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Temporal symmetry solves all quantum paradoxes?

  1. Jun 21, 2010 #1
    Excerpted from an article by U. of Hawaii Physics Professor, Victor J. Stenger:


    "As has been known for seventy years, quantum phenomena depend not only on the initial conditions of an experimental setup but also on the final conditions. This observation already signals that the quantum world is time-symmetrical. Quantum phenomena do not distinguish between "initial" and "final." These are commonsense designations that can be interchanged without making any changes in the basic theory...

    ...All the alleged paradoxes of quantum mechanics result from the unnecessary use at the quantum scale of the singular time direction of common experience. No doubt the arrow of time we all experience in our lives is an objective reality. But it can be shown to be a consequence of the statistical behavior of systems of large numbers of particles. The probabilistic behavior observed on large scales does not apply for the small numbers of particles involved in quantum phenomena."


    Source:http://www.colorado.edu/philosophy/vstenger/Timeless/nowhen.html [Broken]

    But if our common sense experience of the world is time asymmetric (I know we remember the past and wonder about the future) in the way we perceive causality, what sense does it make to talk about an objective reality where time is symmetric?

    What are your thoughts on this?

    Thanks.
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jun 21, 2010 #2

    DrChinese

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    Could be. You may want to take a look at some of the time symmetric interpretations of Quantum Mechanics, as there are several. Generally, these do not assign a preferred direction to time - although they each handle things a bit differently. I think one of the more interesting is Relational Blockworld:

    http://arxiv.org/abs/0903.2642
     
  4. Jun 22, 2010 #3
    Thanks.
     
  5. Jun 22, 2010 #4

    DevilsAvocado

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    Good question. I have a slight feeling that Professor Victor J. Stenger may have 'overlooked' that fact that the universe started with low http://en.wikipedia.org/wiki/Entropy" [Broken] (a nearly uniform gas) and developed towards higher entropy.

    It’s true that a single QM particle doesn’t 'care' if it goes this way or that way. But for bigger systems, like our universe, the http://en.wikipedia.org/wiki/Second_law_of_thermodynamics" [Broken] is a fact. Therefore it could be 'problematic' to talk about objectivity vs. subjectivity, when talking about the universe as whole...

    That’s why you never see 1 omelet randomly transform into 4 eggs! :smile:

    Sean M. Carroll has written a very good article around entropy and the arrow of time for http://www.scientificamerican.com/article.cfm?id=the-cosmic-origins-of-times-arrow".

    Does Time Run Backward in Other Universes?
    ab230924-fa4d-9eac-5e5e8d5152c227b1_1.jpg

    (P.S. Google "Does Time Run Backward in Other Universes?" there’s a 'back-up copy' out there somewhere... :wink:)
     
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  6. Jun 22, 2010 #5
    Ah! Makes sense. Thanks for the article ref.

    Is this because of decoherence?
     
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  7. Jun 22, 2010 #6

    DevilsAvocado

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    Well, I don’t think so... (Disclaimer: I’m just a layman)

    It’s just the simple fact that everything strives towards http://en.wikipedia.org/wiki/Thermodynamic_equilibrium" [Broken]. Like the simple fact that when you pour milk in your coffee, if you just leave it like that, it will finally mix completely (due to the fact that the molecules are constantly colliding with each other and the wall of the cup).

    And there are always many more ways for a "system" to "organize" in high entropy than low entropy, like coffee with milk, or an egg:

    [URL]http://www.scientificamerican.com/media/inline/AB230924-FA4D-9EAC-5E5E8D5152C227B1_3.jpg[/URL]

    Now, the really weird thing is that our universe started as a "fresh egg", so to speak, and then everything has gone "down the hill"... :smile:

    To make things even more complex: Gravity plays a major role when deciding what high/low entropy is. And negative gravity (expanding space) is a competitor to gravity. The key point is equilibrium, in respect of +/- gravity:

    [URL]http://www.scientificamerican.com/media/inline/AB230924-FA4D-9EAC-5E5E8D5152C227B1_4.jpg[/URL]

    I think we could summarize it like this: Since our universe started as a "fresh egg", we are now experience the "egg" being "broken" – and our time is running in the same direction as the "degeneration" of the "egg". I think... :rolleyes:

    The big question is: If our universe started as a "broken egg" – would we then experience time running in the other direction?

