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.

  1. Hi Sean, Why are you okay with the block model that supposes all perception and empirical observation of tensed cause and effect is an illusion and at best only logical cause and effect or alternatively no cause and effect at all? Hmm. Peace, Jim

  2. I think the question “Is time real?” is as important or useful to think about as the question “Is position real?” Time changes for a body in the universe, and position changes. There is relative rate of change of position between two objects, and there is relative rate of passage of time between two objects. We can’t just ask “Is time real?” without also asking “Is position or space real?” And the only way we can make any sort of progress on those questions is if we know what’s going on with quantum gravity, etc.

    So is time real or not? My answer: Shut up and calculate…and do experiments, like what science tells you to do. It’s worked so well thus far.

  3. Excellent article.

    I think the use of word ‘time’ is very old but IMO is used to define many different things. For example, ‘time’ is used to mean a moment, a duration in general language. With relativity came the notions of time for a given frame, the proper time, the timeline etc. IMO any discussion of time should start with which definition is being used for the discussion that follows. It is like discussing god(s). First you have to state which specific definition is being used.

  4. Something I find dissatisfying about the “time is illusory” arguments…is that this particular illusion isn’t merely so overwhelmingly powerful, we haven’t even the slightest clue as to how to overcome it. Indeed, overcoming it would mean time travel, and that opens up all sorts of cans of worms like paradoxes and perpetual motion machines and what-not.

    Any physics of time that fails to account for both our perception of it and the immutability of time’s arrow is one that’s not yet complete. It’d be like a physics model of my ’68 VW Westfalia Camper that concluded it should have a top speed of somewhere north of 500 MPH…because it neglects to take into account aerodynamics and other factors.

    Or, a failure to account for time as a very real human-scale phenomenon should be seen as a sign that the model is being pushed beyond its limits and is ignoring something of vital relevance.

    Shut up and calculate, indeed — but check your damn answers against observation!


  5. I would say it is not a simple determinant block but a set of all block possibilities with our present and future path subject to quantum fluctuation of our decisions, though with others of us making alternate ones.

  6. Perhaps the Eternalism model can explain quantum effects as spooky action at a “temporal” distance, i.e. the future affects the past. No need to invoke MWI.

  7. I’m surprised by Smolin’s conclusions. His loop quantum gravity had me visualizing all particles and forces in the same light as GR gravity, as geometrical distortions in the fabric of spacetime. I had assumed that time in LQG theory would best fit the Eternalism model.

  8. Is there an equivalent to the two slit experiment for time? i.e. Would a block universe treat time with uncertainty in the same way as it treats position, and is it possible to test this by seeing interference effects with particle traveling though a fast shutter ( making two slits in time vs position )?
    A block universe makes things much simpler ( what happened before the big bang becomes moot ), but the illusion of time is so powerful its hard to let it go.
    Thanks for the post, I have some more books to read. 🙂

  9. We should be asking, according to Special and General Relativity, if spacetime is real. Why single out time?

  10. I’m with Ray Gedaly. Anything other than eternalism is either experimentally ruled out or requires increasingly elaborate mental gymnastics to justify. And, you get to rescue locality, at the expense of causality. The war is over, and Tralfalmadore won.

  11. Thanks Sean, I just finished reading Smolin’s Time Reborn and thought most plausible.
    I think you have to say a lot more than the passage of time is not real. I have no problem to attribute “reality” to the idea of the universe coming into being, moment by moment, and alternative theories are looking outdated?
    One reason why you no longer like Smolin’s book is that a leading cosmologist has finally called time on two of the leading scientific thories, general relativity and quantum mechanics. To quote from his book,
    “The consequences of applying quantum mechanics to the whole universe was the disappearance of time and people are still arguing about it. The frozen state of the universe is not the only thing that goes wrong when quantum theory is applied to cosmology. There’s only one universe, and we weren’t there to choose an initial state, so the tragedies of quantum cosmology amount to a formidable list.”
    So we have a Cosmological dilemma –Applying the laws of nature – like general relativity or Newton’s laws of motion, to subsystems of the universe work in all cases. Each application is an approximation based on a subsystem of the universe, but the laws don’t work on the whole universe? My second question concerns the laws of conservation where the basic conservation laws depend on the assumption that space and time are symmetric. In which case an unknown cosmological theory will have neither symmetries nor conservation laws?
    Quantum mechanics is likely an approximation to a more fundamental theory. So as much as Newtonian physics, general relativity, quantum mechanics and the Standard model are, they cannot be the template for a fundamental cosmological theory.
    Its all pretty heady stuff for us readers, but I like the idea that our universe is evolving, and Smolin’s ideas solve the question of inflation, still unanswered by the block-universe?
    Maybe its time we all consider something new in the current impass.

