Is Time Real?

I mentioned some time back the Closer to Truth series, in which Robert Lawrence Kuhn chats with scientists, philosophers, and theologians about the Big Questions. Apparently some excerpts are now appearing on YouTube — here I am talking about whether time is real.

In one sense, it’s a silly question. The “reality” of something is only an interesting issue if its a well-defined concept whose actual existence is in question, like Bigfoot or supersymmetry. For concepts like “time,” which are unambiguously part of a useful vocabulary we have for describing the world, talking about “reality” is just a bit of harmless gassing. They may be emergent or fundamental, but they’re definitely there. (Feel free to substitute “free will” for “time” if you like.) Temperature and pressure didn’t stop being real once we understood them as emergent properties of an underlying atomic description.

The question of whether time is fundamental or emergent is, on the other hand, crucially important. I have no idea what the answer is (and neither does anybody else). Modern theories of fundamental physics and cosmology include both possibilities among the respectable proposals.

Note that I haven’t actually watched the above video, and it’s been more than three years since the interview. Let me know if I said anything egregiously wrong. (I’m sure you will.)

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63 Responses to Is Time Real?

  1. Ken says:

    Let’s say you have some number of paper clips which you can count. We say the number of paper clips is a reality because it is information derived from raw data. By raw data we mean something that conforms to certain parameters within a prescribed margin of error. The paper clips are”real” because they can be determined to conform to our parameters and their magnitude can be collated from raw data, that is counting up the actual paper clips. But if something cannot be reduced to raw data, meaning it cannot be divided up to conform to our parameters of measurement but can only be detected as an “emergent property” i.e. information derived from collated information we could not state with certainty that the entity was real because there is no way of knowing whether it too is collated information. Let’s say we want to measure magnetism by the number of paper clips a magnet can pick up. But can we say with certainty that something called magnetism actually exists, or is it simply some type of information we cannot determine.

  2. Pingback: Is time fundamental or emergent? Nobody knows. | Gordon's shares

  3. Closer to Truth is the best series after Carl Sagan’s Cosmos in my opinion. Time is a very tricky business. Until recently I used to think that time is emergent based on non-commutative geometry and its origin is in quantum mechanics. For a physics argument see the Connes-Roveli paper: To my greatest surprise, last year I discovered that along with the tensor product, time is a fundamental ingredient in deriving quantum mechanics: so it must be the other way around. The true nature of time is a problem whose solution still eludes us.

  4. Meh says:

    I would say that our universe required the existence of time in order to be created, so time must be something that exists beyond the limits of our universe while also existing within those limits.

    Your opinion could be that time did not exist before our universe, but my reply would be that if that were the case, then it must have been generated from something which we would require a time variable to fully describe and understand.

    The most accurate thing that I would say is that time is the only thing that is truly conservative throughout existence.

  5. Frank Zollo says:

    The Closer to Truth series is exasperating. Some of the episodes are excellent, exploring dark matter, causation, multiple universes, free will, memory. But others waste time on theological concepts that presume the existence of God because otherwise it’s so obviously stupid to ask questions like “Is God Simple?”, “Is God Self-existent?”or “What Does it Feel Like to be God?”. More alarmingly, some episodes give a forum for intelligent designer proponents (e.g., W. Dembski) or ESP believers to spout anti-science nonsense.

  6. Sagredo says:

    If we use space-time variables, time is real and there exists a world line of a particle. If we use energy-momentum variables, there is no time and a particle has not any history. Classically this means that we use the integrals of motion; they do not depend on time at all. Finally, instead of ordinary time we can use imaginary time. In this case motion disappears. This is a world of Parmenides. This is a world of statistical mechanics.

  7. Doc C says:

    It’s amazing to me how long we have survived and thrived without being able to describe time in an abstract way. Our brains know how to define time even if our minds don’t.

  8. Jack Smart says:

    Just because you don’t understand time Sean, don’t assume others don’t. Time is very clear, and simple. It has two distinct meanings:-
    1. It is an abstract framework for referencing, calibrating, measuring events (yeh, that includes time as a dimension – a dimension is an abstract framework).
    2. And its a mass noun – a non-specific collective term for events (also abstract).
    Two subtlety different meanings in the same word. But both abstract. The underlying fundamental in both cases being ‘events’….

