Most people have heard that Einstein’s Special Theory of Relativity revolutionized our understanding of time. But most people still aren’t aware of quite how profound the consequences of Einstein’s block universe are, according to which our experience of the present as uniquely different from the past or the future, the very idea of time having a direction, of “passing”, is put into question, argues Michael Silberstein.
“The objective world simply is; it does not happen.” – Hermann Weyl
“I am a Tralfamadorian, seeing all time as you might see a stretch of the Rocky Mountains. All time is all time. It does not change. It does not lend itself to warnings or explanations. It simply is.” – Kurt Vonnegut
If you explore YouTube, you will find hundreds of videos and recorded lectures that assert “time is an illusion” given Einstein’s relativity. But is this true? And what exactly do these people mean by “time” and “illusion”? Because these words have multiple meanings, we must ask. My goal here is to help the reader understand in exactly what sense Einstein (or anyone else) might justifiably assert that time is an illusion based on relativity. In so doing, I argue that there are essential features of our temporal experience, such as the passage of time, presence, and direction, that cannot be fully explained in the block universe picture given to us by relativity—our best physical theory of time. And whilst these essential features of time are not themselves illusory, there is a mystery about the status in which relativity theory leaves them.
Presentism vs Eternalism
I will begin with an analogy between the experience of watching a film and our everyday experience of time (Passage, Presence, and Direction). Go back to the days of film on celluloid. Imagine you are a child who has just watched their first such film and you ask your parent, “How does it work”? Your parent takes you back to the projection booth and to your amazement, the film you watched is just a series of photographic stills each framed in black and ordered in a certain sequence. You are deeply puzzled by how these static images connect with what you just experienced. The projectionist jumps in to show you how the trick is done. You learn the film is projected at a rate of 24 frames per second, helping to create the experience of temporal passage. The film reel advances one frame, pauses for a fraction of a second, projecting the image of each individual frame when it is between the lamp and the lens, to create the sense that frame alone is happening now, and then advances to the next frame, and so on. The projector has other mechanical tricks for making the short periods of darkness between still frames, flicker effects, and its own motion undetectable.
From this, you gain an understanding of the mechanism by which a mere reel of film can be turned into an experience of a story unfolding in time. Mystery solved. You realize that the combination of the appropriately ordered frames moving through the projector creates the “illusion” that only the present moment (the single frame poised between lamp and lens) is real (exists) and that the present moment (a NOW-slice) keeps moving from past to future evolving into ever newer present moments until the end. I say “illusion”, because you now know that the movie experience of continuous temporal flow does not reflect the reality of a series of still frames moving through a projector.
There is something special about the character of the present moment. This is what presumably lead Einstein to say that “there is something essential about the Now which is just outside the realm of science.”
If you are the kind of child Einstein was, you might wonder how the trick of time works in the actual world. Just like in a film, life is experienced as a temporal progression in which the present moment seems objectively distinguished as if a spotlight is shone upon it (Presence), that moment seems to objectively flow or pass (Passage) into the next moment, and that passage seems to have an objective direction moving from past to future (Direction). I will refer to these three features of temporal experience as PPD. Presence has two features because it denotes the experience that a certain event, such as Neil Armstrong placing his foot on the moon, is happening NOW (is real), but it also denotes the experience that there is something special about the character of the present moment. This is what presumably lead Einstein to say that “there is something essential about the Now which is just outside the realm of science.
When I say that the Passage, Presence and Direction features of time are “objective” I mean that they are not in any way mind/brain dependent (i.e., these features of the world are primary properties like shape and not secondary properties like colour), nor are they dependent on one’s place in the universe. Moving away from individual experience for a moment, we also tend to assume that, continuing with the film analogy, everyone in the universe is watching the same film, i.e., everyone would agree on the ordering of events (still frames) into past, present, and future NOW-slices, just as the audience in the theatertheatre does. In other words, everyone will agree about which events are real, because the phrase “is real” just means exists NOW. And to say that various events exist NOW means those events are simultaneous (co-exist) with each other. Based on everyday experiences of the sort that film attempts to emulate, we tend to assume that only present events (those that exist NOW) are real, and not past or future events. This is a metaphysical view of time known as presentism.
Einstein and why the block universe is a mistake
But suppose you found out that reality is much more like the film strip laid out in the projector booth in front of the child, whence all the frames (i.e., past, present, and future events in the actual world) are all equally real. This is a metaphysical view of time known as eternalism, also known as the block universe. The irony, as we shall see, is that eternalism follows from special relativity precisely because there will be relativistic reference frames whose observers disagree about the temporal-ordering of events into past, present, and future. That is to say, there will be disagreements as to which events are simultaneous with which. There will be frames of reference (such as planets at great distances from Earth or a spaceship moving by Earth at a large fraction of the speed of light), whose observers will disagree about how to order events in the universe into NOW-slices. In the terms of the film analogy, in the actual world, not everyone is watching the same movie.
