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Why quantum effects disappear at the classical level

  1. May 9, 2013 #1
    I forget where I read it but one author said that mountains of ink have been spilled debating why quantum effects disappear at the quantum level. I don't understand why this is a problem, I think the answer is rather obvious. One poster on another thread wrote: "Technically - classical physics is what QM does on average ... so the math is different."
    That seems to be the answer. If the Planck length were roughly the size of a human being and a scientist was 10 orders of magnitude larger than a human, then if you continually measured a million humans, and your measuring device was rather crude, then each millions of human would be on average the same, even though a quanta (a human being) would not be the same but uncertain.
  2. jcsd
  3. May 9, 2013 #2
    It might have to do with why we don't see schrodinger's cat ever and we don't encounter wave collapse or entanglement in classical life, but I'm uncertain.
  4. May 9, 2013 #3
    to the OP: Why don't you go in public being naked? Well because there are certain things that we do and that we have when we are alone and different things when we are in a society.
    Although some things overlap and some are just used to either one or the other state that a person is in.
    I look at quantum physics with this kind of a viewpoint.
    Even though not a perfect analogy but one brick is a little different than a wall of brick not to mention a house or a highrise.

    Although I must say that it would be a very interesting world if people could "tunnel" through doors before they ever get the chance to open or if humans or cars could "scatter" and so avoid crashing into objects directly.
  5. May 9, 2013 #4
    Shrödinger equation:

    -h²/2M psi + Vpsi = Epsi

    the higher the mass, the smaller the effect (the wave function becomes more 'difficult' to 'see')

    if it comes to heisenberg:
    xp > h, p depends on the mass and h is really small. The effects can't be seen for macroscopic items.

    And then you got Ernesttheorema (I do believe it's Ernest or Erhnfest, something like that) which is as you say: "classic physics is the 'average' of QM"
  6. May 9, 2013 #5

    I like the first part of your post...I'm not so sure about the second part:
    I doubt anybody knows what would result.....

    If you mean that were the size scale where quantum effects become significant, you are getting close....but a remaining problem would be that Planck time would still be 10-43 seconds or so...Planck energy would make us wildly unstable,etc,etc 'at human size',,,,
    In other words, could this universe even exist??....

    Check out the description in the first several paragraphs here:
  7. May 9, 2013 #6
    All essentially quantum mechanical effects are proportional to hbar, or powers of hbar. hbar is really tiny in normal units (10^-34 Joule-seconds), so quantum mechanical effects are not noticeable at normal scales.
  8. May 9, 2013 #7


    Staff: Mentor

    Well actually its a very deep issue that hasn't fully been resolved yet. Quantum effects do not disappear at the macroscopic level eg liquid helium had been described as quantum mechanics writ large and so called bucky balls show quantum effects. The modern view is that macroscopic objects are just as quantum as other scales but due to decoherence behave classically - if you can remove that decoherence - and such is very hard but not impossible - then they will, and in fact do, display quantum effects such as superposition.

    The emergence of the classical domain has been given a lot of attention - see for example Roland Omnes - Understanding Quantum Mechanics - Chapter 11. From that chapter evidently the mathematics of that emergence depends on something called the Egorov Theorem. With that theorem the issue is resolvable - trouble is it only has been proved for some restrictive conditions such as an infinitely differentiable Hamiltonian and some bounds on those derivatives. Research has continued and some of those restrictions have been removed but is still not general enough to cover all the cases physicists are interested in. It is thought this will eventually be fixed up and most don't worry about it, but the fact is, some issues still remain.

    Last edited: May 9, 2013
  9. May 10, 2013 #8
    bhobba's answer is consistent with what I've been reading.

    So are the (commonly heard) explanations given by other posters just wrong?
  10. May 10, 2013 #9
    no...except for

    which I'm not so sure about...

    the posts represent different viewpoints....

    If you search these forums you'll find a lot of other viewpoints....for example, you can get an idea about 'macroscopic quantum mechanics' from solid state electronics ....semiconductors.....
  11. May 10, 2013 #10
    Thanks for that info, I didn't know that.

    Does decoherence require the many world interpretation?
  12. May 10, 2013 #11
    But the duck's answer

    looks so obvious and easy to understand. I guess liquid helium and bucking balls are anamolies, right?
  13. May 10, 2013 #12


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    No, but if you do not want to add collapses or other stuff, you get MWI.
    Metals, semiconductors, superconductors, Bose-Einstein condensates, vibrating cantilevers... quantum effects on macroscopic scales are common. The true distinction between "microscopic" and "macroscopic" in that respect is not the size, but the "unordered" (leading to decoherence) interaction with the environment.
  14. May 10, 2013 #13

    The environment is just as 'unordered' and unclassical as the system being measured.
    The is no such thing as classical environment that causes some systems to decohere upon interaction.
  15. May 10, 2013 #14
    For all practical purposes it can be assumed that interactions cause systems to lose their coherence. But this is misleading and not entirely true. The issue is complicated and unreslved/certainly not in a manner that most would find acceptable/.
  16. May 10, 2013 #15


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    Not if you prepare your system careful enough to see quantum effects, but don't do the same with the remaining world.
    I agree.
  17. May 10, 2013 #16
    For nice insights into some fundamental mathematical differences between QM and classical physics, give this Leonard Susskind quantum mechanics youtube lecture a try....

    tune in at 1 hr and 17 minutes....