    I have mentioned this before, but you don’t seem to want to engage in understanding as its not a physics based solution…

  9. Ramesam says:

    Lee Smolin: Time is fundamental (Time Reborn – From crisis in physics to the future of the universe , Houghton Mifflin) 2013

    Nima-Arkani Hamed: Time and space are emergent (SUSY 2013)

    Advaita Vedanta: Time is imaginary (time is thought = mind)

    Take your pick.

  10. T.E. Oakley says:

    After thinking about Dr. Lee Smolin’s book “Time Reborn,” (2013), I formulated the following argument, which is relevant to the question of whether “time” is emergent or fundamental in physical theory:
    1. Define “time” OPERATIONALLY as “movement” or “change”; without the concept of “movement” or “change,” the concept of “time” is meaningless; thus,
    2. “Time” and “movement” or “change” are EQUIVALENT.
    3. Due to quantum effects, all physical systems are, on a quantum scale, in PERPETUAL MOTION. No physical system can be brought to absolute stasis, that is, absolute 0 kelvin.
    4. Mass and energy are interconvertible: E=mc2, therefore,
    5. All physical systems are reducible to energy.
    6. This leads to E=t ( “E” is energy; “t” is time). “Energy” and “time” are seen to be EQUIVALENT ONTOLOGICALLY AND CONCEPTUALLY; not in any way distinct.
    It thus becomes apparent that, “time” has masqueraded as “energy,” and “energy” has masqueraded as “time.” So we can conclude that “time” is as FUNDAMENTAL as “energy” in any physical system; therefore, “time” cannot be considered an emergent property.
    I have elaborated these ideas in an email to Dr. Smolin entitled “Special Relativity and Time”; also in a email critique of the Trinity College Cambridge Professor Dr. Huw Price’s review of Dr. Smolin’s book “Time Reborn” as it appeared in the journal “Science,” vol. 341, August 30, 2013. I can forward these to you, Dr. Carroll, if you are interested.

  11. If I can take a bite out of it, it’s real; otherwise, it doesn’t exist.


                                                                                              —Ted Nugent

  12. Suppose I am a real stupid cat (what might be true, depending on your perspective). Now, for me time is something that is not measurable, because I cannot count more then 3 or 4 (I am really stupid, remember that?). Well, days and nights follow each other and I can feel the passage of days and nights, i.e., I feel hungry and I feel sleepy sometimes, normally around the same hour every day. My “feeling” of time passing is certainly related to the arrow created by energy transformation (dissipation or entropy) or, in other words, by the irreversible processes that just happen before my eyes. If I were a stupid and blind cat, that would be much more difficult to tell. As I an animal and lucky cat, I am not conscious of the way irreversible processes occur naturally, and the damages they continuously impose on any natural evolution I face, from my early embryo to my dusty destine in the stars. So the question of time being real or not is not really fundamental, I believe. The arrow imposed by the natural energy evolution (meaning free energy dissipation) is. Big and generous bang!

  13. AJ says:

    Ahhh… The “free will” question. Is our’s the only universe where free-will is apparent? Given the choices between free-will being real or a deterministicly derived outcome of God’s computer, I’ll take the former. My two-cents are that if free-will wasn’t real, life would succumb to entropy, wouldn’t it?

    Disclaimer… I know nothing of physics and it probably shows :) Just having a few and philosophizing on a Friday night.

  14. Tienzen (Jeh-Tween) Gong says:

    Sean Carroll, “I have no idea what the answer is (and neither does anybody else). … Let me know if I said anything egregiously wrong. (I’m sure you will.)”

    Is time real? Excellent question. The video is excellent too. It is very honest for Sean to say that he has no idea what the answer is. It is preconceived to say that “neither does anybody else”. Of course, we can disagree with anyone who claims to know the answer.

