Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The relation between classical from quantum vs measurement perspective

  1. Jun 11, 2015 #1

    ftr

    User Avatar

    What is the relation between classical from quantum vs measurement problem. On one hand they seem to be related on the other they seem to be of different nature.

    We always see our screens on front of us and not 100 meters away, that we say is classical object although the screen is a quantum object in the end and it exists even when we are not looking at it . But the measurement of quantum systems says it only takes reality when "measured". I am confused when people discuss wavefunction collapse in regard as to which problem is actually being addressed and the relation.

    Edit by mentor. You cannot bump a thread in less than 24 hours, so your posts have been merged. Please read the rules. It's amazing that after all of this time here you have never bothered to read the rules.

    Since I am not getting any response let me ask a simpler question. Are classical objects considered to be a collapsed wavefunction of the system or the subsystems or what?.
     
    Last edited by a moderator: Jun 11, 2015
  2. jcsd
  3. Jun 11, 2015 #2

    zonde

    User Avatar
    Gold Member

    Personally I think that wavefunction collapse does not connect quantum and classical worlds. I think it's rather Born rule that does that.
    So my answer is no, classical system is not collapsed wavefunction. Classical system can be described by probability while quantum system by probability amplitude.
     
  4. Jun 11, 2015 #3

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    Well first of all collapse isn't part of QM - its only part of some interpretations.

    The classical world emerges because classical objects are being 'observed' all the time by the environment which via decoherence gives them classical properties.

    Remove that interaction with the environment, while quite difficult to do, has in recent times been done, and some strange phenomena emerge:
    http://physicsworld.com/cws/article/news/2010/mar/18/quantum-effect-spotted-in-a-visible-object

    All objects are quantum - but not all objects are interacting with the environment (the vast vast majority are of course - its very very hard to engineer a situation where it isn't) - those that do have classical behaviour. For example a few stray photons form the CMBR are enough to give a dust particle a definite position.

    The detail of all of this can't really be explained at the lay level - at that level the following is about as good as can be done:
    https://www.amazon.com/Where-Does-The-Weirdness-Mechanics/dp/0465067867

    If you are willing to actually do the hard yards and delve into the detail (which of course I think is the best course) then the following is THE book:
    https://www.amazon.com/Decoherence-Classical-Transition-Frontiers-Collection/dp/3540357734

    Thanks
    Bill
     
    Last edited by a moderator: May 7, 2017
  5. Jun 14, 2015 #4

    ftr

    User Avatar

    Thanks Bill. Of course, I read the Wiki decoherance and hence my question. If you look at the part of measurement you find it brief and confusing, other articles give very hard to understand discussions and not real explanation.

    Moreover, as for classical from quantum, in NRQM you find the proton sitting in one place(ie. well defined position almost) and it is the electron which is elusive. So for normal classical objects I don't know what is the big deal the protons are not allover the place like electrons. And so what does decoherence actually says. Now, if it says the electron's position gets defined, that sound like a disaster for the atoms. I guess I am not getting it.

    My understanding is the decoherance tries to explain why we don't find atoms allover the place as to the description of their wavefunction, is that correct?
     
  6. Jun 14, 2015 #5

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    No.

    It explains apparent collapse.

    Thanks
    Bill
     
  7. Jun 15, 2015 #6

    ftr

    User Avatar

    Thanks Bill. But I was hoping somebody addresses my proton argument.
     
  8. Jun 15, 2015 #7

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    The electron is much lighter than a proton so the model is we describe the system using the proton as the origin of our coordinate system. An atom as a whole is modelled quite well as a little ball.

    You can probably find treatments that remove that assumption but the math would be a lot harder and I don't think particularly illuminating of anything.

    Thanks
    Bill
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: The relation between classical from quantum vs measurement perspective
Loading...