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. John says:

    If time isn’t real, then we shouldn’t be manipulating it to describe things in physics. Physics describes things that are real, not imaginary things so that it just turns out that you got the correct answer. Even if Einstein did say time was only an illusion, the person listening to him or Einstein himself could have possibly just had a brain fart that day. If time isn’t real, and it has to altered in some way, then how do we then know that something like conservation of momentum is even true? If time isn’t real, it would have to be something else that is changing over time, instead of time itself. If you wanted to be smart and accept the laws of physics we have now as being correct, then it would be wise to say that time is real!

  2. Jack Smart says:

    Time is a meaningless irrelevance without events. Simple – as I posted before.

  3. Gary Godfrey says:

    Here is a different restatement of the “times arrow” quandary with a suggested answer that we just don’t have the correct fingers to translate macroscopic objects backward in time.

    _____Spatial translations by parameters (x,y,z) obey the axioms of a group. Use your fingers to manipulate an object on your desk to verify that: 1) The product of any two translations is another translation 2) Every translation has an inverse (ie. do it in the negative direction) 3)There is an identity (ie. do nothing) 4) Translations A,B,C are associative A*(B*C)=(A*B)*C.

    _____We d0n’t do a time translation by t with our fingers. Rather, we cross our arms and wait ….. something else causes the time translation.

    _____By special relativity we expect the translations by (x,y,z,t) to all be the same type of mathematical object, since viewing them from a boosted frame mixes them together. We therefore expect time translation to be a group operation. So, we are puzzled when we can not verify axiom #2…Using our fingers we can’t seem to do a time translation by -t. Thus the quandary of “time’s arrow”.

    _____Perhaps this is not because the inverse time translation does not exist, but only because we are not using the correct fingers to do it. For example, 1)In the microworld an electron can be translated in the -t direction by hitting it with a high energy virtual photon (a different type of finger). 2) In the macro world we just haven’t identified a good “finger” to cause the assembly of all the air molecules in a room to translate backward in time and return to the balloon from which they exploded.

  4. FrankL says:

    @Gary Godfrey – And we can verify that the group of spatial rotations is a group (with our fingers) and we can do a rotation in spacetime with our legs – by walking. Or with a plane, a train, or an automobile … or a rocket ship, anything that changes our inertial frame. But entropy is not a function of rotation angle, only translation along the time axis. If we found such “fingers” for many-particle systems, we would be able to violate the second law of thermodynamics. Or, from another perspective, the second law prohibits us from finding such fingers in its regime of validity (large number of particles).

  5. The Thinker says:

    What I’m fascinated about is whether the block universe really exists. If our inertial reference frame can mean our “now” is someone else’s future or past who’s time slice is at a different state due to them having a different relative inertial reference frame, then the past, present and future all exist.

    But one potential problem I see for this come from the EPR paradox/experiments
    that Einstein threw into the bunch. If two entangled particles can change “instantly” at large distances, what if one of the particles is traveling near the speed of light relative to the other particle when it’s measured? If the moving particle’s reference frame is skewed due to its movement relative to the other entangled particle, how can they change “instantly”? This would seem to violate the idea of a relative reference frame in favor of a universal or preferred reference frame.

    This has been bugging me for weeks. Does anyone know how this works?

  6. zephirawt says:

    In AWT neither time neither space are fundamental, as they cannot be defined/measured/observed without underlying space/time.

  7. Tom Clark says:

    Thinker: “If the moving particle’s reference frame is skewed due to its movement relative to the other entangled particle, how can they change ‘instantly’? This would seem to violate the idea of a relative reference frame in favor of a universal or preferred reference frame.”

    Great point, and it’s an empirical question, is it not, as to whether observed entanglements have involved relative movement between particles? If not, perhaps that would lend support to the block universe view in which there is no special now, in which time is a dimension, not something that flows (e.g., see Brian Greene’s chapter The Frozen River in his book The Fabric of the Cosmos). If so, maybe there’s still a way to incorporate such instantaneous (simultaneous?) effects between particles in relative motion into the block universe view.

    Would love to hear Sean’s take on this.

  8. Jack Smart says:

    I get the 11 dislikes – but, you do need to consider this:-
    When Sean talks about “Arrow of Time” he is referring to time in the mass noun context – i.e. Time as a (non-specific) composite of events. (If he was referring to Time as the abstract framework he would need to be more specific – and anyway can an abstract framework have a direction?). What other evidence of ‘time’ is there besides events?