    Your reply much appreciated,

  12. Ironically I was thinking about this the night before this post:

    Why am I alive right now and Lincoln is not? If there is only a “now” and the universe is billions of years old, its seems to be very lucky for me that “now” coincides with the time that I am alive. If the Universe has only one now, and that now is at one and only one point in the timeline of the universe, why am I so lucky to have it coincide with my lifetime?

    I’m not so sure the Anthropic principle applies – ( it can apply to me or Lincoln but not the both of us ). I admit this is very slippery reasoning, I haven’t thought it through completely yet, but it sways me towards the block-universe view. In that view, everyone exists, always, and everyone (including Lincoln) can ponder how lucky they are to be at *their* particular position in time in the block universe.

  13. Without a clear understanding of the physical processes that support the ability to predict you are faced with a description of Time as a pure abstract, as a fourth physical dimension, or as a condition somewhere in-between without a solid footing for either except a faith that your understanding is superior to all others and you will fight to the end that this is so and perhaps deny any and all alternatives being suggested.

  14. I read your essay Sean. It is easy enough to say “well maybe someday we will discover immutable principles, and so time can be emergent”. I’ve read “Time Reborn” and “The Singular Universe”. I think both Smolin and Unger would point to their assumption that such immutability does not exist in the real universe and say that it is the more fecund for the progress of cosmology and physics. In the last 30 or so years physics and cosmology have exploded in what can only be called various metaphysical directions. Assuming time is not fundamental results in a multitude of possible antecedents (to the observed universe) all of which appear equally possible.

    Clearly Unger believes that all of the basic threads of modern physics and cosmology are compatible with their concept of the reality of time, summed up in the idea that history takes precedence over structure. If I understand Smolin’s part of the book he tries (does he succeed?) to show that this is the case by examining such interpretations (of relativity, quantum mechanics, the CMB, etc) as demonstrate their compatibility with their notion of the reality of time.

    Like some of the other comments above, I to find the “block time” idea a little queer when applied to the universe across its actual history. “Block time” is a mapping problem. It comes down to saying that we cannot draw an undistorted “time map” of the cosmos but this does not mean that a “simultaneous now” does not exist in the real universe and that in the real universe, “the future” remains undefined (at least). The mapping problem is real, but should not be confused with what might be the case “on the ground” as distinct from “on the map”. Even Barbour (“The End of Time”) provides a spacial metaphor (the hyperplain) that supports a notion of universal simultaneity.

    At the least all Unger and Smolin are asking is that cosmology take such ideas seriously enough (at least for a time [metaphysical pun there, sorry]) to see what might come of them in terms of new theoretical and experimental strides.

  15. The “problem” of time in cosmology has to be one of the most interesting issues cosmologist can argue about. This is a question as much rooted in metaphysics as in science. The debate, as Sean related, goes back to the ancient Greeks, the debate between those who followed the Parmenides line on the nature of time or that of Heraclitus , where change rules the universe.

    With the discovery of Relativity, cosmologists have perhaps fell in line with Parmenides, given the “block” universe picture provided by the relative nature of time. Each observer , based on her frame of reference has her own “now” slice. There is no way to privilege one moment in time as existing and another not. Nonetheless, no one can deny that our perceptions align with Heraclitus, not Parmenides.

    Perhaps this suggest that time must be viewed in the context of whether we are thinking in terms of what Tegmark calls the Bird or the Frog viewpoints. All possible observers are limited to the Frog viewpoint where change is constantly experienced , but we can construct a Bird’s eye view where reality is a timeless structure.

    Perhaps Quantum mechanics has something to say about this. In a wonderful popular level book, “Timeless Reality, Symmetry, Simplicity, and Multiple Universes” by the late Vic Stenger , the possibility is offered that we can have a Timeless Quantum State given a Superposition of time reversed quantum evolution. In this context, time is real , but there exists a global quantum description that is timeless. As Hawking has argued globally the universe would be self contained and timeless, it wouldn’t be something that becomes, it would just be.

