In a famous letter to a bereaved family friend, Einstein wrote: “For those of us who believe in physics, the distinction between past, present and future is only a stubbornly persistent illusion".  This has been widely interpreted to mean that Einstein’s theory of relativity itself implies that the passage of time is an illusion and that time, like space, has no direction, a position often referred to as “the block universe”. But despite Einstein revolutionizing our understanding of time, nothing in his theory of relativity suggests that the distinction between past, present, and future is an illusion, argues Tim Maudlin.

Albert Einstein had the double-edged gift of writing striking aperçus. Instead of saying “Quantum mechanics is on the right track, but I am not convinced that the laws of physics are indeterministic” he penned to his friend Max Born “The theory provides much, but it doesn’t bring us closer to the mystery of the Old One. In any case, I am convinced that he doesn’t roll dice”. The quotability of “God doesn’t play dice with the universe” has resulted in it being widely accepted as Einstein’s primary complaint about quantum mechanics, which is not the case. It was rather the non-locality—the “spooky action-at-a-distance” in his pungent phrase—that he really found unacceptable.

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Another extremely widely cited Einstein text, also written as a personal note, has caused similar confusion. When his dear friend Michele Besso died in 1955, preceding Einstein himself by only a month, Einstein wrote a letter of condolence to Besso’s family. In it, he said “Now he has departed from this strange world a little ahead of me. That signifies nothing. For those of us who believe in physics, the distinction between past, present and future is only a stubbornly persistent illusion". Since one might reasonably say that time itself is the foundation of the distinction between past, present and future—it is the temporal relation of earlier/later between events that underpins these concepts—this passage has largely been taken to suggest that time itself is, in some sense, illusory. This quote has been the basis of what has come to be known as “the block universe”, which supposedly implies that the passage of time is an illusion, and that time is just like space, another dimension with no inherent fundamental direction. However, there is nothing in Einstein’s theory of relativity to support any of these claims.

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In truth, I think that Einstein never regarded time as an illusion.

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To begin with, if one of the discoveries of physics were that time is an illusion, it would be hard to even imagine a more momentous achievement. But it ought to immediately strike one as peculiar that if Einstein had really thought his theory of relativity implied that time is an illusion, he would only include this claim in a private note of condolence that he had no reason to expect ever to be made public.

In truth, I think that Einstein never regarded time as an illusion. His note is simply a considerate attempt to console Besso’s family who was suffering a recent loss. Indeed, there is a subtly self-undermining character to the idea that such an illusion could be “persistent”, since something only persists if it remains the same through time. Nonetheless, Einstein’s remark is often taken to be emblematic of the suggestion that time is somehow not “real”, that time in the Theory of Relativity has become “spatialized”, that there is no fundamental distinction of earlier/later or past/present/future, that time does not “pass” at all.

There is certainly nothing particular to the Theory of Relativity to support any of these claims – when it comes to the question of whether time is real or illusory, there is no way in which it essentially differs from Newtonian physics. Einstein and Newton do of course disagree about the details of the temporal structure of the universe. For Newton, an “instant of duration” is a global thing: as he says “the moment of duration is the same at Rome and at London, on the earth and on the stars, and throughout all the heavens.” This expresses the idea of absolute simultaneity: that for any given event there is an objective physical fact about which other events in other places take place “at the same time”. For Newton, time is the succession of the global instants. This is the everyday concept of time most of us operate with. If I snap my fingers here on Earth, I may not know what is happening at that very instant on, say, Alpha Centauri but the usual thought is that something precise is.

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To postulate a novel structure for time is not to deny the existence of time or to render it an illusion. There is still a completely objective fact, according to Relativity, that some events precede others in time.

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Relativity eliminates the notion of absolute simultaneity. In its place, with regards to the event of my snaping my fingers, it postulates a three-fold division of other events into those that are past- related, future related, and space-like related to it. The main difference from Newton’s picture is that there will not be just one “moment” on Alpha Centauri, as there is only one Newtonian moment that is simultaneous with it.

But to postulate a novel structure for time is not to deny the existence of time or to render it an illusion. There is still a completely objective fact, according to Relativity, that some events precede others in time. Besso’s death preceded Einstein’s, wherever you are in the universe. That’s part of the reason that Einstein could write a letter of condolence to Besso’s family but not the other way around.