    Whereas when, say, Einstein refers to time, he is referring to specific events, and the abstract framework that we use to connect them.

    So, we have a divergence – two people talking about seemingly the same word, but in fact meaning two different (albeit related) things.

    But I presume you all knew this anyway…(though I’ve never seen the distinction made by any scientist).

    Bring on the dislikes…but word meaning matters

  9. David Redfrost says:

    Time is an artificial framework imagined by monkeys to contain physical processes. Time is abstract and does not really exist except for in the puny brains of the aforesaid monkeys.

  10. FrankL says:

    @The Thinker – “two entangled particles can change instantly at large distances” is not what’s happening. Also “entangled particles” is an oxymoron, just like “the position of a particle in a momentum eigenstate” is an oxymoron. In a momentum eigenstate, it has no position until you measure it, and then it is not in a momentum eigenstate. For the “entangled particles”, you have a system which does not describe two separate particles, and once you make a measurement that distinguishes the two, you don’t have the system any more.

    What you have is two detectors and if you take many measurements you can describe the statistical correlation of the measurements made by the detectors under different conditions, and you wind up with statistics which are impossible – if you assume that the two detectors are not somehow talking to each other. You can show that any such communication must be instantaneous. But if Alice is at one detector and Bob is at the other, they cannot use that instantaneous communication to send messages to each other, because the information comes from comparing Alice and Bob’s measurements, and they must communicate sub-luminally in order to make that comparison.

    A crucial thing is that nothing strange ever actually happens, its only when you say “if Alice had done such and such, and if Bob had done such and such, then such and such would have happened, and that’s impossible, given what actually happened. The idea that you can talk about “ifs” in quantum mechanics is, well, iffy. You measure the position of a particle, is it proper to ask “what if I had measured momentum instead, would the particle still have had the position I just measured but I would not have known it?” As my quantum professor would say, that’s an improper question.

    The idea that using the word “if” is a proper thing to do is called “counterfactual definiteness” (CFD). Maybe using the word “if” becomes improper as you dive into the quantum world and CFD is not valid. But then what happens to free will? Free will is like “If I do this then A will happen and if I do that, B will happen and I want B so I do that”. Without the if, we just do what we do, and free will is, well, an improper concept. This is were I lose clarity. It’s more comfortable to assume spooky action at a distance, but I sometimes wonder.

  11. Randolph Brown says:

    Time only makes sense to the observer of change. The illusion of time helps the observer understand what he observers in the now. Movement that results in change is fundamental. Time isn’t.

    A conversation with my son.

    As we sat on the deck, enjoying the fire we had started in the fire pit, I asked Harrison, my son, what words he would use to describe everything that was happening in the universe at this particular moment. He looked confused.

    He needed help in understanding the question. I said look at the fire, what is happening there?

    We started talking about oxidization and chemical reactions inherent in the fire. How would you describe the fire? We realized there is a lot of information generated by a fire and how to describe it accurately.

    I asked him, as we watched the fire burn, what do you think is happening now to the stars in a galaxy on the other side of the universe? He said “they are moving”.

    Now think about all the galaxies right now, what words would you use to describe everything that was happening in the universe at this particular moment. He said “overwhelming”.

    It was then I began to tap his knee. Every few seconds I would tap his knee. I said it looks like I am touching your knee repeatedly. He said yeh, you keep touching my knee over and over again. I said look at the fire, I’ll touch your knee again. Was that the same as when I touched your knee earlier? Harrison said no, the fire was different. So each time I touch your knee its different? Yes, each time you touch my knee its unique.

    I asked Harrison, “What does it mean that something is unique”? He said “it’s one of a kind.” I asked him “if it’s one of a kind, does that make it valuable”? We considered such…

    Harrison is 11 and a half.

    Every moment is unique, it can never be repeated. Its value is incomprehensible.

  12. Ron Seadler says:

    “It is disgraceful how new age pantheists seek to further their own agendas misapplying the nomenclature of physics.”

    It’s not always easy to tell if they mistakenly believe they have found the ultimate truth, or whether they knowingly believe they have found their cash cow.

    In the 70’s I dallied with various concepts (aka belief systems) of consciousness and reality. At some point I found one that benefited me extremely well, and with it the world made profound sense. I was damned pleased to relate my discovery to the wisest person I knew. She merely smiled and said “That’s another wonderful way to view the world.” For a few minutes I felt her comment was a bit dismissive. Then I got it. I’ll never forget that moment. It truly saved my ass.