    The problem of time in cosmology, might be related to the question of the global energy of the universe. In General Relativity there is no consistent way to define global energy, except in the case of a closed universe, where the total energy is exactly zero once the gravitational potential energy is considered. In a landmark ( but speculative ) paper , David Atkatz and the late Heinz Pagels, “Origin of the Universe as a Quantum Tunneling Event” (1982) argue that only closed universes can tunnel into existence. A quote here might be helpful.

    “A principle conclusion of our investigation is that only a spatially compact universe can originate as a quantum tunneling event. We find this conclusion somewhat disappointing , we had hoped that the universe could originated from flat space, a configuration that truly corresponds to nothing at all. Unfortunately this possibility seems to be ruled out by the positive energy theorem which requires flat, empty space-time to be absolutely perfectly stable :it its lowest energy state. Any deviation from such a geometry must be paid for by a corresponding increase in the action , and changing the action over an infinite volume costs infinite energy.”

    End quote

    We of course know that the universe we live in , to our best measure, is spatially flat and not compact. However, given inflation it’s quite possible to have a globally finite universe that is infinite locally.

    There is one other fly in the ointment. While the inflation process once underway is a “free lunch”, a term coined by Alan Guth, the launch of inflation does require a scale dependent seed energy. This seed energy must be supplied by a quantum fluctuation in the quantum tunneling scenario. If we look at vacuum zero point quantum fluctuations using the particle picture, we have positive energy flowing forward in time and negative energy flowing backward in time. Might this describe the quantum tunneling that creates universes too? Perhaps, and this takes us back to Vic Stenger’s idea in Timeless Reality.

    I have pushed speculation pretty far here , perhaps too far. But maybe there is some sense in at least some of this. Thanks Sean for an interesting topic.

  16. I agree with you Mathew. Time, as it is “understood” above is overly complicated. When you focus only on what is observable about clocks you end up with a reference speed. Not time. That is, every clock on Earth is an indirect reference to what the Earth/Sun system does. Don’t think of “change with respect to time” … think of change with respect to reference change. Since both are observable. And the Earth’s speed is a reference speed for other speeds. I’ve successfully derived the Lorentz transformations only by considering how a reference and target compose a quotient. Where the quotient is a model of a measurement.

    I’d post a link to the article but, it might make this look like an ad for my work.

    \sqrt{ 2 }

    Thought I’d see if you have LaTex working on your blog Sean.

  17. I don’t understand the fundamental difference between these three approaches, they only differ in what exactly is considered the Universe within its phase space. For Nowism it is single point. For Possibilism it is certain path, and for Eternalism, the whole phase space is considered the Universe. All three are valid views but I just don’t see the big difference.

  18. @Ken K: As with the uncertainty principle for position and momentum, there is, indeed, an uncertainty principle with time and energy (See, e.g., Griffith’s text on Quantum Mechanics, Ch. 3.5.3, which was where I learned it).

    However, in a non-relativistic context, it is quite different in terms of interpretation/application: the concept of uncertainty in time is hard to discuss in the same way as position. Loosely, you can think of it as “the time it takes the system to change substantially” (Griffiths pg. 114).

    When you switch from the Schrodinger equation to the Dirac equation (i.e., to a relativistic context), I assume there would be a temporal uncertainty principle in the same sense as spatial (with energy and time), but I haven’t seen uncertainty principles explicitly talked about in general in a relativistic setting, so I can’t say for sure.

  19. Lots of interesting stuff here. BTW, Not too long ago, YOUTUBE had a video about a Nasa engineer working on the concept of a Warp Drive *as hypothesized by Miguel Alcubierre), that would get a specially designed ship to Alpha Centauri in 2 weeks, a distance of over 4 light years! The information as stated claimed that time within the warp bubble would be the same as on earth. This would be achieved by contracting space in front of and expanding space behind the bubble containing the ship. Those on the ship would experience the movement as acceleration. The main ingredient for this warp drive would be exotic matter or possibly Casmir radiation… could such a warp drive possibly facilitate time travel backward, before the drive existed or in the future at least back to when the warp drive was invented?

  20. Cece B.

    Warp drive, as you have described it, is impossible. Consider an
    imaginary box surrounding the spaceship, as well as surrounding the
    contracted space and the expanded space. Consider that box and its
    contents as making up “the system”. Is it possible for any system to
    travel faster than the speed of light relative to the destination
    planet (or galaxy, or star, or whatever)? No. Special relativity
    says no. So warp drive is impossible.