Nonetheless, a rather widespread terminology has taken root in both physics and philosophy that appears to deny this. We are told that the Theory of Relativity—and maybe even Newtonian physics!—postulates a “block universe”, and that in a block universe “time is an illusion” or “there is no passage of time” or “time is just like space” or “there is no fundamental direction of time”. Whatever is meant by these locutions, they seem to be intended to express some astounding discovery or postulate about the nature of time itself. Certain, saying that “time is an illusion” suggests that. There is really nothing that seems to be less possibly illusory than time. Descartes, at the very peak of his skeptical fever in the Mediations, manages to cast doubt on the existence of space but not of time. He never doubts that he is engaged in a process of thinking in which certain thoughts give rise to others, for example.

All proponents of a “block universe” hold that space-time is (at least macroscopically) four-dimensional. But that just means that locating a particular event (such as a particular snapping of my fingers) requires specifying four co-ordinates: a longitude, latitude, altitude and time, for example. Newton would of course not object to that. All proponents of a block universe believe that there is a unique past and a unique future. The past was as it was, and the future will be as it will be, irrespective of what we know about either. This is not an assumption of determinism: que sera, sera is an analytic triviality, not a contentious physical postulate. But if all of this is common ground, what makes the “block universe hypothesis” at all astonishing? Why would pointing these facts out console Besso’s family?

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One scholar who has done us the favor of explicitly defining a “block universe” is Huw Price. In his book Time’s Arrow and Archimedes’ Point, Price makes clear exactly what he takes a “block universe” view to posit: “For now on, I will simply take for granted the main tenets of the block universe view. In particular, I’ll assume that the present has no special objective status, instead being perspectival in the way that the notion of here is. And I will take it for granted that there is no objective flow if time”. Note that Price’s characterization has two clauses, which are quite different. They differ both in content and more importantly in how surprising or revisionary they would be.

To take the first: “here” is what is called a token-reflexive or indexical term. Unlike, say, “Addis Ababa”, the location referred to by a token of “here” depends on where the speaker is when it is pronounced or written. If the speaker happens to be in London, then “here” refers to London, and if in Addis Ababa then “here” refers to Addis Ababa. In that clear sense, “here” has no “objective status”: it refers to different places when spoken at different places. And similarly for temporal terms such as “now” or “yesterday” or “in a month”: obviously what particular regions of space-time are indicated depend on when the token is produced, just as the spatial locutions depend on where. There is no “objective now” in the sense of some moment of time that all tokens of “now” refer to. But so what? Nobody ever thought differently. The idea that “the now” has a “special objective status” in a way that “the here” does not is already universally rejected.

So if the “block universe” is supposed to be something controversial, that has to be in the second clause “there is no objective flow of time”. But what is that supposed to mean? That it is not an objective fact that some events take place before a given event and others after? That would mean that time has no directionality at all. And that certainly would be a revolutionary discovery. But there is nothing at all in physics—classical or Relativistic—that suggests it.

Spatial dimensions have no directionality. One can say that a certain longitude line runs north/south, but it would make no sense that it really runs north-to-south as opposed to south-to-north. But time does have a directionality, indicated by the asymmetric relation before/after. This is a distinction that all physicists—including Newton and Einstein—as well as all everyday folk take for granted. The before/after distinction grounds the cause/effect distinction: everyone would accept that an earlier configuration of the planets, together with the laws of gravity, cause and explain their later configuration, but no one would say the later configuration causes or explains the earlier. It may indicate or allow one to infer the earlier, but not explain it.

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Temporal structure is fundamentally different from spatial structure. And temporal structure—time itself—is not an illusion.

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Every physicist who makes reference to “the initial conditions” of the universe, or the “fate of the universe” presupposes a direction of time. And that is something Einstein did without a qualm. And he never suggested somehow trying to remove that direction or reduce it to something else.

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Price is perfectly aware of this, although he does  try to eliminate any objective notion of temporal direction—which usually underpins the notions of causation and explanation—from his fundamental account of the universe. And no one can prove it can’t be done, although the obstacles are high and the prospects dim. But that is certainly not a project that Einstein was engaged in, or thought he needed to be. The standard reading of cosmology done in accord with General Relativity is that there is an initial state of the universe which then evolves—in accordance with Einstein’s Field Equations­—to later states. The later are accounted for by the earlier. Whether there is an earliest of all, and if so whether it could be explained in any way we could recognize, is a thorny question. But the present practice of physics, including Relativistic physics—takes a fundamental earlier/later distinction, and in that sense a “flow of time” for granted. Temporal structure is fundamentally different from spatial structure. And temporal structure—time itself—is not an illusion.