    There’s the physical reality we exist together in, and there’s our individual conscious point of view. I smartly cling to the rational exploration of facts and evidence. It’s the self-correcting reality we all share. My conscious viewpoint happily morphs and shifts with scientific discoveries, but so it is. There’s probably more personal viewpoints of the universe than there are people to have them.

    It’s also nice to understand precisely why you became an atheist.
    (Disclaimer: Born Catholic with 12 yrs Catholic education)

  13. John Duffield says:

    I think it’s fair to say that time is real in much the same way as heat is real. But like heat, it isn’t something fundamental. If you open up a clock you don’t see time flowing through it like it’s some kind of cosmic gas meter. You see cogs moving. What clocks actually do is accumulate some kind of regular cyclic motion and show you a cumulative display called “the time”. More and more people are appreciating this, IMHO. What’s actually “out there” is space and motion rather than a static “block universe” comprised of spacetime.

  14. Ron Seadler says:

    @John Duffield

    Heat would have no identity outside of time. You couldn’t open a clock without the time necessary to do it in. Feels fairly fundamental to me. I’d bet my future on it. Well, maybe later.

  15. Brett says:

    There are 2 meanings to time in physics(this is my understanding anyway):

    1.) time is change which results in things being different from each other.

    2.) In cosmology, we see the bigger picture. Consider the universe before the big bang as a precise sphere with no differences at all; every single point is exactly the same, called equilibrium. The first change in history occurs and a new property of nature is born, the ability of something to be different. The ability to change. A single mechanism is responsible for this change; the expansion of space. The expansion of space is still occurring. This expansion mechanism means that things get more spread out over time from all the way down to the smallest length possible, all the way up to the entire universe. This is the arrow of time. It is responsible for the 1st property of time in physics. The 1st property listed above can be considered an “echo” of the 2nd property. There are other properties that can be considered ‘echos’…’echoes’? The arrow of time based on entropy is one of them.

    This is why I believe that time is real and it is the most fundamental property in the universe. And now I must get back to my evil lair and plot schemes against Julian Barbour…spreading salt in his yard perhaps, mwoohahahaha.

  16. John says:

    @ The Thinker

    The problem is that quantum mechanics (QM) violates Einstein’s principle that information cannot travel faster than light, and then they just allowed Einstein’s work and QM to say they are correct because they apply to different fields and they are not compatible with each other. Although, now they think that even galaxies can travel faster than the speed of light (FTL) away from each other, and that is in Einstein’s territory of the macro scale. Then they say this is okay, because space itself is actually traveling FTL as though it is an actual thing. Can you imagine? I don’t think it is really fair to “time” to accept “space” as an actual thing when it comes to this issue…

  17. Howard Hunter says:

    Fundamental or Emergent – Ekaterina Moreva and friends claim time is emergent. Interesting ideas.

    summarized here:

  18. FrankL says:

    Just to clarify the two types of time a bit more. Please note that I have not incorporated quantum mechanics into this, so maybe it’s dead wrong. Hopefully an oversimplified beginning.
    1) There is “physical time” or “time1” which is just a parameter, another coordinate in the space-time continuum. It has no direction, only an orientation as reflected in the fact that the microscopic laws of physics favor no particular direction in time. Confining things to classical relativity, time1 is not “superdynamic” in the sense that particles, for example, are seen as world-lines in the spacetime. There is no “motion” of these 1-D world lines but rather they represent the dynamic motion of 0-D particles. There is no absolute before, now, or after.

    2) There is “type 2 time” or “time2” which ideally corresponds to physical time, but with an added “forward” property. It’s a vector rather than a ray.

    Consider 2 particles that approach each other, collide and rebound at the origin, and then move away from each other. Their moment of inertia about the origin is I=m1 r1^2 + m2 r2^2. This moment of inertia function can be seen as a measure of their “dispersion”, so lets call it that. If you look at the dispersion sometime after the collision, it is an increasing function of time2. With regard to time1, the whole thing is two V-shaped world lines in spacetime, and time1 is a whole set of possible axes that could be used to coordinatise that spacetime. There is no “increasing” concept in the sense of any motion of the world lines. The direction along a ray of time can be positive or negative, it doesn’t matter.

