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Measurement in QM

  1. Sep 2, 2004 #1


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    Interested in opinions:
    What is a measurement in QM? I.e. what is needed to make a wavefunction "collapse" and for an instant moment not obey the Schrödinger equation?

    Could this "not evolving according to S.E." be a clue that the macroscopic laws of nature can't (in principle) be derived directly from QM, but there has to come in something else between? (I know there are some respected scientist (can't remember names) who are saying that there may be something "between", but I don't know what they are basing that on.)

    Before going deeper, I would like to hear what you count as a "measurement"?
  2. jcsd
  3. Sep 3, 2004 #2
    OK, I can tell this is going to be one of those long discussions, because there are probably as many opinions on this as there are physicists.

    For me, a measurement occurs whenever a macroscopic device interacts with a quantum system in such a way that some property of the system becomes correlated with some property of the measuring device. According to the Schroedinger equation, this just creates an entangled state between the system and the measuring device (although you possibly have to include the environment in this as well). The question is how do we move from this entangled state to one of the definite states that we actually observe in the lab.

    You can certainly postulate that there might be some new physics going on in this transition. This is the point of view taken in "spontaneous collapse models" and the like. I typically prefer interpretations that describe all interactions in the same way, i.e. ones in which there is nothing special about measurement. A hidden variable theory, such as Bohmian mechanics, is one possibility, but I think I might prefer some yet to be found relational model.
  4. Sep 3, 2004 #3
    It is always a joy to see your perspective, becuase yours represents the attempt by LQG to arrive at some comprehension of this modelling of the early universe using photonic considerations. In quantum computerization how shall you map this?

    So using this technology would have been vital to the complex information that might have arrive from the geometricalization of quantum gravity perspectives using that same method.

    As I pointed out in previous discussion with you, Penrose was instrumental here in asking the question about photon polarization, with intersection capabilties of that early universe information detailed in hypernova events.

    Why Glast is important frommthe LQG perspective and why your field is important in tryig to find th emthod applicable to understanding that polarization. It's not so spooky anymore is it :smile:

    Does this make sense?
  5. Sep 7, 2004 #4
    Um.... no.

    I would like to point out that I have nothing to do with Loop Quantum Gravity or any variety of quantum gravity for that matter. My research area is quantum information, and the implications that it has for the foundations of quantum mechanics.
  6. Sep 7, 2004 #5
    I am not sure this relevant, but Penrose and others claim that consciousness is part of the measurement process. Of course, this is really a matter of philosophy here : you can always state wathever you want on processes where consciousness is not involved. Yet I find this opinion interesting. Besides, Penrose is not any random scientist !
  7. Sep 8, 2004 #6
    That's nonsense !!!

    I followed quite a few courses on QIT (books from Preskill,and others).

    Quantum-information has NO implications for QM, since it is a "child" of QM. A child does not determin the properties of the parent.

  8. Sep 8, 2004 #7


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    Agree. Someone who knows more in detail what Penrose claims?
  9. Sep 8, 2004 #8
    Does Penrose himself knows in more details ? :wink:
  10. Sep 8, 2004 #9


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    He (Penrose) has been pushing for some of this recently (maybe for some 7 years). He has described his ideas in a couple of books ("Emperor's new mind" and "Shodows of themind" IIRC). Basically, he says that the collapse of the wave function can be a gravitationally-induced phenomenon, and that it takes place in the microtubules that give structure (as well as some sort of material transport system) to neurons. Also, he has tried to use Goedel's theorem to show that the activity of the brain has to be explained in a non-algorithmic way. I don't remember too clearly what his claim was in terms of the implications of QM to brain activity.

    I don't think there is much to those ideas,... but of course, I'm not Penrose.
  11. Sep 8, 2004 #10
    No, but a child often rebels against a parent and thinks that it has much better answers to all the questions that its parents spend all their time worrying about.

    Seriously though, quantum information does lead to new lines of thought and questions we can ask about quantum mechanics. For example, what exactly is the feature of quantum mechanics that gives it enhanced information processing power? There is a lot of disagreement about this, and we need ideas from foundations to sort it out.

    Quantum foundations people have often been concerned with pointing out that some features of quantum mechanics, e.g. entanglement, are weird, but quentum information has inspired us to ask "how weird"? That is, we are interested in quantifying the degree of entanglement, contextuality, etc. present in quantum states. Hopefully, this will lead to new insights into quantum foundations itself.

    Finally, some more controversial researchers have suggested that quantum mechanics IS ONLY about information, as a way of overcoming the problems of quantum theory. I don't really subscribe to this point of view myself, but it is definitely inspired by quantum information.

    You will find plenty of this stuff on arXiv, although it is not present in Preskill or any of the other major books on the subject.
  12. Sep 8, 2004 #11
    Yes, he was such a great scientist, what happened to him :uhh:
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