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B Quantum mechanics and the macroscopic

  1. Mar 24, 2017 #1
    I know the key tenants of quantum mechanics, and am not interested in the measurement problem. What we do know is that a particle has a wave function that describes the likelihoods of it having certain EXACT values when we measure it.

    That's all good, but I am still confused how the classical world emerges from an underlying quantum framework. As I understand it, decoherence is different from "wave function collapse". For example, when I press my finger on my desk, are the desk atoms acting like they are being measured and taking exact values to repel my finger,? Or does my finger become part of the same quantum system and nothing has "collapsed"?
  2. jcsd
  3. Mar 25, 2017 #2


    Staff: Mentor

    The likelihood of what having certain exact values?
  4. Mar 29, 2017 #3


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    One way to see how the classical world emerges from the large-scale limit of the quantum world is to understand that quantum effects are only really observable when the quantum object is isolated from interacting with the environment.

    If you try to consider the system-plus-environment as a larger scale quantum system, and calculate the quantum state of the system as part of the larger whole, that state will almost never just be a pure superposition unless it is completely independent of the environment. Since everything we see/hear/touch is due to second/third hand information about particles that interacted with the objects in question (e.g., a table), it is understandable that we don't see quantum effects in everyday life.

    If someone were to describe the state of you-plus-desk, you can imagine that before you touched the desk, the joint state of you-plus-desk factors as the state of you, times the state of the desk. Because of your interaction, the physical systems "you" and "the desk" become entangled, and the quantum state describing "you" is a mixture of relative states depending on the information you obtained from making contact with the desk. Indeed, this loss of coherence due both to your system no longer being in a pure superposition and to this interaction is how measurement is described in "decoherence".

    There's still philosophical issues to resolve, but this perspective of examining your interaction with an object as though you and the object are part of a larger quantum system can be quite illuminating as far as measurement goes.
    Last edited: Mar 30, 2017
  5. Mar 30, 2017 #4

    A. Neumaier

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    No. In this case nothing is observable. Observation requires an interaction with the environment. Measurement results are visible in the environment only if it has interacted with the system.
  6. Mar 30, 2017 #5


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    To be technically correct (the best kind of correct), quantum phenomena are most easily observed when the quantum object is isolated from everything else, except the measurement device.
    Last edited: Mar 30, 2017
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