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

Can decoherence validate the Copenhagen interpretation?

  1. Dec 12, 2015 #1

    I am an amateur enthusiast and this is a direct question (not a proposal, i'm not nearly qualified to offer a proposal). I would like to throw to experts and people who are more qualified, because it's got me a bit stumped. Could I ask for an answer which is as simple as I am please, lols - answers for dummies style would be just great!

    As I have read it from popular reading books - Copenhagen asks us to believe that the wavefunction breaks down for big things but not for small things until they are observed and offers no explanation as to why, and this is the main criticism of it right? Whereas many worlds says the wavefunction never breaks down and all outcomes are realized in separate realities.

    Yet is it a case of we would break down if nothing was observing us, and I mean in terms of decoherence, i.e. photons of light hitting us and collisions between atoms and molecules within our own bodies giving each others positions away?

    Are we, and other macroscopic objects destined to lie around as definite outcomes because of decoherence giving it all away in multi-atomic objects, whereas in the case of the electron there is nothing to observe it if it is in a dark chamber, and in a vacuum (and thus the wavefunction does not break down?).

    Did they know about decoherence in Bohr and Einsteins day when Einstein offered up 'will a sideways glance from a mouse suffice?

    Would decoherence have explained why Copenhagen can work with an explanation as to why big things break down and small things do not break down?

    Can I have answers from qualified people please, it's a genuine question not a proposal.


  2. jcsd
  3. Dec 12, 2015 #2


    Staff: Mentor

    Ok you cover a lot of territory there.

    First decoherence does not validate the Copenhagen interpretation - but it does morph the issue. The issue with Copenhagen is according to it QM is a theory about observations that occur here in a common sense classical world that it assumes to exist in the first place. Its not the only one to do that but what singles Copenhagen out is actually an issue with what probability is - it assumes a Baysian view with the state being subjective:

    The ensemble interpretation I ascribe to has a frequentest interpretation but other than that is pretty much the same:

    As John Baez says - most interpretations of QM is just an argument about what probability means - most - but not all.

    So what does decoherence do? Well it allows a precise quantum definition of when an observation occurs - it's just after decoherence. This resolves Schroedinger Cat immediately. The observation occurred at the particle detector - everything is common-sense classical from that point on. The cat is alive or dead - not in some wierd superposition.

    Secondly it changes the status of the so called measurement problem. It has three parts:
    1. Why do we generally not see interference terms in the world around us.
    2. Why do some observations single out say position as what's being observed rather than say momentum (this is called the preferred basis problem)
    3. The problem of outcomes ie why we get any outcomes at all.

    There is broad agreement decoherence solves 1. and 2. but not 3. Some issues remain such as the so called factorisation problem - but there is a general consensus they are solved. Decoherence gives the probability of certain outcomes but not why we get those outcomes. To answer that you need further assumptions such as what many worlds or Bohmian menanics does. Copenhagen, and most other interpretations stand powerless before 3.

    That's the situation with Copenhagen today.

    Yes decoherence was known in the early days of QM - but we know a lot more these days.

    Last edited: Dec 13, 2015
  4. Dec 13, 2015 #3


    User Avatar
    Gold Member

    Not quite.
    Standard QM gives statistical predictions for measurement outcomes. But statistical quantity does not have physical meaning on it's own as it is derived from many individual events.
    And so we might ask if these individual events are physically different with different initial conditions undergoing (similar) process that leads to unpredictable individual outcomes but highly predictable average
    all individual events start as physically the same but at some moment they become different i.e. there happens special process that randomizes events (wave function collapse).*
    Copenhagen takes later approach and so we have question at what moment we have to apply this special process. We could think that it happens when event has two macroscopically distinct possible configurations. But if two macroscopically distinct possible configurations later overlap we can see interference effects (as in double slit experiment), so it does not work.
    Another strange thing is that with this kind of model non-local wavefunction of individual event is equally real as localized particle. So there is wave-particle duality.

    * Acctually there is third approach that individual events are not statistically independent. Quite probably this is what Einstein meant by Ensemble interpretation.
    Big/small things does not really come into consideration.
    Idea of decoherence is that part of the problem is interference effects between macroscopically distinct configurations. If we take it away it might help to solve the problem. For big things de Broglie wavelength is very small so that we can't test them for interference effects. So it's not like it applies to small things only. It's more that we couldn't see any interference for big things even if there would be any.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook