Quantum Superposition & Philosophy

  1. According to Quantum Physics (or at least my understanding of it), when a particle is not observed it doesn't exist in the way one imagines is to be. Instead, it is a wave function with infinite possibilities (sum-over histories by Feynman), and only when observed it takes on a definite state. This what I got from books that I've read, but its still quite hard to understand as how a particle interferes with itself while traveling through a double-slit and why exactly is it the observer that chooses the fate of Scholidinger's cat in the box?
    One other thing I'm wondering has more to do with philosophy but can be tied in here also. Geroge Berkeley said that if no one looks at a tree, it doesn't exist, in other words "esse est percipi" or "to be is to be perceived". Scientists have found that even particles large enough to be observed by microscopes (bucky-balls) also interfere. So I'm wondering whether George Berkley is right, and a tree doesn't exist if not perceived, but rather a wave function with infinite different possibilities?
    If what I said makes no sense, I apologize, but please correct me so I have a better understanding.
     
  2. jcsd
  3. Fredrik

    Fredrik 10,198
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    Last edited: Nov 26, 2010
  4. not on nonlinear quantum mechanics (unlike standard quantum mechanics).
     
  5. StevieTNZ

    StevieTNZ 932
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  6. i read it time ago.
    i waiting the result on more big object like:

    http://www.fqxi.org/community/articles/display/103

    Tejinder Singh
    http://arxiv.org/abs/0711.3773

    argues that quantum theory is intrinsically nonlinear, and goes to the standard linear limit for microscopic objects. THe nonlinear theory goes to the classical limit for large objects, but departs from linear quantum mechanics for mesoscopic objects.

    Because of the nonlinearity, the lifetime of two superposed states is no longer infinite. It decreases as the number of atoms in the object under study increases, going from an astronomically large value for microsystems, to extremely small values for macrosystems. Thus somewhere in between, the superposition lifetime ought to be measureable in the laboratory.


    ----------------------
    in any case the final theory have to be nonlinear.
    read:
    https://www.physicsforums.com/showthread.php?t=374854

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    http://arstechnica.com/science/news/2009/10/quantum-gravity-theories-meet-a-gamma-ray-burst.ars

    .........A value this close to the Planck length means that quantum gravity models in which there's a linear relationship between photon energy and speed are "highly implausible" That leaves other quantum gravity options open, including those in which the the relationship is non-linear. Hopefully, theoreticians will be able to devise real-world tests for some of these...........
     
    Last edited: Nov 27, 2010
  7. StevieTNZ

    StevieTNZ 932
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    What is the difference between linear and nonlinear quantum mechanics (perferably without all the mathematics).
     
  8. the linearity refers to the fact that there is no change (superposition stay forever, there is no collapse) is "LINEAR" (linear schrodinger equation).
    in nonlinear model things change, there is breakdown of superposition, there is a collapse (nonlinear schrodinger equation).


    ..........suppose you make a quantum measurement of an observable of a quantum system which is in a superposition of two states. The way nonlinearity destroys superposition is as follows : when a measurement begins, one of the two states starts to grow exponentially, while the other starts to decay exponentially......................
     
    Last edited: Nov 27, 2010
  9. Fredrik

    Fredrik 10,198
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    I wouldn't call that "destroying" the superposition. What you're describing is just a superposition that leads to predictions that are practically indistinguishable from the predictions derived from an eigenstate. This wouldn't get rid of the philosophical problems associated with the (IMO misguided) assumption that the wavefunction describes a single reality.
     
  10. collapse=no superposition
     
  11. Fredrik

    Fredrik 10,198
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    Yes, but "exponentially decaying magnitude of the coefficients of all but one term in a superposition" ≠ collapse.
     
  12. and

    double answer:

    ............The growth/decay process can be said to be `complete' over some calculable time scale tau. Now suppose we were to suddenly take away the measuring apparatus `during' the measurement, [i.e. after a time less than tau since the start of measurement] the state we will be left with is a certain superposition of the two states of the kind that is not seen in ordinary quantum mechanics. It is a sum of an exponentially grown state and an exponentially decayed state - this prediction is different from quantum mechanics. If we feed such a state into a second measuring apparatus, `quickly' after the first partial measurement, the outcome will be different from that predicted by quantum mechanics....................
     