Newton vs Einstein
To see why let us take a short interlude into the physics and the metaphysics of time. In Newtonian mechanics, we have three dimensions (3D) of absolute space in which objects move as a function of absolute time, i.e., everyone is watching the same movie, no one anywhere in the universe will disagree about the temporal-order of events. In Newtonian mechanics everyone agrees on what exists instant by instant, so NOW-slices are the same for all observers. That means the proper time for an object, such as time as measured on each observer’s watch, is the absolute time.
Given this equality between proper time and absolute time, the Newtonian model for the path of an object through space is a one-dimensional curve in 3D space where absolute time is the curve parameter. For example, if you are using Cartesian coordinates [x,y,z] to locate an object’s location or an event in space, an object’s path would be specified by the three functions [x(t),y(t),z(t)] where t is the time parameter. A NOW-slice of all the coexisting (simultaneously existing) objects and events at time to would be all the objects’ locations and events given by [xn(to),yn(to),zn(to)] for each object or event “n”. Of course, Newton was simply modelling his own everyday film-like experience of space and time. Newton just assumed his temporal experience contained no misperceptions and no false beliefs based on such misperceptions.
In Einstein’s special relativity, Newton’s 3D space plus absolute time parameter t (3+1 model) is replaced by a 4-dimensional (4D) model where space and time are fused into what is called spacetime. Special relativity follows from: 1) the light postulate which says the speed of light is the same in every inertial reference frame, and 2) the relativity principle which says the laws of nature are the same in every reference frame. In special relativity, the worldlines for all objects can no longer be given as a set of parameterized curves in one 3D space because observers can disagree as to what objects and events coexist to uniquely establish a NOW-slice of 3D space. In special relativity, simultaneity is no longer absolute; it is relative to each observer. That means each observer has their own 3+1 film-like model for their own NOW-slices.
If there is an event such as the supernova explosion that is experienced by different observers, but they do not agree as to when that event happened, the event must just exist statically, timelessly, in order to be experientable from both these different spatiotemporal perspectives
Combining all of these different 3+1 models into a single self-consistent 4D model (called Minkowski spacetime) requires the addition of a time dimension with its own coordinate t. Events are now located by a set of four coordinates [x,y,z,t] in the 4D spacetime, rather than three coordinates in 3D absolute space with a particular value of the absolute proper time parameter. This means that the path of an object in special relativity is now a curve in 4D spacetime given by [x(T),y(T),z(T),t(T)], where T is the proper time for the object. The 3+1 model is one of coexisting objects moving in space as a function of time in accord with our film-like experience. The 4D model, on the other hand, represents objects in terms of their worldlines in 4D spacetime and it cannot be carved uniquely into 3D collections of coexisting objects and events.
Accordingly, the life of observer A would be a curve in 4D spacetime that begins at birth and ends at death as parameterized by observer A’s age (their proper time). Obviously, no two events on that worldline are simultaneous for observer A (i.e., all such events are timelike—inside or on observer A’s lightcone). But according to observer B, observer A’s 16th birthday might coexist with a supernova explosion in a distant galaxy, while according to observer C, that same supernova explosion coexists with observer A’s 20th birthday. Thus, from observer B’s perspective, A’s 16th birthday party and the supernova explosion are simultaneous events. Whereas from observer C’s perspective, A’s 20th birthday party and the supernova explosion are simultaneous events. And in spacetime, observer B and C are both right. Observers B and C might even meet up in the future and discover this very disagreement.
Spacetime and the Structure of Reality
With Carlo Rovelli
If there is an event such as the supernova explosion that is experienced by different observers, but they do not agree as to when that event happened, the event must just exist statically, timelessly, in order to be experientable from both these different spatiotemporal perspectives. And while observer A’s 16th and 20th birthdays are not simultaneous events from any “ant’s-eye” reference frame perspective such as A, B, or C, from the 4D perspective they must both be equally real because there are frames of reference (B and C) with respect to which the supernova explosion is simultaneous with each respective birthday party. Everything said here applies to all events in spacetime. All of this constitutes a powerful argument for Eternalism.