    In time2 terms, after the collision, the dispersion is an increasing function of time. Now look at entropy. In time2 terms, the entropy is an increasing function of time for the same reason. The collision between the two particles is a little bang, the origin of the universe is a big bang. For entropy, in time1 terms, there is no “increasing” concept. Just as there may be a parameter (dispersion) with reduced information about the two particles, so entropy is a parameter with reduced information about a huge number of particles. Note that for a single isolated particle, no increasing function of time can be defined that cannot be reversed by a suitable redefinition of the direction of the space axes. Not true for the 2-particle dispersion. Or entropy.

    When you look at things by time1, there are no “superdynamics”. Anything explained in terms of a forward direction in time, is using time2, not time1. With regard to time1, there is no before, now, or after, there is no cause and effect, there is no motion, there is no second law, there is no theory of evolution, there is no spooky action at a distance.

    So what is the deal with time2? Lets look at a human observer from the time1 viewpoint. He or she is a huge mass of twisting and turning world lines of all sorts of particles, obeying microscopic laws, converging at some spacetime distance from the big bang, diverging on the same side, but at a further distance. If we could read and interpret every one of those lines, we would see every experience of that person, every dream, etc. And we would see the equivalent of their ideas about “consciousness” and “before” and “now” and “after” and “forward flowing time” (time2). What is that all about?

    Well, I don’t know what it’s all about except to say that its an emergent thing resulting from the microphysics of the world lines. In other words, time2 is time1 with an emergent “time-forward prejudice” that may well never undo itself for organisms further away from the big bang.

    So with this view, the interpretation of the Bell inequalities as suggesting spooky action at a distance is a result of our time-forward prejudice.

    Now I will read Howard Hunter’s links and perhaps deny that I was ever serious about the above.

  19. Frame Jumper says:


    You state: “Consider the universe before the big bang as a precise sphere with no differences at all…” Actually, the true believers (those who want a job after graduation and hope to get their papers published by the establishment) tell us that there was no “universe” before the big bang. There was absolutely nothing, no space, no time. Nada. Zip! Or at least that’s the story IF, of course, you’re a “big bang” and miraculous “inflation” promoter. (This outta draw “dislikes.”)

    Right on, Randolph, “Time isn’t.” Like the illusion of time in a movie, it’s just change in the Now – motionless, timeless frames, i.e., your “taps.” This explains pretty much everything including why photons do not experience time (because they only exist in the Now), why clocks “run” at different “rates” (Einstein), why “time” in which matter exists becomes “shorter” as you approach the “speed” of light, and even why particles disappear from one location only to instantaneously reappear in another without traveling the space between (in a movie, in one frame it’s on the right, in the next frame it’s on the left; simple, actually). Perhaps this also explains why a single photon appears to behave the same as many if you stop (one of those misleading “time” concepts built into the language) and think about it.

    If it were possible to travel at light speed (it is not, according to physicists), you, being matter, would enter the realm of the “eternal now” where time truly does not exist – not up for discussion. All of your matter and your motion through time (?) will have been converted to energy. So, are we all just “light beings” (Kalen J. Craig’s, “bundles of sine waves”) in a holographic universe emerging out of some (Bohm/Mandelbrot) implicate order? Aren’t we, after all, just atoms appearing and reappearing?

  20. John says:

    Stephen Hawking would say that there is no before the Big Bang, as if there is no time before the Big Bang. If there was a transition between there being no time and time, then time would have to be an actual thing that emerged from the Big Bang.

    One growing idea about the early Big Bang is that everything consisted of mostly energy. Then if there was no mass then there wouldn’t be a singularity that the Big Bang would have to escape from. If there was no mass then everything in existence would travel the speed of light. If everything in existence traveled the speed of light, every particles frame in the universe would be contracted. If moments close to the Big Bang had the same laws of physics that we have now (that I find very likely), then spacetime wouldn’t have been really experienced by anything in the universe until the first time energy was converted to mass, so I believe that time is an emergent property that stems from the Special Theory of Relativity itself.

    So then I would have to side with Prof. Hawking and say that there actually wasn’t a “before the Big Bang”, and time would have had to have emerged sometime afterwards.

  21. Brett says:

    “If there was a transition between there being no time and time, then time would have to be an actual thing that emerged from the Big Bang.”

