The Reality of Time

The idea that time isn’t “real” is an ancient one — if we’re allowed to refer to things as “ancient” under the supposition that time isn’t real. You will recall the humorous debate we had at our Setting Time Aright conference a few years ago, in which Julian Barbour (the world’s most famous living exponent of the view that time isn’t real) and Tim Maudlin (who believes strongly that time is real, and central) were game enough to argue each other’s position, rather than their own. Confusingly, they were both quite convincing.

smithsonian-mag The subject has come up once again with two new books by Lee Smolin: Time Reborn, all by himself, and The Singular Universe and the Reality of Time, with philosopher Roberto Mangabeira Unger. This new attention prompted me to write a short essay for Smithsonian magazine, laying out the different possibilities.

Personally I think that the whole issue is being framed in a slightly misleading way. (Indeed, this mistaken framing caused me to believe at first that Lee and I were in agreement, until his book actually came out.) The stance of Maudlin and Smolin and others isn’t merely that time is “real,” in the sense that it exists and plays a useful role in how we talk about the world. They want to say something more: that the passage of time is real. That is, that time is more than simply a label on different moments in the history of the universe, all of which are independently pretty much equal. They want to attribute “reality” to the idea of the universe coming into being, moment by moment.


Such a picture — corresponding roughly to the “possibilism” option in the picture above, although I won’t vouch that any of these people would describe their own views that way — is to be contrasted with the “eternalist” picture of the universe that has been growing in popularity ever since Laplace introduced his Demon. This is the view, in the eyes of many, that is straightforwardly suggested by our best understanding of the laws of physics, which don’t seem to play favorites among different moments of time.

According to eternalism, the apparent “flow” of time from past to future is indeed an illusion, even if the time coordinate in our equations is perfectly real. There is an apparent asymmetry between the past and future (many such asymmetries, really), but that can be traced to the simple fact that the entropy of the universe was very low near the Big Bang — the Past Hypothesis. That’s an empirical feature of the configuration of stuff in the universe, not a defining property of the nature of time itself.

Personally, I find the eternalist block-universe view to be perfectly acceptable, so I think that these folks are working hard to tackle a problem that has already been solved. There are more than enough problems that haven’t been solved to occupy my life for the rest of its natural span of time (as it were), so I’m going to concentrate on those. But who knows? If someone could follow this trail and be led to a truly revolutionary and successful picture of how the universe works, that would be pretty awesome.

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84 Responses to The Reality of Time

  1. The Drinker says:


    What do you think of this ArXiv paper that appears to show the first experimental evidence that the universe is indeed static and that time “emerges” from quantum entanglement? This might be the first empirical evidence that the B-theory of time is actually true. See below.

    Arxiv paper:

    Any thoughts?

  2. bostontola says:

    Still speculative, but recent work on shape dynamics derives time asymmetry in GR, and the Purification Principle axiom underlying QM may also yield time asymmetry.

  3. Alice says:

    I’m on board with Presentism or Eternalism, but I just can’t get on board with Possibilism. Presentism and Eternalism could really be viewed as a distortion of the same thing. I realize possibilism is likely the result of someone overthinking quantum mechanics. They think that because there are many probabilities possible, then time doesn’t exist until one is picked yada yada yada. But do they really think that space will collapse on itself if one of those possibilities doesn’t occur? Or that space doesn’t exist until one occurs? It seems like that would be a misunderstanding of QFT and inflation as well.

    My argument against Eternalism is that it hasn’t happened yet and it isn’t currently still happening. It isn’t real yet, and even though it was previously reality, we now live in a different reality. Some might say that we don’t live in that “world” yet and we live in a different “world” than we used to. As though there are perhaps “many worlds”.

