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A festive analogy for interpreting quantum mechanics

  1. Dec 25, 2014 #1
    While browsing this festive morning I came across Peter Woit's "(his Oct. 3rd 2011 'Not Even Wrong') statements that: "the fundamental problem of the interpretation of quantum mechanics (is): why don't we see superpositions " and that: "the confusing question is ....how classical behaviour emerges during a measurement process" . To me this looks like a good summing up of the unsettling nature of microscopic Quantum Mechanics, which not long ago (2012) was still puzzling folk as clever as Steven Weinberg (Arxiv 1109.6462v4).

    When puzzling it can help to consider analogous mysteries. It struck me that in this case we are always facing a rather similar macroscopic one, the mundane but mysterious distinction between past, present and future. We recall the fixed past, live in an ever-changing present and strive to predict an unknown future. The collapse of the wave function in Quantum Mechanics seems to me rather like, and perhaps just as mysterious , or as familiar as this macroscopic transition from past to future we continually experience.

    Perhaps we should find Quantum mechanics no more mysterious than everyday experience; superpositions are just descriptive guesses of how the intangible microcosmic future might turn out? Or is this characterisation just festive optimism?
  2. jcsd
  3. Dec 25, 2014 #2


    Staff: Mentor

    I think people get far too worried about the principle of superposition. It simply means the pure states form a vector space. It doesn't mean particles are literally in two places at once or anything like that. The reason we don't notice it is well known - decoherence.

  4. Dec 25, 2014 #3


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    Nice to find your post Christmas morning... It's a thought I've been pondering for a long time. Superposition is a very reasonable way of describing the future -- as a bundle of more and less likely possibilities for what might happen next, given the current factual situation. And the "collapse" is just what we call "the present moment" at the quantum level. It involves the selection of a subset of possibilities to become new facts, which creates a new situation, a new superposition of likely possibilities for the future.

    So I agree that this particular aspect of QM doesn't contradict our real-time experience of the world. The problem is, both in daily life and in physics, we're not used to thinking about the world from this "real-time" perspective. We usually think about it objectively – that is, from no point of view, as if we could stand outside of space and time and see things as they are in themselves, without having to be there and interact with them.

    Obviously this is a useful perspective, basic to how we do science, since it gives us all a common framework for describing given facts. It works fine for classical physics, which treats the universe as a vast body of precisely definite fact. It's not so good for thinking about how new facts come into being, in a quantum world where facts exist only to the extent they're actually defined and determined through interaction.

    I think the great obstacle to understanding QM -- or for that matter, to understanding the physical world we all experience, moment to moment -- is that we need to develop a framework for describing physics "from inside", from the standpoint of a participant in the interactional environment. Unfortunately there's a widespread assumption that if we're not describing things objectively, then we must just be talking about our own subjective experience, i.e. the world "in our heads".

    Wheeler was talking about a "participatory universe" many decades ago, and there are some more recent approaches, like Rovelli's Relational QM, or Ruth Kastner's Possibilist Transactional Interpretation, that point in this direction. But I'm not sure if anyone has tried to connect this temporal "transition from past to future we continually experience" with quantum measurement. (As you know, in discussing Relativity it's normal to dismiss the present moment as something that exists only in our minds.)

    Thanks -- Conrad
  5. Dec 25, 2014 #4
    Neither am I! A fly in the ointment with this approach, one that worries me severely, is the scaling of quantum phenomena by Planck's constant. I have no idea why h has the numerical value it does -- perhaps there's a a reason for this, rather than just happenstance. Endless mysteries.

    Have a great 2015 discussing and unravelling some, all.
  6. Dec 25, 2014 #5


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    The microscopic origin of the second law is a fundamental question of classical thermodynamics and statistical mechanics. This is usually explained by special initial conditions. One can get an effective arrow of time for of observables measured on a subsystem from unitary time evolution given a large Hilbert space, and interacting subsystems. However, one then has the measurement problem, which is that one needs an observer to get definite outcomes, and we have to explain the arrow of time for the observer. Bohmian Mechanics is one approach to solving the measurement problem, and makes quantum mechanics look like classical statistical mechanics. How then does the arrow of time arise? Analogously to classical kinetic theory, there is an H-theorem for de Broglie dynamics within the Bohmian picture.

    http://arxiv.org/abs/1007.3957 (Effective arrow of time from unitary evolution)
    http://en.wikipedia.org/wiki/H-theorem (Boltzmann's H-theorem for classical physics)
    http://www.sciencedirect.com/science/article/pii/037596019190116P (Valentini's H-theorem for dBB)

    That is just standard quantum mechanics. However, here there has to be an arrow of time from outside quantum mechanics, because a measurement outcome is the subjective assignment that a "macroscopically irreversible" definite event has occurred.
    Last edited: Dec 25, 2014
  7. Dec 26, 2014 #6
    I am always surprised when people ask why don't we see superposition. The very point of uncertainty principle is that we don't see it. If we did, there would be no uncertainty. What does it mean "to see" the superposition? Is it to be able to measure the state vector twice, undisturbed by the first measurement? The question really means "why are our senses not magically exempt from uncertainty." And the answer is: because so.

    The second problem (how does the classical world emerge from QM) is a bit more difficult. In my opinion QM breaks somewhere in the macroscopic realm. And the thing that breaks QM in my opinion is high temperature. QM is not the theory of small things. QM is the theory of cold things. There are macroscopic things that behave according to QM laws if only they are cold enough (liquid helium). At the same time there are tiny things whose behaviour is almost completely classical (i.e. particles in the Sun) just because they are hot. And the definition of decoherence is "interacting with hot environment".
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