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I Decoherence and time reversal asymmetry

  1. Oct 16, 2016 #1
    Layman question(s), but I hope not too stoopid -- many thanks to anyone with the patience to read and attempt even part of an answer, or share a possibly relevant link! I've got time today to follow and read links...
    1) Saw a recent 'popular' article discussing that darn Cat as if still a 'paradox' -- Whaaaa? What about "Decoherence => Cat solved: bam!"?
    2) Here at the Forums, I quickly saw that the Cat is (almost always, at least) either alive or dead -- as I thought I remembered. However, I still have a question or 2 that I have not found answered in threads on decoherence, or perhaps answered at a proficiency level above my head (https://www.physicsforums.com/threa...ork-does-it-require-the-second-law-of.474411/ -- "Any proof of the H-theorem contains an assumption that breaks T-symmetry by hand." I just about half-get that ...).
    3) Question: quantum-level processes are reversible, '2nd law' or thermodynamic processes (statistically) not so -- are asymmetrical in their evolutions ( btw, the Feynmann lectures on this -- still interesting e.g.

    SO what is the current speculation or evidence + reasoning about how (and why) reversible (time-symmetrical) quantum processes *decohere* to 'non-reversible' (time-asymmetrical) thermodynamic processes? And am I just ignorantly leaving something out, or is this still a *mystery*?
    4) Sub-Question 1: Lots of 'macro' systems have been superposed, like that beryllium atom -- but has anyone superposed distinct states of a non-reversible (e.g. thermodynamic) system? I was assuming that this *can't* be done ... kind of by definition .. and could we by any reasoning possibly detect (say) a cat both alive and dead, if only very briefly?
    5) Sub-Question 2: Is there (whether possibly or 'yep, someone has observed it') a 'level' where both quantum (stuff is reversible) and thermodynamic (not so reversible) constraints are in play -- so that, as someone suggested somewhere, the cat might be alive+dead but only in very (1/10^10^10...almost forever) rare instances; for some reason I visualize this as like a foam or something, with the different 'levels' interacting but not really blended...in time => this might be like an oscillation ... but how would something that has decohered then 'recohere' (revert to time-symmetrical or whatever -- I may have a fundamental misconception in here somewhere, or not)? And could that happen without energy-input/entropy-increase (perhaps in the globally considered thermodynamic system of scientist-experiment-grant proposals)? And what constraints of scale (RNA but not ribosomes, maybe) or other constraints might there be on such a *foam* or *oscillation* or whatever -- and why?

    MY CONSTRAINTS: not a physicist, so any jargon above basic will probably elude me or need explaining the first time (like, is t-symmetry = time symmetry?); and my once brilliant math aptitudes have atrophied over the past 40 years to only the most basic algebra.

    BONUS: Discussions of 2nd Law seem to tend toward a 'statistical' interpretation -- 'Well, it might reverse itself, but it's just really, really unlikely!' That works if we're thinking of asymmetry and not absolute irreversibility. But for the egg to unsplatter itself would seem to require some input of energy (i.e. not a straightforward 'reversal'), or a suspension of basic laws like gravity .. mmm? My impression is that the 'anything is possible, some things are just super unlikely' explanation, is ok for things like gases in boxes, but for rebonding glass molecules, or reconstructing an egg? It seems really lame, or requiring two diff. universes to interact, or something ... Mmmm? 'Within THIS universe, no splattered egg will ever spontaneously reassemble itself, without a further (entropy-balancing) input of energy into the egg-floor-room system, so that the 2nd Law asymmetry of a system is never violated in specific cases of generally thermodynamically time-asymmetrical processes' or something -- ??

    [Moderator's note: discussion of the bonus question has been moved to a new thread:


    Please respond there to that particular question. The title of this thread has also been edited for clarity.]

    Thanks, Charlie
    Last edited by a moderator: Oct 20, 2016
  2. jcsd
  3. Oct 17, 2016 #2


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    Just a suggestion: Could just pick one specific question to ask, rather than asking so many in one post? It's hard to know where to begin in answering your post.

    Let's just pick the issue of Schrodinger's cat and decoherence. In quantum mechanics, there are two different ways for a system to be in a combination of two different states: superpositions and mixtures. The big difference between the two is that superpositions can have interference effects involving the two different possibilities. The presence of these interference effects demonstrate that the system is not in either state, but is in some new state formed from a combination of the two. In contrast, mixtures have no interference effects. Therefore, in the case of a mixture, it's possible to interpret the situation as: the system is actually in one state or the other, and we just don't know which, but it's not possible to interpret a superposition that way.

    So where does decoherence come into play? Well, decoherence is an irreversible interaction between the system and the "environment" (basically, the rest of the world) that destroys interference effects. So it has the effect of turning a superposition into, effectively a mixed state. So after decoherence has taken place, it's possible to think of the cat as being "really" either alive or dead, and you just don't know which. But this is a little bit misleading. Decoherence does not actually choose between a living cat and a dead cat, it just destroys the interference effects that would lead you to say that the cat is in a superposition between the two states.
  4. Oct 18, 2016 #3
    1) Thanks! The distinction between superposition and mixture is helpful, and especially the reminder about interference effects -- which if I recall correctly were an early or even first clue that superposition was happening? (those slits and interference patterns...it's a wave -- it's a particle -- it's a wave and a particle! )
    2) I was thinking as I posted, 'too many questions' -- I started with one simple one but they multiplied .. I'll try to sort and post one at a time soon.
    3) My understanding was that the cat is actually not superposed, because it's not interacting with a quantum-coherent system -- as a thermal process the cat would 'crash' (so to speak) any quantum coherence (cause the collapse of the probability wave function) if there was an interaction -- is that a mistaken interpretation?
  5. Oct 18, 2016 #4


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    As I said, there is a technical distinction between a superposition and a mixture. Because of decoherence, a cat can't really be in a superposition of dead and alive, but it could be in a mixture. So decoherence doesn't explain why only one possibility is actualized.
  6. Oct 19, 2016 #5
    "Dechoherence killed the cat"

    Thanks again. Ok, my understanding is that a quantum-coherent system, upon interacting in a detectable way with a thermal-mechanical system (like the cat-killing device) decoheres, almost by definition -- for the cat to be killed, a particle must interact with the detector, which entails the localization of the particle and the collapse of the prob. function to a single state, right?

    The only way to superpose the two detector states is to have a ψ that is both intact and decohered (or however that is best phrased). Can part of the ψ function decohere in this experiment, and part of it not? Lol, can you have a 'mixture' of superposed and decohered wave function?

    So, as I am seeing it, the system of [decaying material + detector + cat] should only be in one of two states:

    1) No particle emission => detector not triggered => wave function propagating ( => cat alive)
    2) particle emission => detector triggered => decoherence ( => cat dead).

    So -- not arguing, just asking -- it seems to me that decoherence does in a sense explain why only one state of detector or of cat is actualized -- because for there to be a dead cat, there must have been an interaction and a wave function 'collapse', and an end of any superposition. Because triggering the detector = collapse of ψ, there is no state of the experiment where there is both a coherent wave function AND a triggered detector / dead cat. Only one cat-state or the other is actualized because the cat can only be killed by a decoherence event (particle detection). Alive = not decohered. Dead = decohered. Is that reasonable?
    Last edited: Oct 19, 2016
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