Simply put, there is no unique way to carve 4D spacetime into individual 3D distributions of coexisting objects and events in order to create the individual frames for an objective film shared by all. Given that the world is 4D spacetime and not Newton’s 3+1 model, we can reasonably say that our film-like experience of a 3+1 world is a misperception, and some of our beliefs based on that misperception are false. Some use the word “illusion” to denote this. We have seen that with spacetime, just as with the entire reel of film in the projection booth, as viewed by the child, all events (frames) are equally real. But unlike the reel of film in the projection booth, events (frames) in spacetime have no objective ordering into NOW-slices. The irony is that the latter fact is the reason for the former fact. It is because the structure of spacetime demands that there must be many individual films with different temporal orderings involving some of the same events, that all events are equally real.
Think how strange this is. Within spacetime there is no ant’s-eye observer’s reference frame (such as A, B, or C) for whom A’s 16th and 20th birthdays are simultaneous events. Yet, when we look at the spacetime diagram of all this, as in the 4D perspective, one must grant the equal reality of A’s 16th and 20th birthdays. This is the case because it is the only way to explain how both observers B’s and C’s claims can be true.
So, is time an illusion then?
In a recent piece, Tim Maudlin grants that spacetime is a block world as I have defined it, but he does not think it warrants the word “illusion.” In his own words, “when it comes to the question of whether time is real or illusory, there is no way in which it [Minkowski spacetime] essentially differs from Newtonian physics.” Some philosophers go further and suggest that there is no discernable difference at all between the Presentism of Newtonian mechanics and the Eternalism of Einstein’s spacetime. We have already seen that such claims are false. Just think of time-dilation and length-contraction in relativity, wherein we learn that the elapsed time between events and the length of objects are frame-dependent. Regarding the very same events and objects, there will be observers on various reference frames who will disagree about these properties, and both will be right! This highly counter-intuitive state of affairs could not happen in Newton’s world. I would say all of this constitutes a pretty significant difference, and we are not done yet.
You might ask, since Maudlin and I do not disagree about the facts thus far, only whether to call this state of affairs an “illusion”, do we disagree about anything else? Yes. I think that the block universe of spacetime lacks PPD, and Maudlin does not. I will explain my position first and then reply to his.
So, why does the block universe of spacetime lack Presence, Passage, and Direction in a way that Newton’s model does not? As Maudlin himself notes, the Newtonian model assumes absolute/objective simultaneity relations and that time is “the succession of the global instants” from past to future. Unsurprisingly then, Newton’s physical model of time grounded in everyday experience is often associated with or interpreted in terms of Presentism. Presentism in full holds that: 1) Only the present is real; 2) Time comprises an “A-series”: there exists an objectively present moment, and that moment continuously and objectively vanishes and is replaced by a new one, as time moves in the direction of past to future, and 3) the “A-series” that is built into the structure the world is both necessary and sufficient to explain our experience of PPD. In other words, Presentism is the view of time the child has after watching the film and before going back to the projection booth. If Presentism is true, then the statement that “time is like a river” is not a terrible analogy.
Einstein didn’t think time was an illusion
Sometimes Presentism is characterized as the claim that there are objective tensed facts, i.e., facts about the pastness, presentness, or futurity of events. The A-series provides a dynamic theory of time because once an objective distinction between past, present, and future is granted, then it is the case that events must continually change with respect to their pastness, presentness, or futurity. Presentism is taken as the champion of our everyday experience of time.
The point is that spacetime poses an even greater threat to Presentism than alluded to so far. The worry is that the Eternalism of spacetime seems not to contain anything like PPD as conceived by Presentism. That is, there is no universal and objective NOW-slice in spacetime, no explanation for the experience that the present moment is ontologically special (Presence), and there is no literal passage of NOW-slices (Passage) from past to future (Direction). Regarding Direction, there is no reference frame-independent fact about which of two space-separated events is the earlier one and which the later one. Thus, the block universe of 4D spacetime is not logically compatible with Presentism as defined.
The Newtonian 3+1 model, viewed in terms of Presentism, has the resources to explain PPD because there is a physical analog for each. We experience Presence because only the objectively present moment is real, we experience Passage because the present moment literally passes away and becomes another moment, and we experience Direction because everyone in every reference frame agrees on how to carve the universe into NOW-slices.
