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Bizzareness and decoherence

  1. Feb 16, 2016 #1
    Hi guys,

    just a quick question. This is an idea that came up to my mind while thinking about decoherence and stability of macroscopic objects in the world. As we know all superpositions and interference effects are destroyed in a enormously tiny timescale, and as Mfb mentioned it is meaningless to talk about a quantity that is in superposition in that short time interval. Now this may or may not imply bizzareness, but is it possible that all particles of a cat or a table can be in a superposition of different composition in that tiny instant, for instance a cat that is in a state of being in a normal state with its particles constitued as we see it and a state of let's say its neurons being in its legs, or whatever, just that those particles that make up her body are differently ordered. I hope somebody can clear up this bizzareness and explain. Thank you
     
  2. jcsd
  3. Feb 16, 2016 #2
    If macroscopic objects are in such a state for 10-bazillionths of a second or whatever
    why worry? Once things get to the Planck scale, nobody knows what's happening anyway.
     
  4. Feb 16, 2016 #3
    It is bizzare, just that
     
  5. Feb 16, 2016 #4

    bhobba

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    Of course not. Macro objects do not come into existence instantaneously then decohere. They are there all the time constantly being decohered. As the object with localised position spreads it interacts with the environment and that stops the spreading keeping it in a definite position.

    Thanks
    Bill
     
  6. Feb 16, 2016 #5
    I don't understand it, simply I don't. Logic says this - if decoherence is such a fast process any superposition that occurs is destroyed in a range of a smallest fragment of time. What it means to constantly decohere? Do superpositions of properties occur or not?
     
  7. Feb 16, 2016 #6
    Dear durant35

    Decoherence only causes apparent collapse. In principle, the superposition (macroscopic object entangled with the environment) still exists.
     
  8. Feb 16, 2016 #7
    Yes, I know that. The thing I don't understand is the consequence of this for everyday objects. When we see or measure an object we always find in a definite state, so if we focus solely on that object and its properties does or does not the object have a definite state at each instant? Thats what I don't understand regarding decoherence.
     
  9. Feb 17, 2016 #8

    bhobba

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    Yes - but it isn't not a pure state. Its a mixed state. Mixed states can be interpreted as being in a certain state with some probability. This is written as ∑pi |bi><bi| which says its in state |bi><bi| with probability p1. Trouble is that's not how it was necessarily prepared ie it was not necessarily done by some process randomly selecting some state |b1><ibi| with probability pi. If it was done that way its called a proper mixed state. But that not what happens in decoherence - it mathematically exactly the same - you cant tell the difference - but it wasn't prepared as a proper mixed state. That's why its only called apparent collapse. Can we assume it is a proper mixed state and actual collapse occurred. Of course you can - and if you do all problems in QM gone. Its called the ignorance ensemble interpretation (ensemble comes from the frequentest probability interpretation of the pi - its a slight variation on the ensemble interpretation). The thing is it an interpretive assumption - how does it become a proper one - blank out. There are all sorts of ways it could, BM, MW, Nelson Stochastics, primary state diffusion. It may even be nature is simply like that - nothing deeper going on at all. That's where ignorance comes in.

    Thanks
    Bill
     
  10. Feb 17, 2016 #9
    To us we see it in a definite state; it's description by Quantum Mechanics has it being in a superposition. This is where we meet the measurement problem.
     
  11. Feb 17, 2016 #10

    bhobba

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    That's not true. After decoherence its not in a superposition - its in a mixed state. Note every pure state is a superposition of many other states and in many different ways. When referring to decoherence here in the macro world we are referring to superpositions of position because the usual radial symmetry of interactions decoheres into a mixed state of position ie the |bi><bi| of ∑pi |bi><bi| are position eigenstates. 100% for sure it is not in a superposition of position.

    Thanks
    Bill
     
  12. Feb 17, 2016 #11
    ut according to the MWI, it *is* a superposition (the mixed state is just an approximation). According to the MWI, unitary evolution is the only kind of evolution there is, and a unitary transformation can't turn a pure state into a mixed state.

    (Bear in mind that I'm not necessarily agreeing with the MWI; I'm just trying to be clear about what it says.)

