Much has been written about Einstein’s theory of relativity and how it contradicts the philosophical notion of ‘the present’. Most of the literature concerns the special theory of relativity (SR) as opposed to the general theory (GR) which also takes into account the effects of gravitation due to the ‘bending’ of space and time, whose complex four-dimensional geometry is thought to account for the motion of objects. Instead of taking space and time to be distinct entities (if they can be called entities), both theories posit a single four-dimensional manifold known as spacetime, which can be ‘sliced up’ in various ways depending upon the motion of the observer. Thus, what for one observer might constitute space or ‘space-like relations’, for another may constitute time or ‘time-like relations’, with important consequences for their interpretations of simultaneity, mass, distance, and so on. Einstein’s crucial insight was that no matter which frame of reference you happen to inhabit, the laws of physics remain constant. In particular, light appears to travel at precisely the same velocity away from every observer regardless of how fast they are travelling; i.e. it does not, contrary to conventional experience, recede more slowly from an observer who is travelling in the same direction.

From a philosophical point of view, several distinctions should be noted. Firstly, the four-dimensional geometry of relativistic spacetime is quite different to the four-dimensional picture of space and time often employed by philosophers in defending temporal parts theory, for example. The former — sometimes known as Einstein–Minkowski spacetime — is non-Euclidean, i.e. it contains ‘bends’ or ‘kinks’, whereas the conventional conception of time as a fourth dimension is generally thought of as being ‘flat’; i.e. it designates a fourth ‘time dimension’ in addition to the three regular spatial dimensions. In Minkowski spacetime, however, there is no time dimension, since time is relative to an inertial frame and so what constitutes time (as opposed to space) will differ between observers travelling at different speeds. The parallel between the two forms of four-dimensionalism is suggestive, but ultimately obscures radical philosophical differences, and in particular the distinction between spacetime as a unified entity and more traditional notions of space and time as independent dimensions or systems of co-ordinates.

Furthermore, much has been made of the fact that SR and most formulations of GR (see below) do not privilege any particular inertial frame, and so posit no universally agreed upon standard of simultaneity since observers will differ upon what appears to be simultaneous depending upon how fast they are travelling and in what direction. This is largely (entirely?) due to the fact that light — or any other kind of information, for that matter — takes time to travel due to the principle of locality (no action at a distance), and so what each observer perceives as simultaneous will depend upon their relative motion as characterised by a particular inertial frame. Since light travels at the same rate relative to all observers, as described above, the only objective fact that can be universally agreed upon is the causal structure of the universe, which is multi-dimensional, but not its absolute temporal order, which can only be calculated relative to a given observer. Since such observers — people, for example — generally move around or are accelerated due to gravity and other forces, even a single observer may occupy many different inertial frames throughout their lifetime and so cannot give a fully ‘objective’ description of the unfolding of events.

Relativity theory — and particularly SR — is often thought to contradict the very notion of ‘the present’, as constituted by an instantaneous ‘snapshot’ of or ‘moment’ in the life of the universe, or as interface between past and future, as suggested by Aristotle. Since there is no privileged frame of reference, or so the argument goes, there can be no privileged ‘present’ moment, but rather which events are happening now is relative to the observer’s frame of reference. Many philosophers have thought that this overwhelmingly endorses the ‘eternalist’ or ‘B-theoretic’ picture of temporal reality in which all events, past, present and future, are seen as being stretched out in an unchanging and atemporal four-dimensional manifold, just waiting to be encountered by us as we travel through the time-dimension of our particular inertial frame. However, it seems to me that this conclusion is far too hasty since it glosses over some important methodological and philosophical principles.

In the first place, it is far from clear that GR (as opposed to SR) does not contain a privileged inertial frame. It is possible that the distribution of matter in the universe does in fact fix a particular frame of reference, which may then be used to define a single universally agreed upon standard of simultaneity, or ‘present moment’. Or so physics tells us. Although I am not sufficiently knowledgeable about GR to pronounce one way or the other, it seems at least plausible that there are elements of GR which, unlike SR, lend themselves to a more conventional picture of space and time, and so this is an option that should not be ruled out prematurely. Furthermore, we must bear in mind that both theories of relativity are fundamentally incomplete since they do not take into account of many important phenomena, especially at the sub-atomic scale, where the physical model of space and time becomes even more surprising. Recent attempts to unify relativistic theories of the large-scale structure of the universe with quantum mechanical accounts of its small-scale structure appear to be getting stuck on precisely this question of what constitutes space and time, resulting in radical differences in the way that these entities are treated or explained. For example, the ‘background independent’ theory of loop quantum gravity provides an account of how space and time themselves are structured and created, whereas string theory presupposes a complex 11-dimensional geometry within which all energy and matter exist. What causes space and time to exist, or why they should possess this particular geometry, is not something that the theory itself explains (and all the worse for it, according to some physicists). Such wide divergence of opinion gives sufficient reason — for philosophy if not physics — to reserve judgement on such delicate matters as simultaneity, and so jumping on the SR bandwagon to pronounce the death of ‘the present’ would be somewhat precipitous, if not downright philosophically irresponsible.

