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When does quantum take over classical?

  1. Oct 13, 2011 #1
    When does quantum "take over" classical?

    There's seems to be a lot of information about how relativity "takes over" classical mechanics when certain speeds are reached, like at a certain % of speed of light, etc. I haven't found much information regarding when quantum mechanics takes over for atoms within a system. I have found the Quantum Realm from wikipedia but if I try to google that term produces a whole load of strange websites and I can't decipher what's garbage anymore.

    Any advice on where to start looking?

  2. jcsd
  3. Oct 13, 2011 #2
    Re: When does quantum "take over" classical?

    Generally when dealing with particles, though there have been quantum mechanical evidence (wave interference) even with a buckyball ([itex]C_{60}[/itex]) I cant find the paper, but this is the abstract I think: http://www.aip.org/pnu/1999/split/pnu453-2.htm [Broken]
    Last edited by a moderator: May 5, 2017
  4. Oct 13, 2011 #3

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    Re: When does quantum "take over" classical?

    Quantum mechanics never "takes over". It's always correct - in principle, one could use QM to calculate the trajectory of a baseball, but of course that's impractical.

    Relativity doesn't "take over" either - it's always valid. You need to consider it at progressively lower and lower speeds as you want progressively more and more accuracy in your calculation.
  5. Oct 13, 2011 #4
    Re: When does quantum "take over" classical?

    Thanks for the response. I'm confused by this article:


    The article makes it seem like there is a defined line between the classical and quantum. It's pretty obvious that I don't know anything about this topic but I can't find anything online relevant to my question, which is why I'm asking for help to search.
  6. Oct 13, 2011 #5
    Re: When does quantum "take over" classical?

    Looking at the article, what the author probably means by 'the border between quantum and classical' is the scale and temperatures where classical physics no longer gives even a semi-accurate description of the situation.

    The experiment in question involves forcing a macroscopic object to exhibit quantum irregularities, so the border is where it stops acting classically and starts acting 'quantumy'.
  7. Oct 14, 2011 #6


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    Re: When does quantum "take over" classical?

    A good example would be some sort of oscillator with a resonance frequency f0 of a few GHz. At high temperatures where h*f<<kb*T (h is planck's constant and kb Boltzmanns constant): the "population" of the different "levels" in the oscillator (which modes are active, they are not real levels since they overlap) can be described using a Boltzmann distribution. However, if you operate the oscillator at a temperature where h*f0>kb*T you have to use the quantum mechanical description (the Schroedinger equation to get the levels, and the Fermi's Golden Rule for the population).
    In the intermediate region where h*f0 is approximately equal to kb*T both descriptions work (and you can also use some funny "hybrids", the SE for the levels and the BD for the populations).

    Note that there are quite a few systems where this crossover can be seen experimentally (including mechanical oscillators), so it is not a hypothetical scenario.
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