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Can an observer be in a superposition?

  1. Sep 26, 2010 #1
    If I had a measuring device which causes decoherence of some quantum experiment:

    Is it possible to put (somehow) this observer/measuring device into a superposition of two different states?

    So instead of the wave function collapsing to ______/\______ it would collapse to __/\___/\__ (or something like that).
  2. jcsd
  3. Sep 26, 2010 #2


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    One has to keep track of what one means with the observer, in cases where we obviously have more than one observer. As soon as one speaks of a wavefunction, there is always one implicit observer (or observer frame) related to the wacefunction.

    If you ask, can observer A be in superposition relative to observer B - in principle? Then, yes, but then then observer A is simply treated as any physical system from the point of view of B, because in QM there is somehow only ONE implicit observer. If there are more than one observer, they need to communicate to compare results, and this communication is then nothing but a physical interaction, itself subject to the laws of physics, including QM.

    How stable this superposition in a given environment is a different question, but it principle any system can be in superposition. In practice superposition of massive objects aren't stable and decay fast, that's we don't see it.

    But an observer isn't in superposition relative to itself, that wouldn't make much sense.

  4. Sep 27, 2010 #3
    It is interpretation dependent. In MWI observer is always in superposition.
  5. Sep 27, 2010 #4


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    This is true. But to avoid confusion I think it's worth noting that there are several observers involved also here. In MWI, there is an implictit observer the "birds view", which is somehow a hypotethical birds view observer that sees all the frogs in superposition. *THIS* implicit bird-observer is however not a real observer and does not interact with the system.

    Some people like this, some don't :)

  6. Sep 27, 2010 #5


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    In QM there is no specific property "of being in superposition" without qualification. A mode may be superposition of two other modes but then any two of the three may be considered a superposition of the other two. It is more a relative property of the observables than a property of the object being observed.

    The wave nature of a particle manifests from the momentum (defining sinusoidal wave functions) being a superposition of all the positions at which the particle might be observed. Dually the position is a superposition of all possible momenta. So a localized particle is in a superposition of momentum modes and a particle moving at a known momentum is in a superposition of all possible positions.

    Observers are macroscopic objects (whether people or devices). The measurement process is necessarily thermodynamic. A measuring device must amplify a quantum signal to a classical scale in order to record it as classical information.

    Let me also point out that to observe a superposition one must perform many experiments to see the interference pattern or its equivalent. To see for example Schrodinger's cats in a superposition you would need a beam of cats cooled to near absolute zero...(an example my thesis advisor enjoyed using) and a set of measuring devices capable of making quantum scale measurements of those cats. As such "dead" vs "alive" becomes rather meaningless as you must freeze the cats.

    It is possible in principle to get the cats to manifest superposition but not in any way where they can act as observers themselves.
  7. Sep 27, 2010 #6


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    This is not possible in practice because, due to locality of interactions, decoherence always happens in a localized basis.
  8. Sep 27, 2010 #7


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    One other comment. It is possible in principle that "the wave function collapses to something like _____/\____/\____." One must construct a measuring device observing such a mode.
    The fact that it is not localized is not an issue (except in the design of your device). Remember the collapse of the wave-function is simply the projection onto the observed mode indicating we now know that the system will subsequently be measured in that mode.

    One means is to use a probe similarly superposed position wise. For example consider an electron (to be measured) by passing over a double slit through which a photon is to pass and scatter off the electron. If the photon, by virtue of passing through a series of diffraction grating is assured of "passing through both slits" and then by subsequent diffraction gratings is re-localized so information about which of the two slits it passed through is ignored but whether it did or did not interact with the electron is indicated by its position (or polarization or some other measurable feature), then we can say we have observed the electron "in a superposition of being at both slits" whenever the photon so indicates and we then "collapse" its description (wave function) to so indicate that we now know this.

    In spite my earlier comments we could say the answer to your question is "yes". However the measurement is not just the interaction of the photon but subsequent observation of that photon so "the observer" includes the whole of the system gratings, double slit, photon and all.

    Until the photon is observed as described, it can potentially be remixed with the electron, and then a subsequent observation made which indicates the electron passed over one of the two specific slits.

    It helps if you think of the wave function of the quantum as indicating not "how it is" but how it connects to the observer and environment. Until and unless you define this connection the wave-function for a quantum is ambiguous.

    Always insist on the word "superposition" be explicitly connected to some observables. (Here you are implicitly meaning "superposition of positions") Again "being in a superposition" is relative in the same sense as is "happening earlier" for spatially separated events in SR.

    And here's an even simpler device to make your quantum's wave-function "collapse" non-locally. Measure its momentum. It will then collapse to a wave function something like:
  9. Sep 27, 2010 #8
    Thank you! I am starting to understand!
  10. Sep 27, 2010 #9
    According to MWI, when an observer enters a superposition, then he sees a wavefunction collapse.

    One must say here, that any object is always in some kind of superposition. An object cannot be localized and have definite momentum at the same time. So, it must be either a superposition of position states or a superposition of momentum states, or both.

    Sadly, most interpretations pretend that there is a "classical world", with definite momentum and position, MWI included.
  11. Sep 29, 2010 #10


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    The following...
    Should read:
    (I was using exactly the kind of sloppy language which gets people confused. IBYP.)
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