophers and scientists have suggested that time is in fact a dimension that is in many ways analogous to each of the three spatial dimensions.4 Just as London, Rome and Berlin all exist simultaneously in different spatial locations, the events of our lives (including our inevitable deaths) all exist together, but in different times: that is, they are located at different time-coordinates in four-dimensional space-time. The response to the inconsistency noted above is simply that time does not progress at all: it is merely the physical ordering of a set of stages (or temporal parts) that all exist together as one connected object.
There is a rough analogy here with watching a film in the cinema.5 We perceive it - with all of our senses - as something that is progressing: it starts with a beginning, draws into the middle, and eventually comes to an end. However, in truth the Universe is actually more like the film reel used to produce the display; there are many static slices of time, each akin to one of the filmstrip’s frames, and all existing together. Just as no moment of a fictional story or film can ever be described as ‘present’, ‘future’ or ‘past’, other than in terms of the illusion created when it is played to us, we claim that this is true of the rest of the Universe too: there is no special moment that is singled out as the one true present.
The theory of time as a dimension goes by various names such as static time, four- dimensionalism and eternalism (because nothing ceases to exist), and was born out of the physics developed at the start of the twentieth century. It is compatible with Einstein’s theories of special and general relativity: indeed, if an observer could travel at any speed through a given point, then any three-dimensional Universe intersecting with that point would be shown to exist. This strongly suggests that the Universe is four-dimensional.
According to this picture of things, then, there are actually no objective differences between those instances that we perceive as being the present, the past and the future of the Universe: they are all equally real. This paradigm is (often tacitly) supposed for the theme of time-travel that has become common in science fiction: without it the notion of traveling ‘to the past’ is unintelligible. The inhabitants of the past also believe they are the ones living in the present; in fact, we are both equally correct.
There is some similarity here with the intersubjective preeminence we experience all of the time: despite the overwhelming feeling that we in particular are at the centre of things, we know that there is really no underlying metaphysical distinction in kind between ourselves and other people. Each temporal part of ourselves is individually conscious of just one moment, and under the illusion that it is this instance alone that is real; but just as we are not the only centres of experience in the spatial world, the same is true for position in time too. And thus there is a static counterpart to a seemingly emergent reality.
3. Some Philosophical Objections
There is a strong psychological compulsion to dismiss offhand the idea of a static Universe, as every waking moment we seem to be presented with nothing but emergence and change. It seems that phenomenology - the study of the direct experiences of conscious individuals - could not possibly allow for it to be true. It is also often claimed that tensed statements are essential for time, or that that there could be no time at all without these absolute designations of tense - that is, the terms ‘past’, ‘present’ and ‘future’, as applied to a particular event. However, we shall in fact find that a dismissal on these grounds is unwarranted.
Suppose particles persist through time: that is, if they exist in one moment then they are always present nearby in the next. Then the difference in their position through the temporal stages is still described by our usual system of physical laws, which in fact make no mention of tensed facts, and already treat time as a dimension. We can simply define motion as the patterns that these particles form across successive moments of time, with larger structures that are built up out of these particles receiving similar treatment. Other concepts can be treated in much the same way: change is defined as ‘qualitative difference in temporal parts’,6 and causality becomes a certain directed pattern between these temporal parts, rather than the usual picture of one event being brought about by another.
More generally, we may speak of adopting a tensed perspective or way of speaking that describes certain features of the world from a tensed point of view. We do this safe in the knowledge that our tensed concepts can be formally translated into tenseless ones along the lines of the examples above: in particular, the definition of any process whatsoever is simply modified to refer to the sequence of temporal parts that it was previously said to have consisted of under the conventional tensed view.
Following on from this, we are in a position to harness the explanatory power of evolutionary biology. Any naturalistic biological explanation can be transformed between the two perspectives in exactly the same manner as a physical one.7 In particular, the development of a subjective appreciation of time passing can still be explained in evolutionary terms, and the description proceeds in exactly the same way as accounted for under the tensed view.
Humans and other animals have evolved to perceive the world a certain way: our perceptual faculties are designed to steer us through a certain set of challenges involving medium-sized objects moving at medium-speeds. It is for this reason that we perceive objects as continuous solids, even though modern science tells us they are mostly empty space. Similarly, the construction of a consciousness that perceives change and has a certain attitude towards time is an invaluable addition to the arsenal of evolutionary weapons at our disposal for survival and reproductive success in the world. Indeed, change is not even directly experienced in the same way that, say, seeing the colour red is, but simply inferred by comparison with memories acquired by our previous temporal parts. Each such temporal part of us is simply constituted so as to eternally experience the feeling of passage.
Given that time does not progress, one might then wonder why conscious processes always run forwards: the temporal ordering of states we have described is not, as of yet, equipped with a direction. The answer is again provided by physics - the second law of thermodynamics states that the disorder or entropy of an isolated system tends to increase with time. For instance, if a gas is released into one corner of a large room, it will gradually dissipate throughout it. As organized states become more disordered a local ‘arrow of time’ emerges. Temporally directed phenomena often exhibit a similar increase in entropy, whereas those that look the same forwards as they do backwards - such as the orbit of an ideal satellite - do not. As Austrian physicist Ludwig Boltzmann (1844-1906) pointed out, you can’t unscramble an egg. It is therefore almost inconceivable that there are conscious beings perceiving the world in reverse, where this arrow of time has its current orientation. This relationship between time and disorder is perhaps what makes it so challenging to comprehend complex emergent phenomena that seem to break the convention, such as the increasing complexity that marks the evolution of life.
Once the perception of change has been accounted for we see that there can be no valid phenomenological objection to four-dimensionalism, because it does not claim to impact upon the subjective impression of passage in any way. The tensed perspective is more than an idiomatic manner of speaking: it is a physical way of speaking and thinking. Experiencing the Universe as a static manifold is an impossibility because every thought necessarily has temporal extension: necessarily involves change. All discourse is issued from this tensed perspective: even our description of the objective reality of space-time is contained within it. This description can again be analyzed tenselessly - but only from another vantage point that also involves tense and change.
1This problem was first recognised by the Scottish philosopher J.E. McTaggart in his influential essay ‘ The Unreality of Time ’ in 1908. His proof runs along similar lines.
2We are used to describing motion as dx/dt - but what of change in time itself?
3Known as the Rietdijk-Putnam-Penrose argument. See H. Putnam, Time and Physical Geometry (1967), or the Andromenda paradox in Penrose’s ‘ The Emperor ’ s New Mind ’ for a novel illustration.
4A third popular view of time is an asymmetrical growing Universe, where the present and the past exist but not the future. However, amongst other problems, this view is still subject to the above argument against the tensed view. McTaggart himself concluded that time does not exist.
5 A possible flaw is that in fact time seems to be continuous, and can be divided into arbitrarily small intervals. Special relativity also suggests that there is no canonical division of space-time into space and time. These questions will not concern us here.
6As put by David Lewis in ‘The paradoxes of time travel’, American Philosophy Quarterly, 1976
7This does not depend on the assumption that biology can be reduced to physics, although if that were true the point would be more obvious.