    A common argument against Nowism would be that we know the past happened and we can predict, with an ever increasing amount of certainty, what will happen in the future. Some might say that we know 100% what happened in the past and that we could only possibly know what will happen in the future with 99.99999+++… % certainty; which means the future of the eternalism cube is already a little… jello-esque while the present and past are solid. If we didn’t know what happened in the past with 100% certainty, then that might suggest that information is lost. But that’s in magical physics land where black holes never merge and the conditions are always perfect to describe a phenomena exactly like a textbook would. Outside of some universal wave function, this might be true, the past might be 100%. But inside some universal wave function, we of course would experience some form of information loss because we (or a particle if you would like) can’t measure ourselves. And then we go down the philosophy rabbit hole again if you like. If you accept the rhymes I-am-a spittin'(he’s definitely a white guy), then the probabilities of past and present become more equivalent. At most, both could be 99.99999+++… % certain and we do lose at least some bit of information due to our experience as a part of the wave function instead of an observer of the wave function. So the cube is pretty jello-y except for a nice little slice at the present. The only thing that you are 100% certain of is the present. There is no information to loose or any probability to guess. And finally; fuck aioli.

  4. Marc Geddes says:

    The picture presented by Eternalism (Block Universe) is by far the most coherent and consistent with what we know about physics. Einstein’s general relativity basically *requires* that all moments in time be eternally present in order to make sense of the fact that different observers can see different time orderings. And then the work of Julian Barbour builds on that in a very natural way. Eternalism would also be implied by Tegmarks ‘all is mathematics’ multiverse theory.

    The alternatives presented by Smolin and co. just aren’t coherent in my view.

    As a mathematical Platonist, eternalism is basically *implied* by my philosophy of the world – in order for reality to be fully objective and explicable , there *must* be a level of description that contains timeless, universal truths (the platonic level).

    So yes, time (in the sense of something that ‘passes’) is an illusion.

    As far as I can make out, the illusion of the passing of time arises because it is not possible for a mind to keep track of all the information. We have to summarize the data , and information gets lost. This is what I call the ‘system level’ of description (as opposed to the ‘platonic level’).

    At the ‘system level’ of description we are viewing reality in terms of ‘systems’ (things defined as having inputs, processes and outputs). For the sake of this mode of description, our minds don’t worry about the origin of the inputs or outputs – we are focusing on the processing, and it is this ‘processing description’ that gives rise to the illusion of the passing of time.

  5. Bee says:

    A lot of this discussion goes back to a misunderstanding about the supposedly ‘timeless’ nature of a particle trajectory. You draw a curve in a space-time diagram, and then you ask, well, which moment is “now” for the particle? Oh, the particle doesn’t know what is “now”! So then you jump to the conclusion that physics cannot explain why we experience something like a “now” and now (haha) you have something to write papers about.

    The problem with this argument is that we, in contrast to a pointlike “thing” (I don’t even want to call it particle) have a memory. Memory isn’t anything that has to do with consciousness, there are many physical systems (think molecules, some condensed matter systems, etc) that have ‘memory.’ It’s easy enough to model this and once you have something with a ‘memory’ this something can have a ‘now’ experience, which is the most recent thing in memory – at each moment of time. So there is really nothing about the ‘now’ that science can’t explain, and I wish people would stop drawing upon this wrong argument.

    I believe that the only reason this non-issue has attracted so much attention is a remark by Einstein that was conveyed by Carnap, I wrote some words about this here.

  6. Gary Godfrey says:

    Mulder’s comments: “I think the question ‘Is time real?’ is as important or useful to think about as the question ‘Is position real?’ “…. “We should be asking, according to Special and General Relativity, if spacetime is real. Why single out time?” are illuminating and can be extended a bit.

    Like spatial translation, time translation is a transformation that one object A does to another object B. Mulder would argue that according to SR, an observer boosted wrt A will see A do a combination of time and spatial translations. Therefore, time translation is as “real” as spatial translations.

    These spacetime transformations empirically are elements (labelled by t,x,y,z) of a Lie Group. The group of transformations that A can do to B is actually larger, and besides time (t) and 3 space translations (x,y,z), includes 3 boosts (vx,vy,vz), and 3 rotations (thetax. thetay, thetaz) for a total of 10 parameters. This is known as the Poincare group.

    The extension to Mulder’s comment is that time translation is as real as any of the other (x, y, z, vx, vy, vz, thetax, thetay, thetaz) group transformations that object A can do to object B. What makes time translation stand out as weird is that object A does it differently and only in the t>0 direction.