    I have no idea... :uhh:

    All we can be sure of is that if "living creatures" where possible in such a universe – it would be the most terrible horror movie ever! People would crawl up from the ground, as they start their life. And walk backwards for 75-85 years, to finally "disappear" as babies!? :eek: (:biggrin:)


    P.S. QM particles will always run in any direction they want, regardless if the "egg" is broken or fresh.
     
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  8. Jun 22, 2010 #7
    No, it will just last for eternity in thermodynamical equilibrium. Random fluctuations will still be possible, though. Maybe our part of universe is just such a gigantic statistical fluctuation...

    However, I suspect that we have macroscopic time because of spacetime expansion. If the volume is constantly increasing, the gas filling the universe will never reach equilibrium.
     
  9. Jun 22, 2010 #8

    DevilsAvocado

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    I’ll guess you’re right. Sean M. Carroll also speculates around this, and draws the conclusion that "we" would interpret a "backward universe" as backward, but "they" wouldn’t notice any difference at all...

    But what if the laws of nature also where different? So that "systems" strived away from equilibrium? What would happen then??

    Could be, but I think we can be pretty sure that in a collapsing universe – time is not running backwards.
     
  10. Jun 23, 2010 #9
    I don't think that is accurate. Certainly, if a system in a thermodynamic state A evolves unitarily to a state B, then the entropy of B is greater than the entropy of A. But there is no reason at all why B should be at a later time than A, since the in-between evolution is time symmetric!

    I don't like the big bang explanation for this. Even if there was a good reason for the entropy of the universe to be very low at the BB, why should that affect us? I don't like the idea that something on a cosmological scale can affect our every day scale - ice cubes and eggs and things. Surely the entropy of the universe is completely dominated by black holes, in which case who cares about broken eggs, we should see entropy increase in both directions in time almost equally!
     
  11. Jun 24, 2010 #10
    Not if time is only in our minds and if "increasing entropy" and "time" are the same thing (the second law is as much about "increasing entropy" as it is about "time"). Saying that entropy should increase in both directions of time is like saying that time should flow into the future both when time flows into the future and when it flows into the past.
     
  12. Jun 24, 2010 #11

    JesseM

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    Given an isolated low-entropy system around today, you don't need any cosmological considerations to derive the fact that its entropy is likely to increase in the future as long as it stays isolated, you can derive it with statistical mechanics alone. The problem is that because of time-symmetry, you can use the same laws of physics to "retrodict" what the state of a system was in the past if you know the state now, and exactly the same statistical derivation implies the entropy should have been higher in the past--which for most low-entropy systems is not the case! In fact if we trace the history of a typical low-entropy system backwards, we usually find that the low entropy is a consequence of its interaction with some even lower-entropy system in the past, and this chain of decreasing entropy does go all the way back to the Big Bang. Here's a discussion from Roger Penrose, starting on p. 317 of his book The Emperor's New Mind:
    He goes on to explain why the visible light photons coming in have lower entropy than the infra-red ones radiated back out (basically just because the infra-red ones have lower energy so there must be more of them, which means the energy is spread out over more 'degrees of freedom' when it goes out than when it came in, which implies higher entropy). He also explains that the low entropy of the sun must be due to the contraction of an even lower-entropy nebula, and that ultimately the existence of diffuse collections of gas such as nebulas can be traced back to the initial smoothness of the distribution of matter and energy shortly after the Big Bang.
     
  13. Jun 24, 2010 #12
    I am kind of familiar with Penrose's argument from The Road to Reality, but I don't really buy it. The 2nd law is so utterly unquestionable from a statistical point of view that I feel like the time asymmetry we see needs a much stronger argument than one that just looks back in time like that. So what if we seem to have fairly low entropy now from the sun or whatever, there is still nothing stopping entropy from getting even lower in the future!

