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. Brett says:

    You could just as easily say that motion is the result of time since there is no comprehensive understanding of what time is. I could say that a lack of symmetry in nature is a product of time. If motion is the result of a lack in symmetry, then motion is a result of time.

    What is causing the change which motion is an emergent property of? My answer would be time.

  2. FrankL says:

    @John Duffield – I think time is as fundamental as space. If two firecrackers go off in front of me, I don’t need the concept of motion to tell me they went off at different times, or the same time, any more than you need the concept of motion when you hold your hands out and see the space between them (or not, if your hands touch). We need a clock to measure time, just like we need a ruler to measure space. We don’t need a clock to sense simultaneity or lack thereof, any more than we need a ruler to sense whether your hands are apart or touching.

  3. John says:

    @ Brett

    I don’t know how I would send you an email or retrieve one from this blog. The short answer is that electromagnetic energy can just “#$%*” itself or I mean intensify itself.

    Brian Greene talks a lot about a growing problem in physics in his latest book where they are unable to determine the total energy of a particle traveling back and forth inside of a box. It is keeping him from finding solutions to a problem in String Theory regarding the cosmological constant. When trying to solve this problem, they get answers that result in infinity. Then they throw this theory out because they are not allowed to get these kind of answers.

    I think there may be some kind of truth too it, and the conservation laws where basically philosophized into science based on Newtons Laws. Then I don’t know of any actual experiments that took place that reflected energy in a box, basically because the ability to do this had not been invented yet. I find it discomforting that we hold so strongly to classical principals in QM where nothing works classically at all.

    I think there would have to be some kind of free energy mechanism at work near the moment of the Big Bang. If there wasn’t then we would all be made out of some kind of “God Dust” that had these properties of always existing and never having to be created. Then believing in our selves and the world around us could become unscientific, lol!

    Then energy could be anything you could stick in the “E” side of E=mc^2. In a small closed universe there could be more interaction between particles. If the universe grew in size then there would be less interaction. If there was a free energy mechanism from interaction of particles then this process would stop just from the universe growing. At a time like now when a particle traveling the speed of light cannot even go around the universe, there would be no indication of this process still occurring at this time.

    Then particles traveling the speed of light do not experience time because when something would travel the speed of light SR would say that the proper time would be zero…

  4. Gary Godfrey says:

    @FrankL – Yes, if there is an operator whose expectation value is entropy, then that operator commutes with rotation of the system and does not commute with time translation of the system (ie: as you say the entropy of a system changes as you move the system through time). However, this does not mean that an appropriate finger can’t move a macroscopic system backward in time. The 2nd Law of Thermo just implies that the entropy of the finger would have to increase more than the entropy of the system moving backward in time decreases.
    As you also point out there are several more continuous (Lie) group operations that can be done to an object. I think the full list (excluding gauge transformations) is:
    3 space-space rotations (about the x,y,z axis)
    3 space-space parallelepiped strains (eg: done by a gravitational wave)
    4 squash strains in x,y,z,t (eg: done by a gravitating mass which stretches in t and squashes in r)
    3 space-time parallelepiped strains (also known as boosts in the x,y,z directions)
    3 space-time rotations (I don’t know what does these…it’s just a guess, they are there to fill out SL(5))
    4 covariant translations in x,y,z,t
    4 contravariant translations in x,y,z,t
    These 24 operations and their products might form the mathematical group SL(5) (if translations don’t commute). My intent in expanding your list to 24 operations was to point out that time translation is a group transformation, just like the other 23. Time translation is as real as any of the other 23 operations that can be done to an object. Time translation is not special, an illusion, or mystical. We only think time is different because we empirically don’t have any control of its passage or its direction. Perhaps this is because something like the mass of the universe causes times passage, and our puny mass just can’t influence the time translation very much.
    In order for an object to change, some combination of these 24 operations must be done. All 24 parameters are necessary to specify how the object has changed; t by itself is not sufficient. We document the continuous change of an object by writing down the values of these 24 parameters as they build up from 0 (the identity of the group). It may be convenient to specify where we are along this path in 24 parameters by a length along the curve (call it the evolution parameter T). If only translations were being done for this piece of the path, the evolution parameter dT along this infinitesimal piece of the path would be dT^2=dx(covariant u) dx(contravariant u).
    So, in answer to the original question ‘What is time?”, there is a time (a number t) that specifies the group transformation that corresponds to what we do by only waiting. However, in addition, there is an evolution parameter (a number T) that keeps track of where along the path in 24 parameters the object is. If only dt is done and not any of the other 23 transformation, then dT=dt.

  5. FrankL says:

    @Gary Godfrey – I see what you are saying. Its sort of a Maxwell’s demon kind of thing, except rather than simply decreasing entropy, it does it in an extremely specific way, so as to reverse the micro-processes by which the system arrived at its high-entropy state.

    Classically, the demon would have to reverse the velocities of every particle, but quantum mechanically I’m not sure. If you had a box with two particles, one at a high energy eigenstate, one at a very low eigenstate, after time, with collisions, the wave function would reflect a more likely energy distribution. I’m not sure how to restore the original wave function, or if its even possible in principle.

