Time Talk

By Sean Carroll | September 18, 2012 2:14 pm

With The Particle at the End of the Universe scheduled to come out in November, most of the popular-level talks I’ll be giving in the near future will have to do with the LHC and the Higgs boson — and quantum field theory, as part of my secret agenda to get QFT accepted as part of the mainstream pop-sci vocabulary. So I’ll be giving fewer talks about the arrow of time, at least near-term. I thought I’d commemorate the occasion by sharing the slides I used for a recent version of this talk: “The Origin of the Universe and the Arrow of Time.” Not that I’m by any stretch done talking about it, but hopefully the next time the occasion arises I’ll have the energy to make up new slides from scratch.

  • LH Ld Elon

    axis of time, 1, axis of space, 14x infinity.
    So space has many directions.

  • LH Ld Elon

    Oh and that space travel thingy, seems like a total paradox of time throw back.
    So you travel there, 100,000 thousand years ago, and evolve with the planet, forgetting earth, one day, you find yourself hunting others like yourself in space with scopes to see the cosmos, then to overcome all boundaries and insight of matters regarding travel, finally coming across a tiny blue sphere, you will one day may call home, only traces have shown there is something alive down there{earth}, and they have become dominant, and are warlike creatures.
    And this, the knowledge that they are the same, might not be known, and so, the human, wipes out, the human, lol, paradox.
    It also means humans are very alone as they set themselves up to be thee only ones here, think that thee other is Alien.
    How would a cow change, what of a monkey, a whale, an ox?
    I can feed 5000 thousand with one fish, even half a fish, the largest fish in the sea, a whale, simples, hahahaha

  • http://Artifactshore.com Bill

    What?! I thought this was a talk about slow cooked bacon in space!

  • MarshallB

    I’m not telling you any secrets, Sean. You can’t keep them!

    But seriously, thanks for the slides. The specific multiverse you showed was new to me. Such cool stuff is the reason why this is my favorite blog. Looking forward to the book!

  • ed hessler

    Thanks for taking time to talk and think about time and now, for posting the slides of a recent talk. I’m glad to have it available. It took time for me to finally get to your book and I still regret not taking advantage of the discussion of the book you let through CV but so it goes.

    Seeing this today reminded of a grook by Piet Hein who never seems to grow ancient and worn to me. Here ’tis.

    I’d Like to Know

    I’d like to know
    what this whole show
    is about
    before it’s out.

    So, I’m glad you are helping me to know a little about the past, present and future since I won’t be around a few billion years from now or whatever it takes for us to know.


  • Sean Samis

    A small quibble with slide 31 which says: “You remember the past and not the future because the early universe had a very low entropy.”

    Hmm. Don’t we remember the past because we were there, and its attributes (whatever they were) imprinted themselves on us, but we have not experienced the future yet as it has not yet happened, so it has not yet imprinted itself on us?

    sean s.

  • Joe

    If the arrow of time depends on the increase of entropy, then wouldn’t the arrow of time reverse in a region of space where entropy is decreasing? In fact, doesn’t the formation of memories (storage of information) typically involve a decrease in entropy? How does my physical neighborhood know about the entropy of the whole universe? Perhaps this is answered in Sean’s books…..which I confess to not having read (yet).

  • Sean Samis


    I believe part of the answer to your question is that “regions of space where entropy is decreasing” are usually pretty small, and because the decrease in entropy is not recreating earlier conditions, but new, lower entropy conditions. If the arrow of time is reversed, not only is entropy decreased, but the configuration of matter is restored to a prior configuration, not a different configuration.

    For instance, decreases in entropy resulting from the formation of memories don’t represent a reversal of the arrow of time because the because the memories created are not a recreation of a prior condition, but the creation of a new condition.

    It is not necessary for any region of space to “know” about what’s happening in other parts; it is only necessary that events in near-by “regions” drive any entropy-reducing changes locally. And often, “near-by” is a “near” even in human terms, much less cosmic terms.

  • Joe

    Sean Samis,

    Not sure this helps. The smallness of the region should have no bearing on the arrow of time, as long as we are macroscopic. And I am not talking about reversing the particular path that a system takes in time (i.e. time-reversal), but reversing the arrow of time itself. I see how the local application of the laws of thermodynamics (accounting for energy/entropy fluxes into and out of a region) might lead to a “forward” arrow of time even if entropy is locally decreasing. But I have trouble seeing the connection between the cosmological integral of entropy and the local (thermodynamic) arrow of time.

  • Sean Samis


    If actual “time reversal” does not occur, then I think it not sensible to talk about the “arrow of time” being reversed. Local decreases in entropy are consistent with the arrow of time being fixed from past to future; especially if the “region” of entropy decrease is small. If one speaks of localized decreases in entropy, I don’t see how it is useful to call that a reversal of the arrow of time; I think it only confuses things.

    I think the arrow of time is fixed more properly by the fact that time-reversed events are highly improbable. A land slide down a mountain is not extraordinary, a land slide back up the mountain would be miraculous. It may be that, at the quantum level, the arrow of time is less clearly directed, but at higher scales it definitely is.

    My analogy is to the surface of liquid water. At scales higher than molecular scale, a water/air interface (surface) is quite clear and definite. At molecular scales, where water vapor mixes with liquid water, and air gases with both, the boundary is more ambiguous.

    Likewise, “now” is the interface where “future” becomes “past”. At a quantum level, events can and probably do run bi-directionally. At higher scales, this effect is washed out and time proceeds uniformly in one direction because even at a quantum level, some sequences of events are probabilistically favored over others: hence the arrow of time. “Time reversed” events don’t happen not because they can’t but because as the scale increases, the probability of the reversal becomes infinitesimal.

  • Sai

    HI, I’m new here. Pleased to meet you all.

    I would like to propose something new.

    This is my theory: first of all ” Time is not only related to space but also related to energy”. Fact: Gravitational time dilation effect: Gravity affects matter, there is no proof it affects time (as in time dimension). Therefore, time must be related to both energy and space. (this is my proposal, I know its contrary to popular belief, but present understanding doesn’t answer unsolved puzzles. Therefore a change in perspective was needed)

    What is time?
    Well to answer this first: How is time observed?
    By change (includes motion). Only with change we can know whether time has passed or not; whether something has aged or not. Change also includes motion.

    What causes change?
    Force causes change (also motion). Forces such as: gravitational force, strong force, weak force, electromagnetic force.

    Okay, Time is observed by change. But what change?
    Observations: Basic: 1.Take a clock and observe. when one second is passed, the clock produces a sound (tick). This sound (tick) is actually energy that is released. As time moves forward, the clock releasesemits energy. This is change.

    2. Electronic gadgets: Take a cell phone or laptop and observe. After a period of hours the gadget looses its power (energy). This means that the gadget is releasing energy constantly. What energy is released? It is mainly photons, thermal radiation (sound) etc.

    Other examples: Human body (ask yourself why we need food, water and oxygen to survive?)

    Sorry guys, I’m late for my job. I have to go. If you want to know more please read and look at the following:

    Published paper on time: http://iosrjournals.org/journals/iosr-jap/papers/vol1-issue5/E0152327.pdf

    Next: http://2.bp.blogspot.com/-0IaF_LqXsA4/UE85LKdNeAI/AAAAAAAAADE/b6nzlwx68FI/s1600/My+flowchart+pic.png

    Thank you, and kindly note: English is not my first language. I have made errors in presentation… but the point I would like to convey is genuine. Thank you for understanding!



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About Sean Carroll

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


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