    There is *One* who definitely knows the answer, the *Nature*. Today, Nature has two parts in human knowledge, the part we know and the part we don’t. Yet, if a squire of Nature shows us a portrait of Nature which match *all* points of the part we know, it will be a creditable portrait until we can find one point of mismatch. Then, the answer of this question “is time real?” in this portrait can be taken as one possible answer. One such portrait is available at “Litmus test for the final physics ( )”. Is this true?

    Truth sits here silently, blocking all detour attempts. Anyone who is sincerely seeking for truth must face this silently sitting truth squarely. I have showed one very, very simple truth, the Alpha equation at Of course, it was easily buried alive with three simple *dislike clicks*. Yet, can these dislike clicks make that simple equation spelling out a different number? Truth sits here silently, giggling at all those great efforts of dislike clicks.

  15. H.H. says:

    Ok, so if I’m following everything correctly, what we experience as “time” is actually entropy. Entropy is possible because the Universe started at a high state of energy. As this energy decays to the point that it can no longer do any work, entropy effective stops, since everything is maximally disordered. At this point, change no longer occurs, so time “stops.”

    In other words, our Universe is a shaken snow globe, and time runs out when the last flake settles on the bottom.

  16. FrankL says:

    I watched those videos and I have a question. One of Sean Carroll’s statements was that the universe is possibly a local drop in entropy in a “multiverse”. A massively unlikely drop, but in a large enough multiverse, it can easily happen. He also said that when the universe reaches equilibrium, maximum entropy, there will be no time. I wonder about that. If we take a much smaller universe, a container of gas at equilibrium, there will be a certain maximum possible entropy, but the gas will practically never achieve it due to fluctuations, each of which drops the entropy below the absolute maximum. This is not a violation of the second law which holds only in the limit of infinite size, it is just a consequence of the finite size of the system. Suppose for simplicity its a monotomic gas of classical particles that interact only by collision. Then we can plot the entropy as a function of time using the usual formula. There will be rare cases of relatively large drops in entropy, but the entropy will generally rise after that back to a reasonable value. My question is this – will the fluctuations have a “signature” which indicates the forward direction in time? I mean, a drop in entropy probably won’t have a particular signature, but won’t the recovery tend to be exponential “decay” towards the maximum, which would allow you to determine the forward direction of time?

  17. Bob Zannelli says:

    The problem of time in physics shows up in the Wheeler De Witt equation , which is basically the Schrodinger equation for the whole universe, the Schrodinger equation of everything. This equation takes the basic form of HY=0 ,where H is the Hamiltonian operator and Y is the wave function. This equation has no time parameter.

    There are several things we can say about this. Operationally time is what measured on a clock, a clock being defined as some cyclic process. It’s obvious from this that the notion of time is dependent on the division of observer and observed. Therefore if we write an equation that encompasses everything no such division is possible, therefore the global state is timeless.

    Another way to think of this is in terms of Decoherent histories. As demonstrated by Don Page the Decoherence functional is time symmetric. This would seem to mean that we should get of Decoherent histories that evolve in both time directions. However, all observers can only experience what is for them a history evolving toward their future by definition. When the Decoherence functional is applied to the whole universe, we get a CPT symmetric state which when summed over removes time in an analogous that the superposition of left and right circular polarization ( giving us Horizontal and Vertical Polarization) remove any circular action of the EM wave.

    Finally, as demonstrated by Atkatz and Pagels , only closed universes can tunnel into existence ( locally our universe is open due to inflation but closed globally) A closed universe ,unlike an open universe DOES have a defined energy in General Relativity which is zero. Given that energy and time are conjugate variables zero energy means no time evolution. ( Strictly speaking this isn’t true because time isn’t an operator since locally energy is bound from below, but this wouldn’t apply in this case.)

    So based on the above, the “existence” of time is based on all observers being local, no God’s eye view being available.

  18. Frame Jumper says:


    You forgot Julian Barbour, “The End of Time, The Next Revolution in Physics” For him, “Time truly doesn’t exist.”