    But the fact that there was a space without time that turned into a space with time would itself be a function of time. A change occurred. The universe without time and the universe with time would be parts of a larger event in which time was required in order to have occurred. Doesn’t energy require a time variable? I would agree that as far as we are concerned in our universe, there wasn’t a “before the big bang”, but that it doesn’t mean that nothing was happening. It just means that it is beyond our understanding of nature and physics to describe or comprehend a space in that state. In which case, we should just consider there to have been nothing before the big bang. I think it’s sort of a paradox, there was something, but for all intents and purposes, we could accurately say there was nothing. Every possible equation we could use to describe the universe before the big bang would be completely ineffective and utterly pointless. Every answer would be infinity and there would be no boundaries, the only state in which the universe was truly unified. If you can tell me why that changed, please send me an email with the answer and await a very “special” cake baked only for you. You could say that it was a state in which the universe was “pure energy”, but what does that mean? Isn’t that the exact same thing as I described above; nothing but null field values at every infinite measurement?

    How did the discreteness and variability come about? That is when time started. What disrupted the perfect system?

  22. The Thinker says:

    @ John

    Not sure that answers my question.

    @ FrankL

    Is it just a measurement problem, or is this a real phenomena that travels faster instantaneously? Scientists pretty much agree it is instantaneously. If that is the case, what inertial reference frame does the instantaneous change take place?

  23. FrankL says:

    @The Thinker – there is no instantaneous change. It’s not a case of “look, I changed something here, something changed over there.” In other words, you cannot send information using the Bell inequalities.
    A very simplified (but still kind of hairy) thought experiment might help. Suppose a machine put two balls onto two chutes, one leading to Alice, one to Bob. The balls can carry instructions. The balls fall into a box with two doors, and some mechanism in the box will then read the instructions, and paint the ball black or white, depending on the instructions and which door is opened, and then deliver the ball to whoever opened the door. The only time Alice or Bob can see the color of the ball is by opening a door. They notice that no matter which door they open, its a 50-50 chance they see a white or a black ball.
    If they both open the left door, both balls are the same color. If they both open the right door, the balls are again, always the same color. If Alice opens the left door and Bob opens the right door, the colors do not match. Ok, it’s easy to model whats going on. The balls coming down the chutes have either BW instructions or WB instructions. BW means if the right door is opened, paint the ball black, if the left door is opened, paint the ball white. WB means the opposite. If BW and WB balls are sent with equal probability, you reproduce all the results for the above three scenarios.
    There is one case we did not consider – Alice opens the right door, Bob opens the left door. The above model says they won’t match, but what actually happens is that they match. This destroys the above model. If the above experiment were quantum mechanics, then quantum mechanics would correctly predict all four of the above results. How do we explain the above results?
    The only way to explain the above results is that somehow each box must know what door was opened on the other box before painting its ball. Even if Alice and Bob are separated any distance, by a million light years, and open their boxes at the same time, the above results always happen. The communication must be instantaneous.
    Notice that Bob cannot send information by this means. Alice opens a box, sees a white or black ball with 50-50 probability, what is she to make of it? Nothing. Over and over, thats all she sees. Only when Alice and Bob share their results, by light-speed (or less) signals, can they compare results. And even then, one run gives nothing, they both say “ok, nothing strange happened, results are as predicted.” Only when they share the results of a number of runs, involving all four possible outcomes, do they say “yes, the predictions are correct, but that’s spooky” and “there must be some information being transmitted instantaneously”
    Actually, there is another way to explain the results – deny counter factual definiteness. In other words, the above results are explained by the fact that Alice and Bob have no choice in the matter, no free will, the universe is a Laplacian pre-ordained clockwork, they were bound to open the doors that they did.

  24. Is time real and fundamental if the speed of light in vacuum is not invariant?

  25. John Duffield says:

    @martenvandijk: It’s real, but not fundamental. The speed of light in vacuum varies with gravitational potential. Check out the coordinate speed of light. However the locally-measured speed of light doesn’t vary because you calibrate your rods and clocks using the local motion of light. Then you use them to measure the local speed of light. So you always measure 299,792,458m/s

    @Ron Seadler: Heat is heat. When something is hot, you experience it. It’s real, it’s empirical. But we know it isn’t fundamental, and instead is an emergent property of motion. A hot gas is made up of fast-moving molecules/atoms.

    OK, hold your hands up a foot apart, and you can see the gap, the space between them. Space is real, and empirical, and fundamental. Then waggle your hands, and you can see them moving. Motion is real and empirical and fundamental too. But you can’t see time flowing. You can see literally see things moving through space, but not through time. And when you open up a clock you see cogs moving, not time flowing. What clocks do is “clock up” some kind of regular motion and show you a cumulative display that you call the time. So you don’t need time to have motion, you need motion to have time. It’s that simple: stuff moves, sh*t happens, that’s it.