    Consider that object B is an apple, and object A is (you + the rest of the universe not including the apple). Whereas object A’s finger (your real finger) does (x, y, z, vx, vy, vz, thetax, thetay, thetaz) transformations to the apple in either plus or minus directions, time translation is done by object A’s yet unseen “finger” in only one direction (t>0) by object A simply waiting. The (t<0) group transformations mathematically exist, but object A’s “finger” responsible for time translation for some reason doesn’t do them.

  7. phumurn says:

    The Present define a cross-section in our univers the past define the course(reason=>of the present) of action the future define the actual univers wich includes in it’s self all the posibilitys …

  8. dave says:

    I think it’s very cute how everyone thinks Sean will answer their (sometimes long) questions.

  9. Bob Zannelli says:

    Alice says

    My argument against Eternalism is that it hasn’t happened yet and it isn’t currently still happening. It isn’t real yet, and even though it was previously really, we now live in a different reality. Some might say that we don’t live in that “world” yet and we live in a different “world” than we used to. As though there are perhaps “many worlds”.


    Alice you’re relating your subjective sense of now , nobody has any disagreement about that. But you’re failing to take into account the new reality presented to us by Albert Einstein’s Special Theory of Relativity. Common sense is often not a good resource for understanding reality.

    To understand this new way of thinking , you must know about what physicists call a now slice. When you think of your personal now , there exist a snapshot, a mental freeze frame image of the entire universe. Every place in the universe is at the same now you are at, that is an event on a distant planet occurs at the now you are experiencing. Of course, you can’t actually observe anything on your now slice, now, because information transfer is limited to the speed of light.

    According to Newton, and this comports with your subjective sense of now, every person in the universe is on the same now slice, so in this picture the past is a memory and the future hasn’t happened yet. But Einstein ( and many experiments) has taught us this is wrong.

    Given any group of observers their relative now slice is given by;

    delta[t] =(delta[v]/c^2)*x

    Where delta[t] is the time difference, delta[v] is the relative velocity between observers, c is the speed of light and x is the distance between observers. Given any relative velocity and distance between observers, there will a different now slice. So for example, imagine that you have a relative speed of 1 meter/sec ( about 2.2 MPH) toward an observer who is at a distance of 10 billion light years. Your now slice will rotate to a point in time approximately 33 years into the future of the other observer. This means that an event that is happening now for you won’t happen for another 33 years for the distant observer. If you reverse direction and walk away from the observer , maintaining the same relative speed , your now slice rotates 33 years into the past of the other observer,. An event you experience now, happened 33 years ago for the distant observer.

    I think you can see by this , that the word exist can’t be just applied to your own personal now. Events in the future and events in the past exist in the same sense that now exist for you. This is called the Block Universe concept. As I pointed out common sense can lead one astray when we try to understand the nature of reality. If you’re amazed by this, you’re not alone.

  10. Cece B. says:

    Hi Dave, Thanks for responding! …

    Wrong as it may be, as I understood it, the “warp drive”avoided the violation of Special Relativity by virtue of the fact that SPACE can do anything it wants, and scrunching space up front of the warp bubble (a black hole?) while stretching it behind, is how the warp bubble is supposed to achieve the apparent speed, not by actual acceleration. even though those aboard the ship could possibly perceive movement or acceleration…

    Dr. Larry Krause (Physics of Star Trek) did a demo of the idea with a pop bottle at some conference. And purportedly, Ron Mallett at the U of Connecticut is trying to use lasers to bend light to specifically achieve time travel…

    If these concepts can be realized, assuming they do avoid violating relativity by bending light or folding and expanding space, wouldn’t you think they might be capable of achieving time travel, at least from the time onwards from when the gadgets were turned on?


  11. simon morley says:

    PLEASE Professor, some academic rigour is needed here!!!

    You can’t speculate about something unless you can do at least one of two thing regarding the subject:-
    1. Either wholly define the thing, the word, you are speculating about,
    2. Or give hard, empirical evidence of its existence around which to build your hypothesis.

    Time is either a tangible reality, and you have evidence to back it up, or it isn’t and is hence abstract, i.e. merely a word, in which case its definition is as we (its creators) determine by how we use the word. THIS IS NON-NEGOTIABLE!!!

    There is no evidence for the existence of time (as a property, as a force, or flow or anything else tangible). None.
    The only evidence we can observe are events happening (a clock ticking is a series of events – caused by energy differential, and conveniently calibrated; all you evidence is change, not time).
    So, go back to the dictionary, and reduce all the multiple (and confused) uses of the word time, and you come down to two root definitions, subtly, but distinctly different:-

    1. It’s the abstract framework by which we index, reference, calibrate events (and change) i.e. the t in science.
    2. It’s a non-specific set of events. (when we say, for instance, “time passed” ALL we actually mean is a sub-set of events happened – that is the ONLY empirical evidence there is for “time passing”)

    So, Time is not a universal, fundamental; events are the fundamental. Without events, time is redundant.

    This definition of Time is comprehensive and water-tight (try it). There is no more speculation of “What is Time”…it’s answered!!!!!!! It didn’t need a book to explain.

    So when you speculate about Time, which meaning are you using??? Or don’t you know (or care??): Smolin never offers a definition – neither do Hawking, nor your good self for that matter.
    What has academic rigour come to???

  12. Tony Patti says:

    My theory is that time, as we know it, and as it applies in classical physics, exists entirely in the past. Our neurological construction of time is created based on our ability to remember and record past events, and compare them with events that are in the future of past events and in the past of past events.

    I think you can argue about many possible interpretations of time, but they all take place in the past. Our ideas of the future come from observations of the past, also, but the assumption that time exists in the future is nothing but an unprovable assumption. The future does not exist, because it hasn’t been created yet.

    The past, since it has already been created, persists in the future. To disrupt this accumulation of mass and energy would take more energy than it took to create it, throughout time, and a paradox in itself. To assume that something already created in the past is already pre-created in the future is an assumption, proven only by examining the past and noting persistence from one moment to the next.

    I’m interested in quantum states precisely because they are observations of time coming into existence. The closer we get to observing the absolute present, much like absolute zero, the closer we get to seeing the universe in the process of coming into existence.

    I wish I had the mathematical skills to run even the crudest tests on this idea, if it’s even possible. The only proof we have of the existence of the future will always remain in examinations of the past, and I find it difficult to understand why that is sufficient simply because it is the only possible evidence we have.

  13. Bob Zannelli says:

    dave commented on The Reality of Time.

    Bob Zannelli,

    Good luck getting Sean to reply to that one!


    Dave I don’t know why he would need to reply, what I describe is just standard Special Relativity. I am not proposing anything new.

  14. Simon, you write

    “There is no empirical evidence of time being tangible, concrete or ‘real’ – none. Time doesn’t cause anything; it’s not a force, or a stream or a flow. If it has no causal impact, it has no existence, other than in our mind.”

    Isn’t speed an observable? Why would I not be able to use one speed as a reference speed to measures other speeds? Do I need a clock to measure speed if I can use one speed to measure another speed? Same for acceleration.

    The usual response by most is that one does not know the magnitude of the reference speed and so the argument is flawed. Of course if that were true then the same would apply to a reference length.

    The meaning of “time” in the usual sense is superfluous. But the language we use that relates to the subject carries a lot of baggage with it. I.e., words like “duration”, “event”, “past” etc. inherit the classical meaning of time. Spoken language is convenient, however, so I tend to use the common meaning since conventional meanings save time.

  15. Swami says:

    Maybe the passage of time can be thought of as movement through addresses in an infinitesimally fine grid of space-time. It’s fascinating to read about changes in the appearance of atoms when temperatures are near absolute zero and particle movement slows dramatically. This could be relevant, or maybe not.

  16. Bob Zannelli says:

    Alice says
    swing and a miss, Bob.


    No one can convince anyone of anything who refuses to engage the points being made. It wasn’t missed, it went over your head.

  17. paul kramarchyk says:

    Time is the count of “ticks.” Where the user defines the tick. A tick could be the spin of a planet, the swing of a pendulum, or the leap of an electron and subsequent electromagnetic splash. In any case, time is nothing more than the count of ticks. No tick, no time.

  18. Tom S says:


    I don’t think your idea is complete. Don’t you also need some “tocks”?

  19. simon morley says:

    When you observe speed, in what way do you observe time? I mean actually see something tangible called time. Describe it if you see it.
    What you “observe” is the differentiation of momentum between two objects, or a calibrated measure. Sure, that needs Time the abstract framework – but that is all it is, the abstract framework.

  20. Magnema says:

    Personally, I take what is (possibly; I’m not entirely sure whether it is distinct from the block universe) a fourth stance. (Skip to the end for the abbreviated version.)

    I’m going to assume we have some laws that describe the universe which form some PDE over spacetime. The block universe, in my view, has redundant information, which can be seen as follows:

    Suppose the universe satisfied the Laplace equation in three spacial dimensions (this universe does not have time). Then, there are a limited number of possibilities, but suffice to say that (in flat spacetime; my manifolds class hasn’t gotten to PDEs on manifolds yet, but I believe existence and uniqueness also holds for elliptic PDEs on manifolds) the value at every point is uniquely determined by the value on the boundary of the space. (If the space is infinite, you have to take more care, but everything can still work.) Thus, I would argue, in this universe, it suffices to have only the boundary exist, for given that everything else falls out naturally.

    Although time has a more complicated relationship (the Einstein Field Equations, in particular, are nonlinear and hyperbolic), I think that it might still end up being clear that the solution to the world is uniquely specified by less data than the entire world contains (e.g., by the initial conditions, as most classical physicists assume), in which case I take the stance that once you have specified sufficient information to determine the universe, you are “done” in some sense. (Under this definition, existence is not uniquely defined – the initial value [IV] “exists” and the ending value [EV] “exists” are both valid statements, resulting in the same universe, but since you can’t specify the IV and still have the EV be undetermined, they can’t both be true at the same time, in some sense. I don’t, however, take this to be a problem; it’s akin to trying to distinguish between diffeomorphic manifolds: there is no meaningful distinction to make, as all the relevant properties are the same.)

    TL;DR: I take the stance that all that exists is the information of the universe. I don’t think it is useful to have a notion of “physical” existence on a metaphysical level. In this sense, since one can specify our universe with the laws and an initial condition, the universe is, exactly, these two things (without, granted, a unique way to specify “initial” condition).

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  22. Alex R says:

    A lot of words, but not all that much physics, that I can see. I can’t see anything that convinces me that there is any difference between these different metaphysical pictures of time. Why should I not take pretty much the same stance on this that I do on interpretations of quantum mechanics: “shut up and calculate“?

  23. Paul Torek says:

    Bob Zanelli, I found your Bird’s Eye/Frog’s Eye View explanation helpful, thanks.

    simon morley, you say there are two root definitions of time, and I’m partial to your first one: time is the t in science. And then you say, Time is not a universal, fundamental; events are the fundamental. But it seems to me “fundamental” is a property of a model; it does not necessarily make sense to apply “fundamental” vs “derived” to the reality, even if it’s a good model. In your model, you can derive all the “time” statements from “event” statements. OK cool, that’s interesting and important. But all I heard Sean say was that time exists. And existence is binary. It does not come with a lesser, “derived” level and a greater, “fundamental” level.

    Magnema, can’t I pick a point in the middle, just as easily as the initial or ending value?

  24. Niranjan S says:

    Shut up and calculate asks scientists to not be armchair philosophers. But as long as the counter intuitive nature of quantum physics or time travel does not paralyze or confuse us, we *should* hypothesize and chase conceptual ideas.
    Shut up and calculate is good advice to remind us that philosophical objections cannot challenge a mathematically/experimentally correct theory. But its a defeatist and unscientific approach to scientific progress in general. We should not use Mermin’s words so out of context.

  25. Magnema says:

    @Paul: Indeed, middle values work just as well – there are many ways to specify the information. So long as you have provided sufficient data to completely describe the universe, you are done.

    @Niranjan: indeed; I feel like many physicists use that justification to dismiss philosophy out-of-hand, to say “screw the truth, I just want what works,” which I don’t believe was the central intent. “Shut up and calculate” should be used as “I’m just interested in different questions, so I’ll ignore your questions about what is ‘actually happening'” or “I know neither how to predict what I see nor what is actually going on, so I’ll worry about the former first” – quite explicitly NOT, as it is so often used, to say that “the calculation is all that is meaningful to discuss.” You could argue the last point, but that would be an argument you would have to make, not one you can take for granted.