However, I think that a stronger case in favour of the philosophical theory known as ‘presentism’ (the doctrine that only what is present exists) can be made. The alleged counterexamples to presentism arising from relativity theory generally come in one of two varieties. The first involves the observation that simultaneity is relative to a given inertial frame, and so consequently not all observers will agree on what is simultaneous, as described above. This, however, seems to be the case due to the fact that, as has already been noted, information takes time to travel, and so the way the universe seems to a given observer will depend upon what information they receive and in what order, rather than being able to see the way that things really are at another spatial location as it happens. This does not, it seems to me, contradict the notion that there is a way that things really are at some particular time in every other location, but merely reinforces the difference between appearance and reality with which we are all familiar (i.e. it only seems to observer A that observer B’s light flashed later than her own due to the information from A’s location arriving at B after it was originally emitted). Thus we do not have to construe reality or simultaneity as being relative to a particular inertial frame, but can differentiate between reality, which is objectively real, and appearance, which is relative to one’s particular perspective, in order to explain the relevant phenomena. (Perhaps this response trades on a naïve interpretation of relativity theory, but if so I’d be interested to hear where it goes wrong.)

The second type of alleged counterexample to presentism involves the fact that identical clocks travelling at different speeds will run faster or slower relative to one another. A clock, whose mechanism can involve any physical process, travelling at 95% the speed of light, for example, will measure a shorter duration of elapsed time than one that is in freefall (the closest thing that relativity offers to ‘absolute rest’). Time, in effect, will pass ‘more slowly’ according to how fast one is travelling up to the logical maximum of the speed of light, at which time would effectively stand still (although there are good reasons why an observer with non-zero mass cannot accelerate up to this speed, since mass also increases with velocity). This phenomenon has been observed to follow Einstein’s theory to a high degree of accuracy, and so must be accounted for by any future ‘complete’ theory of physics, should such a theory turn out to be possible. Such cases are generally explained in terms of the faster clock taking a ‘different path’ through the four-dimensional structure of spacetime such that less time has elapsed whilst covering more space. Since one’s combined velocity through spacetime cannot exceed the speed of light — the fastest speed that there is — travelling faster through space slows one’s rate of passage through time, and so clocks run slower when travelling at speed than those which are travelling more slowly through space, and therefore faster through time.

It is far from clear, however, that the four-dimensional explanation is the only one available. It would, I submit, be possible to account for the relative slowing of the clock in terms of alterations to the physical processes occurring within the clock that is travelling at high speed — it’s increase in mass, perhaps — without invoking the notion of objects traversing different paths through four-dimensional ‘reality’. Of course, such an explanation would need to avoid the obvious circularity of saying that time ‘slows down’ for rapidly moving objects, since this presupposes that time passes at a particular rate that can itself ‘slow down’ or ‘speed up’. However, such changes can only be measured relative to a secondary time scale, since the notion that time itself passes at some particular rate, e.g. one or two seconds per second, is absurd, as many philosophers have observed. However, this does not mean that such an explanation is impossible. Provided that the rate of change is properly indexed to other changes that occur (or would occur) in matter travelling at slower speeds relative to the rest of the universe, the need for an additional fourth dimension of reality is avoided in favour of a single three-dimensional reality, events within which are governed by a single set of inertia-sensitive laws. (Again, I do not pretend to have an in-depth understanding of the physics involved, but this kind of solution appears to be at least prima facie plausible from a philosophical point of view. I invite argument from my physicist friends as to why such a formulation should be considered impossible.)

So far, I have given two reasons why I think that it is too soon to give up on the notion of ‘the present’ as a unified, coherent and simultaneous state of the entire universe: (i) the incompleteness of current physical theory, and (ii) the possibility of accounting for relativistic phenomena in other ways than positing four-dimensional Minkowski spacetime. However, there is a third and perhaps more important way in which the presentist may perhaps resist the claim that SR and/or GR refutes their theory, which concerns the very notion of ‘the present’ that such arguments employ.

