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Superposition principle explained

  1. Aug 7, 2012 #1
    I'm taking an introductoruy course on QM and qubits and my professor is frustrating me by using anthropromorphic terms to decribe superposition. He says, " the electron doesn't make up its mind to be in the ground or excited state until the measurement". Can someone give me a better explanation?
  2. jcsd
  3. Aug 7, 2012 #2
    The wave function dictates the statistical probabilities.The wave function smears all possible conditions of the system and only upon some kind of measurement ie. A collapse of the wave function, will some particular state of the system be measurable.
  4. Aug 7, 2012 #3
    I'd suggest you look up the basics, uncertainty principle would be a good start, before you involve yourself in probability and superposition and all the confusion that imparts. I don't think it's that much of a headache to think that a wave remains in a state that is indefinable until measurement, just because that is a given. How can one express an epistemology of system one has no knowledge of without assigning variables to an unknown that are possible and "random". To put it simply after all, if I could not look at the ripples in a pond, but I could touch them, I would see that once I had imparted energy to the "water" the ripple patterns had changed because I touched them, I would have no knowledge of the intitial system because I could not see it. Feynman lectures are always an amazing place to start. You can find them in many places, Google them. :)
  5. Aug 7, 2012 #4


    Staff: Mentor

    QM is a theory about the results of measurements. All you can do is predict the probabilities of what the outcome of a measurement will be until you actually do it. Then it instantaneously changes to another state that implies different measurement probabilities. This is the collapse of the wavefunction issue and is a deep mystery - although many people including me think decoherence resolves it - but it is controversial. Quantum objects never make up their mind about anything - what the theory describes is the probabilities of measurement outcomes. What it 'does' between measurements is interpretation dependent but certainly it never 'makes up its mind'. My view is between measurements it does nothing other than be in a 'state' that determines the probability of measurement outcomes if you were to actually do one - at rock bottom that is what QM is about - predicting the probabilities of observational outcomes.

    Of recent times I am starting to form the view the way we teach it is not quite correct. We should be teaching it from first principles in an axiomatic way rather than closely follow the historical development and experiments. Check out:

    Basically it's a new type of probability theory that allows continuous transformation between pure states.

    Last edited: Aug 7, 2012
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