    The idea that time itself is the same thing as increasing entropy is nicer but it doesn't really have a good theoretical underpinning, since time seems to be completely symmetric at a fundamental level.

    There was an interesting paper recently by Lorenzo Maccone that tried to explain this. He said that any process that decreases entropy is necessarily decoupled from any observer, in such a way that the observer has no record of the higher-entropy-in-the-past state. I think it relied on quantum effects so I'm not sure it works completely on the classical scale. I prefer the idea that time asymmetry comes from some fundamental rule like that though.
     
  14. Jun 24, 2010 #13

    DevilsAvocado

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    Thanks JesseM, very interesting and Roger Penrose are brilliant, as always.

    There seems to be 'several' http://en.wikipedia.org/wiki/Arrow_of_time#Arrows":
    • The thermodynamic arrow of time
    • The cosmological arrow of time
    • The radiative arrow of time
    • The causal arrow of time
    • The particle physics (weak) arrow of time
    • The quantum arrow of time
    • The psychological/perceptual arrow of time
    Which are all, more or less, linked to the http://en.wikipedia.org/wiki/Second_Law_of_Thermodynamics" [Broken].

    One 'peculiar' fact is that if the particle physics (weak) arrow of time would have pointed to the opposite time direction, our universe would be made of anti-matter rather than matter!

    I think that the one thing that causes most 'confusion' is why we can remember yesterday, but not tomorrow. My guess is that it’s due to a combination of all arrows of time and maybe mostly causal, thermodynamic and psychological/perceptual arrow of time.

    To create memories of the past we need to 'consume' low entropy, thus increasing the total entropy of the universe. Therefore we can’t remember a (universal) higher entropy state than the current state, when we started the memory process ("now").

    To make an allegory:
    If time is Route 66 highway, and 3D space is our car (with very big tank), and gas is entropy, and low-entropy is full tank, and high-entropy is empty tank.

    Now, if Big Bang is our car with full tank in Chicago, and we are heading towards Los Angeles (Heat death :wink:), it’s no 'miracle' that we have to use half tank to get to Amarillo, and we can’t obviously visit Los Angeles before Amarillo.

    Yet, we all agree that neither Chicago nor Amarillo generally comes before Los Angeles in time, or vice versa. They are just different places on the map, and all depends on where you started.​


    Sean Carroll (Caltech) and David Albert discuss this interesting matter on Bloggingheads.tv:

    Time’s Arrow – Why is the past so different, in so many ways, from the future? (12:28)

    1z2m0dw.jpg



    P.S. TheAlkemist you were right! The quantum arrow of time does depend on wavefunction collapse/decoherence!
     
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  15. Jun 24, 2010 #14

    DevilsAvocado

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    I would say it makes all the difference you can think of. Slightly higher entropy at BB, and we would not exist discussing this matter...
     
  16. Jun 24, 2010 #15
    Pineal Gland contains nanocrystals, and melatonin which creates your day and night pattern of time. The sun warms the earth and the axis tilt provides us with different seasons for the different distances we are from the sun during our orbit, which we perceive as one year and 365 days. You have to see now that there is much more to time than what we actually perceive. Time is the product of the entire system as a whole, working in light to communicate to our eyes and produce a colour spectrum through the electromagnetics (electrons) of the particles themselves.
    Yes we create our own skewed concepts of perception.
     
  17. Jun 24, 2010 #16
    You're just describing some examples of the same fundamental 2nd Law, and there is no need to do so. This is not a skewed perception, and we don't rely on our physiology alone to interpret reality. I have no idea what you're getting at, and it seems a little cranky to me. Obviously we conform to an increase in entropy throughout our biology, but your examples seem a little odd.
     