    What about this old post from PeterDonis?
     
  13. Feb 17, 2016 #12

    bhobba

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    That's wrong.

    The off diagonal elements are not quite zero - but so low to be undetectable - but that does not mean its in a superposition. In fact since its entangled so it cant be in a pure state hence can't be in a superposition.

    Before drawing conclusions like the above you need to study the detail so you have the background to make correct ones.

    Thanks
    Bill
     
  14. Feb 17, 2016 #13
    It wasn't my conclusion of course, I'm not competent enough to type down something like that. It was from another forum member and I just wanted your opinion about it because everything you say makes sense pretty much. So if we assume the ignorance ensemble approach combined with decoherence, can we conclude that each macroscopic object has a definite property at each instant on the timescale (but a mixed state ofc), nature doesn't really care and collapse has actually happened (there are no macroscopic superpositions in conditions we live in) ?
     
  15. Feb 17, 2016 #14

    bhobba

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    Of course we can.

    But again I stress we are dealing with the property of position.

    As I have said don't get worried about the fact theoretically its not zero. It's the type of thing that happens in applied subjects all the time.

    Thanks
    Bill
     
  16. Feb 17, 2016 #15
    I understood the thing about. So other properties are basically derived and the same as in classical world because the object is really-well localized?
    The main thing I was stressing about (and you know it) are the Schrodinger cat-like properties (like dead or alive, or the shape of the object). So all objects are constantly decohering and the position basis or location is well defined at any instant, and from follows that the other properties of the object are definite (or classical) at any instant. Why does the decoherence time get mentioned so much then in textbooks so it leaves an impression that the objects properties in fact are in a superposition but then decoherence destroys the superposition?
     
  17. Feb 17, 2016 #16

    stevendaryl

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    Just to clarify something: Every quantum state can be interpreted as a superposition of other quantum states. So it doesn't really make sense to say that decoherence gets rid of superpositions. What it does is prevent interference effects among macroscopically distinguishable states. So you can never observe interference effects between a dead cat and a live cat. But there are still superpositions involved.
     
  18. Feb 17, 2016 #17
    If we understand decoherence as a collapse, no as an apparent collapse, (like Bill mentioned), the superpositions go away, right? How would you describe the superpositions that are not macroscopically distinguishable, can you give me an example?
     
  19. Feb 17, 2016 #18

    bhobba

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    Decoherence isn't just for the quantum classical transition - it has practical applications as well. Now dont ask me about those because I am not into it, but its important for that, in particular they want to prevent it in quantum computing.

    Thanks
    Bill
     
  20. Feb 17, 2016 #19

    stevendaryl

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    Another point of clarification: When you ask whether some subsystem--such as an electron, or a molecule, or a cat--is in a superposition of states, that's a little ambiguous. In MWI, there is a single wave function for the entire universe, and that wave function is in a superposition of possibilities. But subsystems don't have a wave function, in general. If two subsystems are entangled, there can be a wave function for the composite system, but not for each subsystem.
     
  21. Feb 17, 2016 #20

    stevendaryl

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    I don't know if "go away" is the right phrase. The way I understand decoherence as applied to Schrodinger's cat is this: Originally, you have a uranium atom that is in a superposition of decayed/undecayed states. The superposition quickly "infects" the rest of the universe. You don't have a cat that is in a superposition of live and dead, instead, you quickly have the entire world in a superposition of a world with a dead cat and a world with a live cat. The implications of the uranium decay spread out at the speed of light. In one "branch", there is a decayed uranium atom, and there is a dead cat, and there is me, crying over my dead cat, and there is Schrodinger patting me on the back and telling me that my cat died for the good of science, etc. In another cat, the atom is undecayed and the cat is alive, and I'm not sad, and Schrodinger isn't patting me on the back.

    Once a superposition has "infected" the whole world, the various macroscopically distinguishable branches no longer have any effect on each other. At this point, you may as well assume that the branch you're observing is the only one. Or you can go MWI and assume that the other branches exist, as well. It doesn't make any practical difference.

    Sure. A molecule can be in two different configurations of the same atoms, or it can be in a superposition of those configurations. Such a superposition is not of macroscopically distinguishable states.
     
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