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

I Macroscopic wavefunction evolution

  1. Mar 20, 2016 #1
    Hi guys,

    I saw mr. Nugatory's post on one of the older threads which got me a bit conceptually confused so I wanted to ask you a question regarding it.

    This is the original quote: "For most macroscopic systems most of the time, the wave function evolves in a way that makes quantum effects like interference extremely improbable very quickly"

    What is meant by this sentence and how does the wavefunction of macroscopic systems evolve, what is 'the way' that was mentioned in the post, shouldn't the interference effects be extremely improbable 'right away' and not 'quickly'?

    Thanks for the patience, regards.
  2. jcsd
  3. Mar 20, 2016 #2


    User Avatar

    Staff: Mentor

    The wave function of all quantum systems, whether macroscopic or not, evolves according to Schrodinger's equation.
    No. You calculate the probability from the wave function. The wave function is evolving according to Schrodinger's equation, and it if starts in a state such that quantum effects are likely it will quickly evolve into a state in which they are unlikely.

    This explanation is terribly oversimplified, but it's as good as it's going to get if you're not prepared to actually study and understand the subject.
  4. Mar 20, 2016 #3
    I see your point and the oversimplification, thanks for the response.

    But the only thing which makes me confused on this, and the reason why I needed to start the thread is the apparent fact that the objects around us don't start in a state where quantum effects are likely. I understand the fact that the wavefunction will quickly evolve into a state where those effects are unlikely, like let's say in a case of the vibrating diamonds where the excited state last for a picosecond. It seems that the objects are already decohered, so how do they evolve when they don't start in a state where quantum effects are likely, does enviromental interaction keep them stable from interference and quantum effects?

    Thanks for the patience.
  5. Mar 21, 2016 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    I'm not sure if this is what the original poster was asking about, or not, but there is a difficulty with treating macroscopic objects quantum-mechanically, because they aren't isolated systems. They tend to be entangled with the rest of the universe. So either you need to use the wave function for the whole universe, or you have to use some technique to give an effective, approximate description of the macroscopic system alone. But in any case, a macroscopic system that is less than the whole universe will not obey the Schrodinger equation.
  6. Mar 21, 2016 #5

    A. Neumaier

    User Avatar
    Science Advisor

    Strictly speaking, this also holds for microscopic systems, since these also couple to their environment. Except for the dynamics of the universe as a whole, the unitary dynamics is always an approximation, valid only to the extent the interactions to the surrounding can be neglected.
  7. Mar 22, 2016 #6
    The main problem for me is the contradictory input that has been given from various forum members which leaves me in a "Which one is correct?" situation. Also I am sure that my lack of basics contributes to the misunderstanding, but again I catched up with some basics in reading many helpful posts here.

    So for instance this sentence: "
    It purports to answer the question "Why do we not encounter macroscopic superpositions such as the neither dead nor alive cat of Schrodinger's thought experiment?" and the answer is "They decay so quickly that we don't have a chance to observe them".

    Reference https://www.physicsforums.com/threads/decoherence-and-the-dust-particle.854454/"

    Isnt it the case that after decoherence there are no longer macroscopic superpositions like the one mentioned (and I know superposition isn't the right term but you know what I mean), not just that we cannot observe them because of the speed of decoherence? And I'm not again refering to the cat, but to everything else like tables, chairs and so on.. What exactly is decaying in a already decohered macro object?

  8. Mar 22, 2016 #7

    A. Neumaier

    User Avatar
    Science Advisor

    Whenever you get contradictory information from competent members (and it is not resolved quickly) you may conclude that there is a big subjective element in the assessment of your question.

    In the present case, no experimenter in his right mind will describe a macroscopic object by a wave function. Wave functions are good only for very few degrees of freedom. So even when buckyballs (and these are still far from being macroscopic) are superimposed, it is only one degree of freedom that is modeled - the remainder of the buckyball is in a mixed state!

    On the other hand, theoretically one can consider everything - so you find people who treat your question literally and tell you what theory would predict if the object would be described by a wave function (although in practice it isn't), while others say that it is a meaningless question.
  9. Mar 23, 2016 #8


    Staff: Mentor

    Off diagonal elements in the density matrix - which I seem to recall has been already explained to you. You must study the detail to know exactly what that means.

    Unless you are willing to study the detail you must accept what is being said. You will get nowhere worrying about things requiring technical knowledge to understand when you are not willing to get that knowledge.

  10. Mar 23, 2016 #9
    Could you give me a pdf or a internet where the technical details are explained?

    Thanks for the patience
  11. Mar 23, 2016 #10


    Staff: Mentor

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted