By Sean Carroll | June 17, 2009 10:04 am

After the FQXi Essay Contest, I was asked to comment on some of the essays besides my own, but I never did. Mostly because I didn’t take the time to read them all (there were an awful lot), but also because I just don’t know what to say about many of them. In her essay (which I liked), Fotini Markopoulou divides the world in two:

There are two kinds of people in quantum gravity. Those who think that timelessness is the most beautiful and deepest insight in general relativity, if not modern science, and those who simply cannot comprehend what timelessness can mean and see evidence for time in everything in nature. What sets this split of opinions apart form any other disagreement in science is that almost no one ever changes their mind…

That’s just about right (although perhaps there are also other splits with the same quality). Julian Barbour, whose essay finished first in the judging, has famously championed the view that time does not exist, even writing quite a successful book about it. In a recent Bloggingheads discussion with Craig Callender, Barbour talks a bit more about his view.

To which all I can muster is: I don’t get it. There are a set of technical arguments, which for the most part I do get, that can be used to make it seem as if time does not exist. In ordinary classical mechanics, we can perform some formal tricks to remove the time variable from the conventional equations of physics. More dramatically, in general relativity or quantum gravity we can express Einstein’s equation (at least in certain circumstances) in a form where time does not appear. On the other hand, we can usually re-write any of these equations in a form where time does appear (at least, again, in certain circumstances).

But none of these technical arguments are really the point. What I don’t understand — and this is a sincere lack of understanding on my part, not an indirect claim that this perspective is wrong — is what’s supposed to be so great about timelessness. What are we supposed to gain from thinking in this way? What problems is it supposed to solve?

Put it this way: clearly time appears to exist, at first glance. Even the timelessness crowd somehow manages to submit their essay competition entries by the deadline, and finish their Bloggingheads dialogues within an hour. So the claim “time does not exist” certainly doesn’t mean the same kind of thing as “unicorns do not exist.” It must mean (I suppose) that, while we all find time very useful in our everyday lives, there is a deeper level of description in which time doesn’t appear at all; it only emerges in some sort of approximate description of reality. But that approximate description seems extremely valid and useful, including all of the phenomena in the observable universe. Surely it behooves us to take this purportedly-non-fundamental notion seriously, and attempt to understand some of its puzzling features? Moreover, even if “time” doesn’t turn out to be fundamental, why would that tempt you into saying that it doesn’t exist? Protons are made of quarks, but you don’t hear particle physicists going around claiming that protons don’t exist.

The problem is not that I disagree with the timelessness crowd, it’s that I don’t see the point. I am not motivated to make the effort to carefully read what they are writing, because I am very unclear about what is to be gained by doing so. If anyone could spell out straightforwardly what I might be able to understand by thinking of the world in the language of timelessness, I’d be very happy to re-orient my attitude and take these works seriously.

  • http://backreaction.blogspot.com/ Bee

    You are neglecting the coolness-factor of claiming that time doesn’t exist. Or that anything else doesn’t exist which obviously seems to exist for the average human being. Like space, vacuum, free will, etc.

    I find it puzzling though Fotini writes the fact that almost no one ever changes their mind sets this issue apart from other disagreements. In my impression no one ever changing their mind is the rule rather than the exception. If you need any evidence consider how scarcely somebody tells you did you know X changed their mind on Y.

  • Count Iblis

    What is meant is time in the sense of a pointer that points to what really exists: the present moment, while the future and the past do not exist. Time in this sense does not exist, or at least, is difficult to reconcile with modern physics.

    Simple examples:

    In special relativity, you have the Andromeda Paradox:


    In quantum physics, you could consider a big closed box inside which everything evolves according to unitary time evolution. But then the past and the future exist in the same sense as the present moment, because they are all relatated to each other via unitary transformations.

  • George Musser

    Isn’t this similar to saying, I don’t see the point of a statistical mechanical description of heat and entropy, when the thermodynamic one works just fine? Or: I don’t see the point of an atomic description of nature when I can do chemistry at the test-tube level? Physics is about probing deeper, no?

  • http://brokensymmetry.typepad.com Michael F. Martin

    You’ve put your finger on the problem exactly. What is the instrumental value to Barbour’s hypothesis? It’s like intelligent design in this sense — even if true, so what?

    Aside from lack of instrumentalism, there are a few key points in his essay that I believe are either false or partly false.


  • Low Math, Meekly Interacting

    I haven’t read Barbour’s book, and my understanding of this extends about as far as I hear tell the time variable simply drops out of the Wheeler-DeWitt equation, which some folks think is very profound. Perhaps some of these folks are being semantic provocateurs? So maybe time is emergent, and not fundamental. I’ve seen the mass of the proton referred to by some as “emergent”, and I think all agree that the great majority of it isn’t “real”. That’s not to say it doesn’t “exist”, just that it is largely due to the contribution of virtual particles, and not the “real” quarks that make up the proton. So are they really trying to say time isn’t “real”?

  • Eugene

    I think the whole thing is an argument about semantics really. Whether the statement “Time does not exist” is elegant or useless is a matter of taste, and there is no accounting for that.

  • Neal J. King

    I agree with Sean.

    I would say to Those-who-have-transcended-Time:

    “OK, I surrender, you got me, time doesn’t exist.

    “Now please explain to me why time appears to exist.”

    But I have much the same attitude about the much-vaunted many-worlds interpretation of quantum mechanics:

    “OK, I get it, the universe doesn’t have to choose which branch of the fork / eigenvalue / eigenvector to pick, it just takes them all.

    “Now please explain to me how I ended up in this one.”

  • Matt

    What I never understood about the arguments of the “timeless” crowd was why, according to them, time doesn’t exist but space does exist. In relativity, time and space are on an equal footing, apart from a mere difference in signature in the metric. And in QFT, both time and space translations are carried out by unitary transformations—again, they’re on an equal footing.

    So you could just as easily eliminate the time variable as any of the three space variables. But nobody seems to be wandering around wide-eyed saying “The y-direction doesn’t exist!”

    The other issue is that causality is an extremely powerful principle in physics. Nature appears to exhibit a causal ordering of events, at least inside of light cones. So even if a particular choice of time parametrization is arbitrary, that causal ordering is still there—it’s “diff invariant”—and that causal ordering is precisely what most physicists mean by “time” anyway. Do the timeless people claim to have gotten rid of causal ordering? If not, then what are they even really talking about?

    Now, most physicists already fully anticipate that notions of space and time may both begin to break down near the Planck scale. But understanding what’s going on there will require some new theory of Nature beyond relativity or QFT—something like string/M theory or its successor. And that’s not the issue we’re even discussing here—we’re not talking about the breakdown of time (or space) at the Planck scale due to bizarre stringy effects or what-not. What the timeless people claim is that we can already see the breakdown of time—that time cannot be fundamental—already at the level of semi-classical relativity at everyday energies! How do they justify this assertion?

    Sean, I agree with something you once posted about crackpots, that if someone who challenges an accepted idea wants to be taken seriously and have his papers get read, then the first thing he needs to do, right at the beginning, is answer the most obvious questions. The timeless people have never attempted to do that.


    The closer you get to the speed of light, the slower time flows, right, time dilation! If you reach the speed of lght, c, time would stop flowing, true?

  • Count Iblis

    I find it puzzling though Fotini writes the fact that almost no one ever changes their mind sets this issue apart from other disagreements. In my impression no one ever changing their mind is the rule rather than the exception.

    That’s not my exprience. But given that both B and Fotini are at the Perimeter Institute, it could be that the people there are like that. :)

  • http://quantumnonsense.blogspot.com/ Qubit

    “The closer you get to the speed of light, the slower time flows, right, time dilation! If you reach the speed of lght, c, time would stop flowing, true?”

    Yeh and one day soon man might learn how to travel back through time and travel back to the real year 2001! Then we can escape from the black hole that the universe fell into, I could the say the Earth but that would not make any sense.

  • http://www.sonic.net/~rknop/blog Rob Knop

    I guess I’m just not smart enough, but….

    In GR, if you’re working in a spacetime with a metric that’s got one timelike and three spacelike dimensions, how can you make time go away altogether? Isn’t it implicitly lurking down there somewhere?

    Am I just too facile in my understanding of GR?

  • http://www.sonic.net/~rknop/blog Rob Knop

    The closer you get to the speed of light, the slower time flows, right, time dilation! If you reach the speed of lght, c, time would stop flowing, true?

    Actually, not really true– and the problem is that it’s a poorly phrased statement. You *can’t* reach the speed of light, not if you’re a massive particle.

    In fact, you’re always at rest, at least with respect to yourself.

  • boreds

    While I tend to agree with the posting, what would be cool in this situation is if we could invite Julian Barbour to respond, and try to address these issues. To make us see what the point is.

    Is that possible, Sean? No-one sufficiently inspired by timelessness has appeared in the comments.


    (I wanted to end with a good time-related pun, but that was the best I could manage.)

  • James

    There are two kinds of people in physics. Those who think that quantization is the most beautiful and deepest insight in modern science, and those who simply cannot comprehend what quantization can mean and see evidence for continuous divisibility in everything in nature. What sets this split of opinions apart form any other disagreement in science is that almost no one ever changes their mind…

  • http://www.fqxi.org Anthony A.

    Hi Sean,

    I would agree with what Count Iblis suggested: they are not suggesting that *time* does not exist, so much as that *now* does not exist, in the sense of a borderline between that which is past/certain/fixed versus future/undetermined/malleable. Perhaps it would be better to say that ‘becoming does not exist’.

    I think you are supposed to derive comfort from this idea in the sense that all of the good times and people you experienced in the past are just as real as ‘now’ (which is nothing special). Even if this is true, however, the illusion is a persistent one, as someone wise once said.

    Also, while I’m here and since you mentioned the last one, let me note that FQXi has launched a new essay contest — see http://www.fqxi.org/community/essay


  • Christopher M

    I’m decidedly an amateur in these matters and not a technically sophisticated one. But here’s my question. When you drop the time variable from the basic equations of physics to get these timeless models, are you eliminating potentially redundant information, or just recasting equations into a different but mathematically equivalent form?

    To put it another way: what about two instantaneous states of the universe that are identical physically, but exist at different times? It would seem that, if time is “real,” then these two states are truly distinct; but if there is no time (in Barbour’s sense) then they are, quite literally, the same. Maybe this difference would have some further statistical implications?

    Again, as a total non-expert, I’m commenting more in the hopes of being told why I’m wrong than in order to answer the question correctly.

  • http://www.domenicdenicola.com/ Domenic Denicola

    I find James and George Musser’s comments to be the most insightful.

    My perspective (or rather, the perspective I have arrived at after thinking about how to respond to this for approximately 30 seconds) is that timelessness is seen as an insight into the “fundamental structure of nature.” What does that mean, operationally? It means that our ultimate theory-of-everything should not contain time in its fundamental description, and so we can use this as a guiding principle while searching for it. Perhaps a good analogy is with background independence: one might claim that “a fixed spacetime background does not exist,” with the meaning that we should be looking for background-independent theories as we search through TOE-space.

    Obviously time should emerge, so as to reproduce our everyday experience—just as, in James’s example, the continuous world should emerge from the quantum. But this isn’t the point that timelessness advocates (myself included, I suppose) are trying to make, or even care about.

  • Wade Kelman

    Perhaps saying that Time does not exist will have some testable consequences, as does my saying that Magnetism does not have an independent existence, and is not a force in its own right.

    Magnetism is an artifact of continuous electrostatic field lines and the finite speed of light. If the speed of light were infinite, there would be no “magnetic force”, or lateral shift in the direction of the electrostatic field lines of force. That being so, I predict that magnetic monopoles will never be found, because they do not exist. Neither do centripetal monopoles exist.

  • http://www.fundy.com Fundie

    I’m afraid Sean has put his finger on exactly what was wrong with the FQXi essay contest [apart, that is, from the failure to weed out the obvious cranks]. There was way too much vague talk about timelessness and such stuff, and way too little about actual concrete physical models of anything. I was tempted to submit an essay, was dissuaded by the cranks, and was very glad that I hadn’t bothered when I saw who won.

    For sure there is an interesting issue lurking here — but whatever it is, what is needed at this point is a deep physics argument with lots of equations.

  • Eric Habegger

    It seems to me the whole idea of trying to eliminate the idea of time is sort of like trying to negate one’s entire experience as a human being. What’s the point? We are born, we age, and we die, and we define these stages by the ordering of events in our own experience of time. We all experience these things and it seems to me only someone who experiences these things in a significantly different way from most of us would dare claim that time is not an extremely useful concept. It strikes me as a sort of dilletantism bordering on arrogance to think that time should not be a beautiful concept.

    My own feeling is that the commonality of description of time in the quantum and classical world relates to the appearance of change with the passage of time. Things change around us and within us even if we try to avoid it. Similarly I think fluctuations in space time are the equivalent to the passage of time in the classical world, but happening in discrete steps.

    I seldom agree with Sean but I wholeheartedly agree with him on his reaction to eliminating the idea of time. Even if you can reduce time as an emergent property of fluctuations in space related to thermodynamics, what’s the point. A rose by any other name would smell as sweet.

  • http://telescoper.wordpress.com Peter Coles

    I was going to comment on this, but I don’t have time.

  • Steve

    Peter, perhaps you’ll find time later?

  • Elliot Tarabour

    As someone pointed out elsewhere “Time is there to keep everything from happening at all at once”

    I don’t see how you can eliminate time and retain causality in any meaningful way. I am not a professional scientist but I am sort of attached to the notion of causality as a fundamental feature of reality.



    @ Qubit and Rob Knop

    What about tachyons?

  • Count Iblis
  • Chris W.

    Echoing comment 16 (from Anthony Aguirre) I’d say that Barbour’s position is deeply bound up with what is occasionally referred to as the “many-fingered nature of time” in general relativity. In most of our experience, and for that matter in most applications of physics, this hardly comes into play. We can get away with assuming that there is a single, absolute “surface of simultaneity” most of the time [ :) ] without running into serious difficulties.

    By the way, when Sean says “Protons are made of quarks, but you don’t hear particle physicists going around claiming that protons don’t exist.”, he is touching on a profound epistemological issue in the layering of scientific explanations. When one uses an explanation to deny the existence of what one originally set out to explain, one is playing a very dangerous and ultimately self-defeating game. This tends to happen when one tries to turn the explanation into an all-embracing metaphysical worldview.

  • Sam Gralla


  • RD

    Of course they don’t change their mind, there is no time for that. The moment they changed their mind they would have created an new instant of time, thus discrediting their hypothesis.

  • SFJP

    I’d like to answer to both Sean and Matt. I see one and only one interest and reason for considering timelessness: space is isotropic, time is not. While many people think that the irreversibility of the arrow of time is well understood, I believe it is not. To be brief and go directly to the meat of the issue, I’d say that the discrepancy between time reversibility at the microscopic level and time irreversibility at the macroscopic level has never been completely solved. At the end of all reflection on this issue we always get to the same paradox: the law of large numbers, which most of the people think obvious and even mathematically proved, which is not true in a physical sense. If time flow is an illusion, the way we connect instants to form an history that will appear irreversible is then a matter related to the building of memory, and not an intrinsic property of a physical time. There is there the possibility may be to escape the paradox of macroscopic time irreversibility, not by following Babour but rather some alternate approach à la Rovelli about thermal time and/or the role of entropy growth in the constitution of memory. This is a very confused issue, but it is the notion of a true time, being both reversible and irreversible, that remains the most confusing.

  • asdfhgh

    Is there any particular reason why my last comment was deleted? I was simply voicing an opinion; I also happened to do so in a much less derisive way than many of the flat out ignorant comments being posted by a majority of people (the majority of which, I’m willing to bet, have not read Dr Barbour’s papers), which have not been deleted. Even you, Sean, have admitted that you haven’t read the papers in detail. Some of these people (him and he collaborators) have been working exclusively in the field (of gravitation) for longer than a lot of the commenters have been alive – in short, their opinions are worth listening to*.

    My point is this: It’s not about what we can ‘gain’ from thinking of the world of timeless. Time is either an emergent property or it is not. I for one believe that understanding the true nature of time for its own sake is more than worthwhile, whether it exists or not – whether or not we stand to gain something more from doing so should not be paramount. I find it saddening that just because timelessness is counter-intuitive/against the opinions held by some (although, thankfully, not all) of the commenters, they see fit to label Dr Barbour as a crank, with little other justification.

    *Before anyone tries to misconstrue that: By ‘listen to’, I mean exactly that. Just because someone has been working in a field for as long as Dr Barbour has does not mean that his idea could simply be flat out wrong. Regardless of that, he is an expert in classical general relativity and not some readily dismissed crackpot.

  • |John R Ramsden

    Considering the overall evolution of the Universe, from the early inflation phase when presumably lots of events were occurring in short intervals, to the supposed distant future of rapidly expanding space containing nothing but a few black holes, one might almost think that nature itself is working to eliminate time, defined in the practical sense of a causal ordering of events or, what amounts to the same, weaken its effect by spreading it out.

    At the risk of sounding homely, this sort of reminds me of something my aunt once told me. “The older you get, the faster the years slip by”. How right she was, and perhaps as I say the same is ultimately true of the Universe.

  • http://magicdragon.com Jonathan Vos Post

    My teacher’s teacher to the 5th power (through such links as G. E. Moore, Bertrand Russell, Norbert Wiener) John McTaggart Ellis McTaggart [1866-1925] was a Fellow of Trinity College, Lecturer in Moral Sciences, and a Nonreductionist. He was the author of “Studies in Hegelian Cosmology. The Philosophy of Hegel” [Dissertation, 1898; 1901; Garland, 1984]. This work explored application of a priori conclusions derived from the investigation of pure thought to empirically-known subject matter; human immortality; the absolute; the supreme good and the moral criticism; punishment; sin; and the conception of society as an organism. McTaggart was controversial for claiming that time was unreal: “The Nature of Existence” [Cambridge University Press, 1921]; “The Unreality of Time” [Mind, vol. XVII].

  • Ahmed

    Anyone can have valid mathematical claims, but nobody (including Barbour) can explain things like the second law of thermodynamics, in our closed universe, without the arrow of time. He can’t explain basic quantum phenomena either without the arrow of time, even if the more complicated formulatiions work out. Mathematics is only in your head. Energy, entropy and more importantly *information*, are very real.

    All the physics that completely does away with time faces these problems. It is not a lack of creativity on your part to dismiss the way the world works. You are a scientist, an observer. The observable principles of physics (nott the models, the observations) are more worthy of belief than the trickery of mathematics. We can write the math any way you want, we can even reinvent it, come up with entirely different ways of doing things.. that activity is not really interesting in itself, it is just a consequence of how we think. Logic was supposed to be interesting, because it is inherent in the universe, but mathematical systems are not. Your observations and reflections on the world are far more interesting, especially when tthey coincide with some sort of representatiion that we *happen* to comprehend and write down in symbols.

    For e.g, we know for sure that things seem to ‘change’, because things are distinguishable from each other. Why though? Why are things different? Your essay laid it down prretty nicely, and it is even understood since antiquity: things are different because they can be, dancing along the moving arrow of time. And ‘Different’ is a term that implies information portrayed by the world, and information is mysteriously linked to tangible things like energy (see Landaur et al). So, although Barbour can probably *describe* his submission of the paper in a way that answers your question, I’ll bet you he woulddn’t want to stick his hand in a burning flame. If information is physical, the act off classic computation itself defies a truly timeless universe in a tangible, observable manner.

    They just can’t see it.

  • http://guidetoreality.blogspot.com Steve Esser

    Hopefully, the debate will indeed be decided by which sort of picture helps explain things. From my (limited) perspective as a layperson interested in this stuff: it seems more likely that a program which starts with time/causality will be able to derive space and matter as emergent phenomena (as in Markopoulou’s work, CDT, etc.); as opposed to the timeless theory successfully explaining the role time plays both in our experience as well as in our best physical theories to-date. I guess we’ll see.

  • http://eclecticsuniverse.blogspot.com/ Blue Fire

    Timelessness, to my understanding, simply means that time, in and of itself, does not exist as a tangible or otherwise inherent property of the universe. Does time exist inside a movie? No. There is only change from one frame to another. So our universe “proceeds” from one state to the next with a change from one state to the next. A single “frame” would be what we may want to call the present. The “next” frame, our future. The “previous” frame, our past. But according to theory, those frames, all of them, exist. We humans only seem to experience time because we remember the “past” and not the “future”. But that is only a consequence of a change in the state of our brains from a simpler state to another state in which we remember the previous state or frame.

    And a direct extension of relativity shows that person A’s future can be person B’s past due to differences of velocity, direction of motion, and spatial separation. If this is true, then all of spacetime must exist: all of space, all of “time”. Spacetime would be like a huge loaf of bread. The slice of spacetime that I call “now”, could intersect with someone else’s “past.” This depends on how the loaf is sliced by each person, and that is determined by each person to be slightly different according to the comparative warping of spacetime due to relativistic effects.

    So, you can see that this does indeed have meaning-of-life type implications! It would mean that determinism might rule after all. Except in the case of the uncertainty principle, where the details of a slice of spacetime here might depend on the actual result of a wave function collapse in a different frame. But then, all slices (or frames), are defined differently for each observer – yet the whole loaf of bread, or the whole of spacetime, still exists independently of how it may be sliced.

    That’s my story, and I’m sticking to it! :-)

  • Zoltan J Kiss

    Dear Sean and All

    The “time case” is simple:
    No time would mean no event and no event means no time. Timelessness as such would mean no matter as well. The universal event establishing “the time” is the transformation of matter from its mass status into its energy status and the re-transformation of the energy into mass again. This process-re-process is permanent and is in balance with certain intensity differences between the two.

    The three basic parameters are: mass, energy, time.

    The realised by the institutional science conflict between the General Theory and the quantum theory stems from Einstein’s mistake in the time formula (the reciprocal character is missing). Contrary to the “official” Special Theory, the motion speeds up the time flow. With this input, the General Theory is different and today’s quantum theory is also different. And there would be no conflict to speak about.

    The institutional science, with all my respect to them personally, does not listen to this as the science would only be the privilege of those within it.

    There is no space in a blog to prove my points above, but my works “The Energy Balance of Relativity” (trafford.com/06-3261) and the “Quantum Energy and Mass Balance” (trafford.com/08-1547) give the physical and mathematical proof.

    All the best to you all

  • Victorb

    Add me to the list of people who think time cannot truly be discounted, no matter which equations it can drop out of. It’s still there in GR even if you’ve transferred all that energy into x, y, and z. And of course, it’s still there as the underlying basis of perception. One cannot perceive anything at all without time, even the concept of timelessness.


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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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