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Susskind, superposition and interpretation

  1. Oct 13, 2011 #1
    I've watched the Susskind lectures on QM and Quantum Entanglements. Susskind seems to avoid many of the interpretation issues inherent in QM. For example, he never talks about the wave function collapsing due to the involvement of "conscious mind". He does show, mathematically, that wave function collapse in the double-slit experiment is a result of particle entanglement with the measuring devices. I'd like to hear from others who can elaborate about Susskind's position in this regard. I don't want to take his explanations solely on his authority.

    My other question is along the lines of superposition. I have read in many sources that a quantum system "exists in all possible states simultaneously". Susskind doesn't seem to buy this. He seems to discount this by saying that one just can't measure the states of a quanum system in a manner that allows you to say otherwise. So, the question is, is the statement that a quantum system "exists in all possible states simultaneously" a fundamental underpinning of QM or is it just an interpretation of the results of QM predictions?
     
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  3. Oct 13, 2011 #2
    All "collapse" is due to this effect. Wave function collapse and consciousness are unrelated.

    Statements about what happens between measurements are interpretation. Ultimately, anything that isn't about predicting the number that will appear on your measuring instrument's readout is interpretation.
     
  4. Oct 13, 2011 #3

    Ken G

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    There is an awful lot of confusion about what "collapse" is. I would say that entanglement with a measuring device is not the same at all as collapse, it is what should be called "decoherence." Decoherence means that we the physicist have chosen not to attempt the impossible task of tracking how coherences in the original quantum system map into coherences within the macro system. Instead, the physicist "projects" the macro+quantum system onto a much smaller subspace that consists only of "pointer settings" on the macro system, and "eigenstates" of the quantum system. That projection results in what is known as a "mixed state"-- no coherences between the eigenstates are tracked any longer, so the quantum system is no longer in a superposition state, it is described as being in a mixed state.

    All of that is perfectly standard quantum mechanics, not a shred of any interpretation even appears. Also, no "collapse" has occured yet-- that only happens when a single definite outcome is registered. And that is also where the interpretations come in-- the CI enthusiast will say that the mixed state was always in a collapsed state after the decoherence, there was just lack of information about which collapsed state. The MWI enthusiast will say no collapse ever occurs, and the conscious observer is entering into a kind of self-made illusion. The deBB enthusiast will say the system was in a collapsed state right from the start, but had to be treated as a mixed state because of missing information about the initial conditions-- similar in effect to CI except that nothing "random" ever happened, it was all determined from the start in a way that we have no access to.

    So three ideas about what collapse is, three interpretations, all kicking in after the mixed-state decohered outcome that Susskind is apparently talking about. He didn't need to talk about an interpretation because he didn't get to one yet, or else he is using the "ensemble" interpretation which asserts that quantum mechanics is only a complete description of large ensembles, and is just plain incomplete for talking about individual outcomes. Frankly, the more one thinks about these different interpretations, the more suspicious one becomes that none of them are saying anything all that different, scientifically speaking. I agree with The_Duck that the statement you cite certainly involves interpretation, and does not necessarily follow from any quantum mechanical necessity. However, I would say there can be a role for consciousness in collapse, essentially because everything we can say about quantum mechanics stems from the way we do science, and that is all about being conscious-- or at least processing information the way our conscious intelligence does it, it's not exactly clear what details of "consciousness" are really required as we don't even know what consciousness is beyond our experience of it.
     
    Last edited: Oct 13, 2011
  5. Oct 14, 2011 #4
    I don't see any role in consciousness in QM at all, the universe unfolds exactly the same when people are awake, asleep, drunk, dead, it doesn't matter.
    This link to consciousness seems to be over-interpreting the Copenhagen model of QM which is basically formulated to help scientists make correct predictions.
    Wave function collapse is not a process in the real world, it is just a change in what is measured, it just has a bad name that makes it sound like an actual event. I got that from the lectures of Richard Feynman.
     
  6. Oct 14, 2011 #5

    Ken G

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    Do you hold that one of the ways "the universe unfolds" is that closed systems evolve according to the Schroedinger equation?
     
  7. Oct 14, 2011 #6
    Yes, but to take the example of Schroedinger's cat, from Wikipedia you can see that neither Niels Bohr or any many-worlds interpretations consider any role of consciousness at all, least of all Schroedinger:

    "One of the main scientists associated with the Copenhagen interpretation, Niels Bohr, never had in mind the observer-induced collapse of the wave function, so that Schrödinger's Cat did not pose any riddle to him. The cat would be either dead or alive long before the box is opened by a conscious observer.[6] Analysis of an actual experiment found that measurement alone (for example by a Geiger counter) is sufficient to collapse a quantum wave function before there is any conscious observation of the measurement.[7] The view that the "observation" is taken when a particle from the nucleus hits the detector can be developed into objective collapse theories. In contrast, the many worlds approach denies that collapse ever occurs."
     
  8. Oct 14, 2011 #7

    Ken G

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    Neither Bohr nor Schroedinger believed that closed systems had to evolve according to the Schroedinger equation. You just said that they did. That means you hold that closed systems have a wave function, a requirement of being able to evolve via the Schroedinger equation. So your own view of the situation falls victim to the "Wigner's Friend" paradox-- all we need to do is put a conscious being into your closed system that evolves according to the Schroedinger equation, and you have a contradiction. Had there been no conscious observer in that closed system, you never would reach a contradiction in the claims you've made. So we see that consciousness does indeed play a role-- it is exactly the place where your views break down. Many people do not recognize that their stance is internally inconsistent, and that is often where consciousness becomes important for finding that.
     
  9. Oct 14, 2011 #8
    Susskind did discuss this a bit. His position was that the concept that the cat can be both alive and dead at the same time is hogwash. As I recall, he explained that the cat, along with Geiger counter, radioactive source, etc. all become entangled and cause decoherence. When the observer opens the box and looks at the cat, s/he becomes entangled as well.
     
  10. Oct 14, 2011 #9

    Ken G

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    Saying the observer becomes entangled sounds a lot like MWI to me, that's just exactly what someone would say who thinks not only that the cat is both alive and dead, but that the observer is entangled with both possibilities as well. Decoherence isn't doing anything to help with that, it only means there is no crosstalk between the dead cat observation and the alive cat observation.
     
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