  21. Dr. Carroll,
    May I make a few observations with regard to your April 3, 2015 blog post: “The Reality of Time.”

    1. You are mistaken to presume that this problem of time “has already been solved,” in the Eternalism of the “block universe;” this being “our best understanding of the laws of physics.” In your “Timelessness” blog of June 17, 2009 you indicated that the world in the language of “timelessness” doesn’t seem to offer us any special scientific insight into our physical reality; nevertheless in your April 3, 2015 posting you say “personally, I find the eternalist block-universe view to be perfectly acceptable….” But, Dr. Carol, this “perfectly acceptable” block-universe gives rise to time travel, which, in turn presents us with paradoxes, which, in turn, bring into question the very logical consistency of general relativity (see: Dr. David Z. Albert’s comments in “Remembrance of Things Future: The Mystery of Time” by Dennis Overbye, New York Times: Science Times, June 28, 2005). Furthermore, Dr. Carroll, as Dr. Smolin has emphasized in his lectures and his book “Time Reborn,” the very conception of our physical laws depends on the notion of whether time is real or illusory. Do you, Dr Carroll, subscribe to a “timeless,” eternal, Platonic, metaphysical world where mathematics and our physical laws really exist? As someone who presides over a science blog headed by the motto: “in truth, only atoms and the void,” you would, it seems to me, be actually an ontological physicalist, not a metaphysician; but I, nevertheless, duly note your heading of the “3 Metaphysics of Time.”

    2. Dr. Carroll, “time” can demonstrably be described now in the language of physics and not just the language of metaphysics; this would indicate a scientific and intellectual progression. Brevity prevents me from elaborating.

    3. Dr. Lee Smolin, in conversation with Robert J. Sawyer, at the Toronto Reference Library, on April 29, 2013 (see link at responded to the question “What is time?” by answering “The activity of time is the process which generates the future out of the present.” Do you suppose Dr. Smolin had his philosophical hat on, or his theoretical physicist hat on when he asserted this definition? Scientifically, I know we can define “time” in terms that are scientifically and quantitatively more exact.

    4. With regard to your “Timelessness” posting of June 17, 2009, you ask : “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?” Well, this “timelessness crowd” is telling you, Dr. Carroll, that you don’t have any choice in the way you are to think about time, fundamentally, since “timelessness,” as in the block-universe, is, to quote you, “our best understanding of the laws of physics.”

    5. Additionally, in your “Timelessness” post (June 17, 2009) Dr. Carroll, you referenced a paper by Dr. Fotini Markopoulou-Kalamara of the Perimeter Institute for Theoretical Physics (the Institute that Dr. Smolin helped establish in Canada) entitled “Space does not exist, so time can.” On page one of her paper she says “What sets this split of opinions [in quantum gravity, the “time is an illusion” group versus the “time is real” group] apart from any other disagreement in science is that almost no one ever changes their mind, there is practically no crossing camps on the issue of time.”
    This is an interesting observation, since, in the realm of science, data and physical proof tend to dissolve differences. To explain this calcification of positioning, I might suggest that much of the theorizing about time is just that: theorizing, theorizing without actual physical predictions which might be entailed in a particular theory, which would dictate very specific, testable outcomes. As an example, in an Edge podcast entitled “The End of Time–A Talk with Julian Barbour” (with John Brockman) dated August 15, 1999, Brockman asks Dr. Barbour: “Is there any way of testing your idea observationally?” (this in reference to Dr. Barbour’s “Platonia,” the static landscape of Nows, presumptively described by the Wheeler-DeWitt equation); in response Julian Barbour says “I cannot as yet see any direct experimental way of testing this particular idea. What is needed above all is development of the mathematics.” Additionally, proof, with regard to Dr. Smolen’s thesis of time’s reality, as far as I can find, devolves to the fact that we have yet to encounter “time travelers” from the future; perhaps I have missed something more sophisticated here, if so let me know.

    6. In conclusion, I’m guessing, Dr. Carroll, that you are comfortable with time being emergent and not fundamental. But there is another possibility, the possibility that time is hiding in physics behind another concept. The American blogger Eliezer Shlomo Yudkowsky proclaimed: “To reduce time, [in a reductionist process] you must reduce it to something that is not time…if you toss out the t coordinate, you are forced to see time as something else, and not just see time as “time.” Well, time is, indeed, “something else.”
    T.E. Oakley

    Sent from my iPhone

    Sent from my iPhone

  22. T.E. Oakley,

    Sean is not going to read your monstrosity. I doubt he’s reading any long comment posted. Unless it’s from a physicist he already knows.