Philosophers and physicists have typically either identified ‘the present’ with a single instantaneous moment of time — the ‘now’ — or a very short duration — perhaps something approaching the Planck length with respect to time. The latter notion, however, may arguably be shown to be incoherent since if the present itself has some finite duration, then it also has a beginning and an end which themselves can either be past, present or future. As Augustine famously observed, since it is absurd to think that some part of the present may itself not be present, this leads to the conclusion that the present must be instantaneous, or possess zero duration. This notion of an ‘instantaneous present’ is expressed within relativity theory in terms of an inertial frame which, as has already been discussed, causes problems for a universal definition of simultaneity for reasons discussed above. However, I do see any reason why the presentist need adopt this notion of ‘the present’ as it stands. Instead, the presentist’s ‘present’ should be expressed in terms of the world’s existing as a enduring three-dimensional whole, rather than an instantaneous time-slice of some notional ‘temporal continuum’ — the very image of time that presentism itself rejects. Instead, the presentist’s ontology should be framed in terms of being, which is essentially temporal (i.e. tensed), and existence, since — according to the presentist, at least — only what is present exists. The being of the present should not merely be contrasted with the non-being of the past and future, therefore, but rather expressed in terms of the being of the world as a unified and co-present whole. To that extent, the name of the doctrine may itself mislead since it characterises the theory in terms of the very picture that it rejects, i.e. the notion of ‘the present’ as an instantaneous time-slice between the past and future, as opposed to what it affirms, i.e. the existence of the world as a whole. Given that the moniker of ‘existentialism’ is already taken, perhaps ‘three-dimensionalism’, ‘endurantism’, or even ‘universalism’ would be a better name, although the former terms are more usually employed in relation to metaphysical discussion of the persistence of objects, rather than as a doctrine concerning the nature of temporality per se (although the two are of course very closely related).

Of course, simply calling presentism by a different name does nothing to answer the challenge that Augustine and others put forward concerning the duration of the present, but when we consider this question in relation to the world as a three-dimensional whole, we can perhaps start to see why it is so misguided. Does the world as a whole have a duration? Well it persists in time, if that’s what you mean, but otherwise the question lacks sense. Which inertial frame is privileged in calculations of simultaneity? I do not think the presentist need take a view on this question, since it only makes sense from within the theoretical framework of SR (and possibly GR), although he does then owe us an account of relativistic phenomena that does not rely upon the four dimensional account of spacetime, but rather accounts for differences in judgements of simultaneity and relative rate of physical processes in terms of the speed that information can travel and the behaviour of energy and matter as it travels at different speeds relative to its surroundings (i.e. a Machian formulation of relativity theory — something which may or may not be possible, depending upon who you talk to). In short, the presentist’s ‘present’ need not be the zero-duration instant that we have come to think of as synonymous with the term, but may take into account a broader and more flexible notion of the world, reality and being itself.

Is presentism incompatible with relativity? I don’t think so. Certain formulations or interpretations of relativity certainly posit the existence of a single unchanging four-dimensional manifold. However, this may be as much an artefact of the way that relativity theory is formulated as opposed to an essential feature of relativistic theories per se. Just as we shouldn’t be misled by Newtonian physics’ use of the variable t for time into thinking that time is itself a distinct dimension, much careful analysis is required before jumping to the conclusion that Minkowski spacetime literally represents how space and time really are, as opposed to merely being a convenient mathematical model for describing the unfolding of certain spatiotemporal phenomena. Moreover, physical accounts of time are prone to treating the past as being entirely symmetric with the future, whereas everyday experience shows them to be anything but. As such, any four-dimensional or eternalist theories is guilty of obscuring the deep metaphysical connections between temporality and modality — i.e. possibility and necessity — since what is time if it is not the process by which the possible becomes actual? This apparent symmetry is perhaps unsurprising since physical theories are based upon the model of past observations, extrapolating these patterns in a way that mirrors the structure of the past in its predictions for the future. However, as a recent paper on the possibility of quantum mechanical time travel admits, ‘looking backwards, the world is deterministic … looking forwards, the future is probabilistic’, and our best theory of the workings of the small-scale structure of the universe, namely quantum mechanics, does preserve an important asymmetry between past and future in terms of the collapse of the wave function. If our intuitions of free will and the openness of the future are true — and why should we think they are not? — then it is the eternalist and not the presentist that has some explaining to do, since only the latter theory is compatible with the non-determinateness of future events, regardless of whether the laws of physics are determinate or (as currently appears to be the case) not.

The moral of this story? That philosophers should be careful not to interpret physical theory too literally since — as most physicists would be the first to admit — such theories merely represent an abstract model of reality, as opposed to a literal description of it. The Quineans amongst you may think that this amounts to much the same thing, and there are times that we should have the courage of our convictions and, á la David Lewis’s modal realism, posit the existence of those entities that the variables in our best physical and philosophical theories dictate (not that I subscribe to modal realism — quite the reverse!). However, in my view, as regards the reality of the past and the future, the jury is still out, and the inconsistency of relativity and ‘the present’ has yet to be conclusively demonstrated. In the meantime, by rejecting the physicist’s or Aristotelian notion of ‘the present’ in favour of a more flexible and inclusive conception of reality as a whole, it remains open to the presentist to describe a view in which reality is not confined to a single durationless time-slice, but in which the abstract entities we think of as ‘times’ are explained in terms of the being and endurance of the world, which is in a constantly changing and self-creating state of flux. In other words, in questions of time, there is still everything to play for.