  18. Jun 28, 2010 #17
    Yesss!!! *pumps fists*

    Just kidding.:biggrin:
     
  19. Nov 30, 2010 #18
    Time can flow only in 1 directon because you can't have an effect causing a cause and you can't go backwards in time because the very fact that you were back in time would be a change in th e past which would not be there in the 1st place but would if you did which would be impossible. We don't need all these nonsensical and grotesque examples of murdering one's parents or grandparents which no one would do anyways. Also, some one going back in time would not go into another branch any more than going forward in time.
     
  20. Dec 1, 2010 #19

    Demystifier

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    Yes, it is shown recently that it works also on the classical level:
    http://xxx.lanl.gov/abs/1011.4173
    (Read the introduction!)
     
  21. Dec 1, 2010 #20

    DevilsAvocado

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    This is VERY interesting!
    I hope I understand this correct; it’s the psychological/perceptual arrow of time that 'resolves' the thermodynamic arrow of time, right? We know what we know...

    Though, David Albert has an 'objection' to this, in a discussion with Sean Carroll:

    Time’s Arrow – Why is the past so different, in so many ways, from the future? (12:28)

    1z2m0dw.jpg


    Albert’s argument is that you can leave your room in a high entropy state (complete mess) and form a memory of this state at t1. You know what you know. At a later moment in time you return to the room, and now someone else has put it in perfect order = low entropy state. Your past memory of high entropy has reversed to low entropy at t > t1 ...


    P.S.
    A pure personal speculation (maybe totally crazy). As far as I understand, the particle physics (weak) arrow of time points to the opposite time direction, for anti-matter, right? We know that our ordinary matter is the rest "left over" after BB, when all anti-matter was annihilated by matter. Now, if anti-matter = reversed arrow of time, we could say that the reason we have the forward arrow of time is because we are made of matter, and not anti-matter... Right...?:blushing:?
     
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  22. Dec 1, 2010 #21

    Demystifier

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    Well, not really. Namely, to HAVE a psychological arrow of time (i.e., the phenomenon that we remember the past and not the future), we need a thermodynamic time arrow in the first place. The paper does NOT attempt to explain why there is a time arrow; this remains a mystery. The paper has a much more modest goal - to explain why the time arrow is UNIVERSAL.

    No! Entropy increases in the same direction for particles and antiparticles.
     
  23. Dec 1, 2010 #22
    I have always had trouble with this concept of thermodynamic arrow of time and it is by no means clear for me that the second law of thermodynamics should universally hold. All proofs so far in the context of the holographic principle contain loopholes and assumptions which may very well not be true. The concept of a psychological arrow of time is much more fundamental in my opinion and does not require an increase of global entropy whatever that may mean. So I disagree with you that there is a necessary logical connection between them.

    PS: as you are probably very much aware, this issue cannot be debated within conventional quantum theory at least not if you take the Von Neumann definition seriously. I know people have been looking for notions of dynamical entropy, but then you are not adressing the increase of total entropy of the entire universe (that one is ZERO and remains so forever). But such constructions typically split the universe into ''(isolated) system'' and ''rest of universe'', perform a partial trace over the ''rest of universe'' and use usual Shannon-Von Neumann on the remaining density matrix. But in those cases, the entropy does *not* need to be increasing in time.

    Now, it's your turn.
     
    Last edited: Dec 1, 2010
  24. Dec 1, 2010 #23

    DevilsAvocado

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    Ahh! Got it, thanks!

    ... but isn’t the No. 1 reason the time arrow is what it is (asymmetrical, forward, universal) due to the very special initial condition of low entropy at BB, bringing us back to square one ...??

    Ouch! I knew I was on exceptionally thin ice here... (I just have to say that to save my face! :biggrin:)

    The source for my misunderstanding was too fast and sloppy reading of an (maybe a little 'vague') text on Wikipedia:
    Shame on me. :blushing:
     
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  25. Dec 1, 2010 #24
    ??? Are you saying here that you have TWO notions of entropy ? Let's make this discussion into a serious one and start by defining entropy and then we will talk. All this babbling about what entropy is supposed to do gets me nervous :rolleyes:
     
  26. Dec 1, 2010 #25

    DevilsAvocado

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