Our temporal experience misleads us into thinking that Newtonian Presentism is true
Let’s return to the film analogy to see the point. Imagine the 4D block universe from the Big Bang to the heat death of the universe is like the entire film reel in the projection booth laid out from the beginning to the end of the movie. In the case of the film, we know what mechanism explains how we get from a series of still frames to a PPD-experience of the film. But what is the commensurate mechanism in the block universe of 4D spacetime? If the analogy between real spacetime and the film reel is a good one (in the sense that each event/frame are equally real, not in the sense of objective temporal-ordering), then why do we experience PPD? In other words, if objective PPD is not there in the universe, some would say our experience of objective PPD is an “illusion” and our belief in objective PPD is false. This is not to say that the subjective experience of PPD itself is an illusion-we really are having such experiences-PPD is not a hallucination or mirage. Given relativity, the experience of the present, the passage of time, and the direction of time, are an illusion in that what you perceive as a primary property given objectively in the external world is just not there in the physics.
Some people claim that 4D spacetime is missing something physical or metaphysical that needs to be added to explain the experience of PPD. Others claim that the explanation for the experience of PPD must lie with cognitive neuroscience, thus making PPD secondary properties, like color. There are several philosophers, physicists, and cognitive scientists who argue that the brain must somehow generate the experience of PPD. But one need only contemplate this idea for a second or two to see the problem. Barring radical emergence, if physics is “frozen” in the block universe, then so are brains. The brain (i.e., the static 4D worldline of a brain in spacetime) cannot be the analogue of the film projector, because its states no more move or flow than anything else in spacetime. The ‘activities’ of the brain are themselves just more events “frozen” in the still-frames, therefore the brain is not like the film projector that brings PPD to the game ‘from the outside’. Falling back on the ‘dynamical activity’ of the brain just begs the question of how a brain in a block universe could generate, produce, or cause any conscious experience, but especially those involving PPD.
Maudlin thinks that the spatiotemporal structure of relativity itself is sufficient to provide objective PPD and thus explain our experience of PPD. He says it is an objective fact that some events take place before a given event and others after. At least it’s an “objective” fact relative to each observer’s frame of reference. As he puts it, “But time does have a directionality, indicated by the asymmetric relation before/after.” All of this suggests that for Maudlin the experience of Passage and Direction are explained by the fact that for each reference frame events are ordered into a temporal succession of the right sort. Regarding timelike events, every observer in that reference frame will agree on the ordering of events into earlier-than or later-than, before and after, cause and effect, etc. So, there will be many local ‘HERE-NOWS, these ‘HERE-NOWS change in succession, and that is what Maudlin calls Passage and Direction. Back to the film analogy, each of these local reference frames is its own movie.
He says, “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.” For Maudlin this fundamental and irreducible earlier/later-than distinction, as given by events having the right temporal ordering or succession is all there is to Passage and Direction. He also thinks that succession grounds talk of earlier events causing or bringing about later events. As regards general relativity, Maudlin notes that cosmology is done from the perspective of the “cosmic time frame” wherein it is very reasonable and useful to take the Big Bang as the beginning of the universe and the heat death as the end of it. There is a sense then in which the cosmic time frame is like the same movie for all observers.
But is Maudlin right that such succession, whether local or global, is sufficient to explain the experience of Presence, Passage, and Direction? I am not convinced. To see why, let us return to our film analogy. Is the carefully crafted ordering of the still-frames making up the film reel sufficient for succession? Clearly Maudlin must say no since one does not experience PPD until the film reel is subjected to the mechanism of the projector. I assume he would say that before the projector is engaged there is only spatial succession. Temporal succession as in PPD only happens after the projector is active. So again, what in the real world does the work of the projector in the reel world? Nothing I know of. I would say in this case what is true of the static reel of film is also true of the static events in spacetime. Merely having events ordered in intuitive ‘before/after’ relations is not sufficient for the experience of PPD.
As regards causation, in the block universe of spacetime, talk of one event “producing” or “causing” another in the sense of bringing something into being that absolutely did not exist before, only makes sense from the “ant’s-eye” timelike perspective of an individual observer. Likewise, it is only from the ant’s-eye perspective that talk of PPD makes sense, that is, experience with ‘the projector running’.
Finally, regarding Presence Maudlin says, “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.” As noted earlier, Einstein, speaking from his everyday perspective, begged to differ. Einstein worried that physics fails to explain the experience that the present moment is special (Presence). I agree, and I see nothing in Maudlin’s program that resolves this.
PPD itself is no hallucination, but our temporal experience misleads us into thinking that Newtonian Presentism is true. However, if relativity is true and complete, then PPD is largely missing from our best theory of time. Maudlin is right that if special relativity did not have the metric structure it does and if the initial conditions at the Big Bang didn’t enable Einstein’s equations to yield a cosmic time frame, that the mystery of PPD would be much worse. But I have little faith that relativity alone has the resources to resolve the mystery of our experience of time.