    Smolin mentions Barbour several times in his new book. And while I am a fan of Lee’s “The Trouble with Physics,” I just couldn’t find anything of any significance in “Time Reborn.” Sorry Lee, but I’m starting to wonder if this (“Time”) is just the next ‘hot topic’ if one wants to sell a book. I REALLY wanted to buy your book, Lee; honestly! Since I have my own thoughts on the subject, speculation about time is of real interest to me.

    Anyone interested in Barbour’s ideas? How about:
    “…memories and records are in fact present phenomena.”, quoting John Bell
    “…the instants in time (the ‘Nows’, my comment) are in some sense finite. (my italics)” I would agree.
    quoting John McTaggert: “If (an event) is past, it has been present and future…thus all three characteristics belong to each event.” So, each ‘Now’ is present, was past, and will be future. Anything you can say about past or future is encapsulated in the present. The universe is timeless and motionless. There is only the ‘Now.’
    “…all the instants we have experienced are other worlds, for they are not the one we are in now.”
    “…time doesn’t exist…Time is change, nothing more, nothing less.”
    “The universe is its own clock.” (The ‘cosmic clock’, or what I refer to as the “universal clock”, is not time as we currently understand it, i.e., solar time. Time exists in the instant, not the other way around. Like the frames in a movie, each frame is timeless and motionless and more advanced civilizations don’t come from a planet far, far away, they have simply mastered ‘frame jumping’ among multiple, multiplexed, multiverses.”

  19. John says:

    If you can bend it or warp it or dilate it, it is real. Otherwise, would be to say that we are just modifying equations on a component that actually doesn’t exist, because there is something else fundamentally wrong with the equations. How could we ever say that we have the correct descriptions of mechanism in physics when we modify all of physics in a part of it that actually doesn’t exist? I believe to say time doesn’t exist undermines Einsteins work indirectly.

    Einstein discovered the correct mechanism that needed to be modified that more accurately described the laws of physics. It wasn’t a magic trick done with math that did that by modifying something that didn’t actually exist. If time didn’t actually exist then those modifications would be needed to be put somewhere else. You would in effect just be manipulating nothing to get a more correct answer.

  20. (Understanding Time in the Pursuit of Science). Time as a component in science is visualized as the sequential progression of the matter distributions that comprise the Universe. The nature of the Universe is perceived by analyzing changes in the distributions of its matter. As a technique to manage and understand the fundamental aspects of the process, sequences of observations are divided into subsets. The discovery and creation of uniform, repetitive phenomena from the rotation of the earth to oscillations of quartz crystals provide the metrics for clocks. This allows subsets to be assigned intervals as a function of reference oscillation patterns. Integrated into this process man has formulated the concept of time.
    The experimental accessibility of observational techniques positions scientists in a light-signal-limited volume of the Universe on the macro scale with sensitivity on the micro scale that can only sample consequences of the population effects manifested by the ultimate components. Hence, only a fraction of universal time, based on the components of matter, can be documented.
    Only the present instantaneous configuration exists and it is never the same for two instants. The past configurations are remember as the past and anticipated configurations are designated the future. Time travel in this scenario is impossible since only the present exists. However, conceptually, science is equally dependent upon present, past, and future.
    Time manifests a record for observations according to which physical laws and theories are developed and documented. Our laws and theories are validated when we are able to predict subsequent phenomena, i.e. the future, from current observations. A reference clock is crucial to analyses of sequences; however, its intervals, often represented as time itself, are passive parameters.
    In totality the distribution of matter contained in the Universe appears to be expanding and the Universe appears to be evolving in such a manner that its entropy is increasing. If we assign this evolutionary property to time, then current time has an arrow, i.e. the increase in entropy. The metric of an oscillator supposedly provides uniform intervals that allow the assignment of intervals to selected sequences from the Universe. The time axis allows the positioning of “instants” in a sequence and if a single body of uniform matter is being followed in a three-dimensional reference frame, it may be utilized in an analogous manner to a geometric axis for spatial positioning with the understanding that time proceeds from the present to the future. When the process is extended to a four-dimensional system, visualization is more difficult.
    Time possesses a mystical quality and becomes a metaphysical entity when we equate it with duration. However, in the practice of science, time need not be considered a mystery. Conceptually, it is the past, present, and future history of the configurations of the physical Universe. The mystery that science cannot explain is how the Universe came into existence.