    A sample:
    In classical three dimensional space, vectors are over REAL numbers;
    in QM, vectors are over COMPLEX NUMBERS> you can multiply vectors by complex numbers.
    Last edited by a moderator: Sep 25, 2014
  18. May 10, 2013 #17


    Staff: Mentor

    No. Its often the case in any field of endeavor that the explanation at a deeper level is different to what is said at a more superficial level. For example in studying electronics they say a current is electrons flowing through conductors but that is not true - its actually electrons and holes where holes are the absence of electrons but due to quantum effects they actually act like particles. Its just not necessary to get caught up in this for basic electronics - but the jig is up when you want to understand how transistors work - then its of vital importance. The same here - most textbooks don't give the full modern answer because it not really required in most applications. But when discussing fundamental issues the jig is up - you need to be more careful.

  19. May 10, 2013 #18


    Staff: Mentor

    Last edited: May 10, 2013
  20. May 10, 2013 #19


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    No - QM operates at all levels - even when planks constant can be taken as zero for all practical purposes which suppresses many - but not all effects. Quantum effects can still remain eg the existence of holes. Most of the time you will not notice them but under some circumstances you can eg if you put a conductor in a magnetic field and measure what is called the hall effect you simply cant explain it without holes.

  21. May 10, 2013 #20
    I've already gummed up the works today so one more post won't hurt.
    If the question is something like, "Why do Quantum effects morph into Classical stuff?", I'll try to get another quote from a physics book I use, maybe tomorrow.

    Consider an infinite square well with an ideal classical marble bouncing back and forth. Where are we MOST LIKELY to find the marble?
    At the sides: It approaches one side, is reflected, goes to the other side, approaches, is reflected.
    4 times vs 2 times in the middle.

    Where does QM say we'll find the marble? Most likely IN THE MIDDLE with the first "ground state wave".
    As we consider greater numbers of "Excited" waves, the amplitudes begin to make appearances at different parts of the well and the result begins to look like the classical form.

    Bohr's correspondence principle states that the behavior of systems described by the theory of quantum mechanics (or by the old quantum theory) reproduces classical physics in the limit of large quantum numbers.


  22. May 10, 2013 #21


    Staff: Mentor

    Yea - that's correct - but must be used with care - quantum effects can, and sometimes do, remain. For example liquid helium is an entirely macro object but requires QM to explain its weird counter intuitive behavior eg:
    -It carries no thermal energy (no entropy): all of the heat energy is in the normal component
    -It has no viscosity: it can flow through tiny holes.
    -It flows towards areas where the helium II is heated. Heat causes superfluid to convert to normal. A flow of superfluid into the heated area cools that area and restores the uniform mixture of normal and superfluid.

    None of this can be explained classically.

    Last edited: May 10, 2013
  23. May 10, 2013 #22
    See what happens when I bring up Bohr. I always get into trouble.
    I have a better version I'll try to get to tomorrow.

    Insomniacally yours,


    PS: You can bring Low Temperature Helium into any QM discussion anytime, I'll probably believe whatever is said. Simply mind boggling material.
  24. May 11, 2013 #23
    g.lemaitre: a quick change of perspective for illustration:

    is of course valid, but could be considered 'backwards'..... it seems the traditional view because most people learn classical first then quantum...but it IS valid to say:

    If each state has a probability via QM considerations... in classical physics the probability [uncertainty] 'disappears' and becomes an 'exact' measurement [state]....and we have an 'exact point'....so with QM we have less information that we otherwise might.

    edit: upon reconsideration, I don't like this description all that much.....but it is fun to turn stuff for a different perspective so I'll leave it posted...
    Last edited: May 11, 2013
  25. May 11, 2013 #24


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    I think this should be a harmonic potential, otherwise classical mechanics predicts the same probability everywhere. In addition, you need something like a harmonic potential to get the highest probability at the borders in QM.

    @Naty1: But if you start with QM, how can you ask how classical effects (what is that) disappear at the quantum level (which was your baseline anyway)? Shouldn't you ask how classical effects emerge?
  26. May 11, 2013 #25
    Try this one. From _Physics_, Wolfson and Pasachoff, ISBN 0-67339836-6:

    Attached Files:

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