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How do we define observe in QM?

  1. Dec 18, 2004 #1
    How do we define "observe" in QM?

    In Qm, what does "observe" means? Observing the result, like the Schrodinger Cat Experiment? Or knowing the exact position of the atom?
  2. jcsd
  3. Dec 18, 2004 #2
    not to nitpick, but you ask:

    what does observe mean?

    does it mean observing or knowing?
  4. Dec 18, 2004 #3


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    Observation is equivalent to Measurement.
  5. Dec 18, 2004 #4
    I agree with Shrumeo on making these distinctions first. Integral gave us a definition of observation = measurement. But to start our analysis, we have to define whether we are simply refering to a single observation of some system, which is indeed a measurement, and in which case our knowledge and observation are the same thing. Or do we mean we are an observer which can review a number of phenomena of the system and, therefore our 'knowledge' about a system can be expanded simply by our theory of it as Einstein suggested. That is, we can get closer to being an 'independent observer' which is not disturbing the system to gather information about it.
    But of course never becoming a 'perfect observer' which knows everything about the system and its physics.

    Personally, I think the 'Copenhagen' interpretation argues correctly that a single experiment can not be separated from the measurement and therefore our knowledge of the system is confounded with our observation, but then tries too hard to extrapolates that all knowledge must then be a sum from all simple measurements and so we have a linear sum of squares of all the errors within our knowledge (not just error within all our measurements). I think we have additional knowledge beyond the knowledge of the measurements since we can now theorize what can and can not be. Further, not all measurements are created equal in the error problem involved in Heisenberg's uncertainty principle.

    Therefore, we may get restricted on a quantum measurement as expected, but we have classically measured confirmation from other concepts, such as from relativity which do not have the Heisenberg error restriction on expansion of our knowledge. I would say the Thomas factor is a simple example of this, we realized through relativity that it is needed to account for the magnetic moment and Dirac calculated it in his relativistic theory which requires no measurement, only the assumption that the theory of relativity applies. He set forth a theory and cared little if direct measurement on the proton-electron pair was possible to confirm it, but that mostly indirect information about the theory of the emissions of the pair was required to arrive at his new calculation.
  6. Dec 18, 2004 #5


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    You've certainly hit on a sticky bit in QM. The observer cannot be separated from the observed.

    Real-world analogy: trying to get a coin out from the behind a cushion on your couch. Every time you pull the cushion back to reach down, the coin slips further down.

    The classic thought experiment involves trying to observe a moving atom. The only way to see it requires a photon (or other probing mechanism) to be emitted from (or bounced off) the atom. Doing so changes the position/trajectory of the atom.

    (Don't think about this too hard without doing some reading about it. It's easy to think you have devised a method to "passively" observe something.)
  7. Dec 18, 2004 #6
    I think the Copenhagen doctrine is correct if we just say that the interaction "could" be known. QM does appear to be controlled by information or knowledge. For example, just knowing the location of a photon at one slit of the double slit experiment eliminates the interference fringe pattern, no matter how slight the photon is perturbed in the experiment. The location of the photon in this case is actually known, even if removed from the human observer by several layers of instruments and processing time. The interesting thing is that the waves know to collapse long before the human knows what they have done. It seems to happen instantly. Some try to argue that a cosmic consciousness is involved. But it is sufficient for scientific purposes to just extent the Copenhagen Interpretation of QM to 'could' observe.

    In experiments the human often perturbs the environment by making a measurement of where the photon is. But in natural processes, very strong perturbations revealing the locations of the photons happen all the time, like at the detector screen, which usually absorbs the photon. But photons are constantly being absorbed by various surfaces and the interaction locates the photon. Sometimes humans get to know this, but almost all the time there is no human intervention, yet the Extended Copenhagen Interpretition would work.
  8. Jan 5, 2005 #7
    Heisenberg would say that the Uncertainty Principle is not intended for understanding the location of a photon which has already hit the scintillation screen. But instead it is an uncertainty concerning what direction it is headed and its position later in time had the screen not interferred with it.
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