  21. Sean, In one of your video clips you say that Earth gives off 20 times more photons than it absorbs from the sun. (a) Does this mean that passage of time has some symbiotic relationship with flow of photons? and (b) How has science established the truth of this 20 to 1 ratio? Thanks.

  22. Meh says:

    whenever I hear the phrase, “we remember the past but we don’t remember the future”, my urge to nitpick grows like the heart of The Grinch. I think this phrase should be exchanged for something slightly different. I would argue that just because we don’t use the same word, does not mean that it isn’t the same thing; but my explanation just proves your point even further. We remember the past and can predict the future; these are the same actions, though we use different words to imply past and future to reduce the amount of words needed to explain our thoughts and prevent confusion. We predict the future(if smart enough and/or evolved enough) based on the information(laws) that we know in the present. We know this information and these laws in the present because of experiences in the past and present (a bird flies when pushed out of the nest because it quickly realizes that the panicked flapping of its wings produces lift).

    So predicting and remembering are really the same action performed in opposite directions from the present. Consider the present moment the origin, the future = positive direction and past = negative direction. Prediction can be just as accurate as remembering, sometimes more so. They are symmetrical. The flaw in this is that we can only predict the future based on past experiences.

    To sum up my addendum, I would still argue that time is more accurately described as radiating away from the present, the present being a point of origin. That origin is admittedly still moving in the positive direction as a sub-interval of a grander iterative function if we don’t interfere, though it does not have to be if we find a way to work around it. If I roll off a cliff, I would naturally fall to my death; unless I decided to roll off a cliff in an airplane. Electrical power is lost when passed through a wire, unless the wire is superconductive.

  23. rocken1844 says:

    It is disgraceful how new age pantheists seek to further their own agendas misapplying the nomenclature of physics. In “Wishes Fulfilled” Wayne Dyer claims that Albert Einstein taught (p.146) “that time is an invention of man and essentially an illusion.” No footnote or reference of course. Dyer then ties this idea in with NDE and dream states. The next sentence “Anita’s NDE unconscious state reinforces Einstein’s idea of no time.” On p. 71, after citing Max Planck (again no reference or context) he writes “You simply know you have an imagination and this imagination is the Source of all being.” Rubbish.

  24. It seems the question as to whether time is “real” depends more on the definition of “real” than on the nature of time itself. There is no question our lives would be different without the invention of clocks to coordinate our daily lives, design our spacecraft, measure our universe, and derive our theoretical physics. Many of man’s accomplishments, especially recent ones–e.g., the GPS, rely on the constant rate of time (at least one lab is working on a clock with an accuracy of one sec. in 300 million years). Time to me is a “real” fundamental feature of nature that has played a central role in man’s achievements since his emergence from the stone age.

  25. FrankL says:

    It seems to me that Einstein did not have much to say about the aspect of time that is being discussed here. He discovered a more accurate relationship between the measured time and space coordinates of events in spacetime, and the implied relationships between physical quantites, (mass, energy, momentum, electric field, etc.). But we do not directly experience these time coordinates written on a piece of paper. In order to imagine experiencing them, (lets say as an inertial observer in special relativity), we have to imagine ourselves as an unaccelerated particle, a straight-line world line in space time, and our consciousness is a point on that line, moving “forward” along that line at the speed of light. A moving plane perpendicular to the line at that point is what we perceive as “now” in space, with the point itself as “here and now”. Now we can reconstruct what our consciousness would experience, given these data points. But Einstein had no comment, that I know of, about this rather ad hoc reconstruction process, or the fact that you must introduce a “conscious time”, along with this reconstruction recipe, in order to get a sense of what your consciousness would perceive given a set of measured spacetime coordinates, momenta, etc.
    We have to distinguish between time as a parameter in a scientific equation, and our experience of time, which, in some “normal waking state” moves forward rather steadily and in which we experience an ability to remember an uncontrollable past to varying degrees, and to predict and control the future to varying degrees. Unless you are an Orwellian politician (“those who control the past control the present and those who control the present control the future”.)