  13. In answer to the OP:

    There is a large difference between a Tree and the bucky-balls of that experiment, and that is that the tree is interacting with it's environment. It may not have any human observing it, but there are still many other things that "observe" it, in the form of being in contact with it, such as the air, and the ground it stands on etc. The significance of these things is that information about the trees existence constantly leaks into the rest of the world, making it impossible to remain in a superposition state.

    You will find that for all the experiments that are being done with large quantum objects, the single most difficult things to accomplish experimentally, is to be able to create enough isolation of the object from the rest of the world, and even "from itself" because even the objects own different internal degrees of freedom can destroy superposition states. The latter usually means that you have to cool the object down to it's motional ground state, which rapidly becomes harder and harder to do with increasing object size.
     
  14. So what you're saying, Zargon, is that the object perceives itself or is perceived by its environment, so that there is no real superposition or it's only momentary which would collapse the MWI.
     
  15. there are no collapses on MWI.
    the objectification is due to a process called "Environment Induced Superselection"
    and the other branches (superpositions) goes according the MWI.
     
  16. By "would collapse the MWI" I meant that the theory would not hold. I take it from your answer it still would.
     
  17. No, I did not mix MWI or any other interpretation into my statement. What I said is true regardless of beliefs, beucase all interpretations agree (as far as we know today) on measurement outcomes, and in this case the probability amplitude for the tree being there is always 1. I was just saying that the reason that such a large object as a tree won't be in a superposition state is because of all it's near-infinite number of degrees of freedom (for example vibrational and rotational modes between the molecules), and because all of the connections that it has to other objects. The wavefunction will never be isolated enough that you could get to a point where no information leaks out, destroying the superposition.


    Also, the tree being there or not suffers from additional problems, such as violating energy conservation. If you look at the quantum experiments that deals with superpositions, you will see that they conserve all properties. An typical example is a single ion being in a superposition of either "being excited" or "being in ground state + there being an extra photon equal to the energy difference between the states". In the tree case you would first have to define a state that has the matching energy/momentum etc. as the tree without there actually being a tree, before one could even begin to talk about superpositions of the states.
     
  18. "Any modification of the apparatus that can determine which slit a photon passes through destroys the interference pattern,[5] illustrating the complementarity principle: that light (and electrons, etc.) can behave as either particles or waves, but not both at the same time.[8][9][10] However, an experiment performed in 1987[11] produced results that demonstrated 'which-path' information could be obtained without destroying the possibility of interference. This showed the effect of measurements that disturbed the particles in transit to a lesser degree and thereby influenced the interference pattern only to a comparable extent"
    This is from wikipedia page:
    http://en.wikipedia.org/wiki/Double-slit_experiment

    So what does this mean, that the particle can in fact be observed, but there is a certain limit on how much? wouldnt that prove Objective collapse theory true?
     
  19. Elvin12,

    Objective collapse is indetermistic, but what about SD? How would that explain superposition?
     
  20. an answer to the OP:

    I think it depends on the interpretation of quantum physics you prefer. If you would agree with the Copenhagen interpretation you would say it doesn't matter because you could never know anyway. I, with my limited knowledge of the subject, think that the tree is in a constant superposition throughout space, and that we just perceive it to be at a particular place at a certain point in time for whatever reason.

    PS. could anyone recommend a book on quantum physics without all the math, something more suitable for someone in highschool or pregrad college? i'd appreciate it.
     
  21. GodPlaysDice,

    I can recommend Quantum World by Ken Ford and Introducing Quantum Theory by McEvoy and Zarate, which I have, but also Understanding Quantum Mechanics by Roland Omnès, Quantum World and Quantum Theory by J.C. Polkinghorne, and Quantum: a Guide for the Perplexed by Jim Al-Khalili, which look to be very good, too, and which I'll probably get also (I have already ordered Quantum Theory by J.C. Polkinghorne).

    These are not mathematical and are fairly short but very substantial so are not heavy going. You can get them through Bookfinder.
     
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