  6. E-motion is caused by electricity.

  7. Adel Sadeq says:

    My theory which seems to reproduce the laws of physics it indicates that time does not exist. We perceive it as such because of the change in the state of the system, very much along Barbour’s scheme. As a matter of fact my theory has a very similar trait of being conformal similar to Shape Dynamics which is a theory derived from Barbour’s scheme, However, my theory looks more fundamental and the time result is just one small part of it.

  8. John says:

    Maxwell’s demon must be responsible for every time my mother makes me clean up my room! LOL, Jk!

  9. Miao says:

    Hi Sean, I am a huge fan of your blog. I am wondering if you have read this article. I would love to hear your thoughts on it; so if you are both willing and able to spare time to write a blog post about it, I would be over the moon!

    Thank you for writing such a fantastic blog — keep up the good work!

  10. Tom Clark says:

    Looks like Sean is an eternalist, someone who takes the block universe view of time, as opposed to a presentist, who supposes that time flows:

    “Carroll is more skeptical [about the flow of time being fundamental to physics]. Rather than attempting to change the block universe to explain our experience of time flowing, he says we should concentrate on explaining human experience in light of what our very successful physics tells us about the block universe. That task, he says, is quite achievable. ‘That doesn’t mean that we’ve done it yet, but I see no obstacle to doing it.'” – from “The now delusion” by Michael Slezak in New Scientist.

    The nature of time, e.g., whether it’s fundamental or emergent, is a matter of what the most successful physical theory tells us, then we go about accommodating the commonsense view of time to the facts. The New Scientist article (not free online that I could find) presents several examples of the reverse: trying to come up with a physics that accommodates commonsense and experience.

  11. I have introduced in physics the difference between endogenous aand exogenous processes. By doing so I have introduced the notion that comparing gravity with pseudogravity (created in an elevator or a rocket) is comparing apples with oranges. Gravity as we experience it continuously is caused by an endogenous process, whereas using an external source of energy to create pseudogravity is an exogenous process. In other words, I don’t think the equivalence principle is right. Likewise, as I understand it the Lorentz transformation is not valid in an endogenous process like gravitation.Gravity is fundamental as a continuous process. Studying the way light behaves under the influence of gravity, I think that it stands to reason that time is fundamental as well. So, if Sean Carroll is an eternalist, I have no reasons tothink he is wrong.

  12. Brody Facoum says:

    “You can see literally see things moving through space, but not through time.”

    No, you literally see things moving through spacetime. The thing is that looking around you, you will mostly see mesoscale things moving much much further along the timelike axis than along the spacelike ones. Where distance along the spacelike axes are measured in c*seconds and c is set to 1, this becomes readily apparent. In spherical coordinates centred on you at some point in spacetime, a ball in one hand will move essentially not at all along the spacelike axes whereas a ball thrown from the other hand might move tens of nano-(c*second) radially and follow a curve of a few nano-(c*second) up and then down in altitude, and not at all azimuthally. However the thrown ball will have travelled perhaps several seconds along the timelike axis; the held ball and you will also have travelled almost exactly the same several seconds along the timelike axis, along with everything else that you see nearby.

    For nearby objects moving slowly relative to one another, the timelike coordinate varies so little that people generally avoid thinking about it. However, those objects are certainly travelling along the timelike axis at a rate much much faster than the rate at which they travel along any of the spacelike axes.

    Indeed, in the example above, the held ball travels essentially no distance at all along the spacelike axes. However, when working out the spacetime intervals in a block of spacetime surrounding this example, the timelike term totally dominates the thrown ball, the held ball and you at t’ compared to all three at t.

    When you’re looking at your mechanical clock, the arms or the gears inside are also moving in spacetime, dominated by movement along the timelike axis.

    Additionally, the reason that everything appears to be moving uniformly in one direction along the timelike axis is possibly a constraint similar to the constraint from Earth’s gravity against movement in altitude when dealing with the sort of energy you can impart to a ball by throwing it. If you increased Earth’s gravity severalfold, you could only ever manage to send a thrown ball downwards along the altitude axis.

    It is wholly possible that the “clocks” (in the most general sense) ticking away in the example above tick essentially at the same rate of one second per second. Your wristwatch and your mechanical clock will agree very precisely over the several seconds of the patch of spacetime local to the ball-throwing. However note that gravitational potential influences the clocks’ ticking, too, and not just the thrown ball’s altitude.

    cf. the comment by Gary Godfrey above.

  13. Glenn says:

    Is time real? Perhaps, this may answer your question? I’ll keep it as brief as possible.

    Time is the quale of duration? Duration is the ‘length existence’ (the period between ‘beginning’ and ‘end’) of an event. An event is an action we observe. When we observe we are perceiving, and when we perceive we are assimilating qualia. Time is simply the perception of duration of an event.

    Time, per se, holds no existential reality of its own.