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Observers and Measuring Devices

  1. Jun 22, 2011 #1
    Just wondering if anyone can explain to me why it is thought necessary to say that consciousness (an observer) is necessary to account for certain things in Quantum Physics (e.g. for the collapse of the wavefunction), rather than just say that a measuring device (with or without a conscious observer) is sufficient.

    I can certainly see that experiments such as the delayed choice quantum eraser experiment, stretch our understanding to the limits, but is it actually necessary to invoke the conscious observer? When various writers on quantum physics say that wavefunctions collapse when "observed", that waves appear as particles when "observed", often the distinction between "observer" as a conscious subject, or as a measuring device which has no consciousness, no subjectivity (in the technical sense of "subjective", rather than the perjorative sense), is left unclear.

    Certainly Schrodinger and his cat was not happy with the idea of a conscious observer collapsing the wavefunction.

    I am a layperson and have no mathematical background, but would be interested to know what you think.
  2. jcsd
  3. Jun 22, 2011 #2
    The ''observer'' effect is an effect made by a scientist in the lab, or a quantum observation. Avoiding what observation we speak of can be a bit lazy, because both types of observation can mean different things.
  4. Jun 22, 2011 #3
    by ''quantum observation'' I mean something like two electrons defining their wave functions.
  5. Jun 22, 2011 #4


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    No one thinks this.

    An observation is simply a measurement. There does not have to be a person attached to it. An electron could measure another electron by interacting with it.
  6. Jun 22, 2011 #5
    It's not true that NO-ONE thinks this. There are plenty of intense debates about this very issue. I am just wondering about this distinction between a measuring device on the one hand, and a conscious observer on the other hand, which seems to be where much of the debate lies. There is something called the Von Neumann Chain which is discussed by theoreticians such as Eugene Wigner, though I dont pretend to be able to understand the discussions at this level.
  7. Jun 22, 2011 #6
  8. Jun 22, 2011 #7
    The Von Neumann chain is basically the laws of the locating an observer with an observed. I'm not very wise on the subject, but I think it's solution is by stating that consciousness collapses the wave function, so where there is an observer there exists alongside the observed, both with amplitude of the wave function squared.
  9. Jun 22, 2011 #8
    Perhaps another way of formulating the issue is in terms of information and what a measuring device actually IS. A measuring device is not just a random collection of particles but an intelligent device designed to provide information. The particular and peculiar ways in which the interposition of measuring devices in some experiments - such as delayed choice quantum eraser experiments - affect the results, involve (arguably) more than just a mere decoherence caused by the bumping together of the particles of the measuring device and of the particles involved in the experiment, but the fact that information is involved. Now, once we have information, philosophically speaking, we are into an arena where we cannot completely separate consciousness (the observer, the subject) from the object (the observed). It seems as if in some sense we are entangled with the experiment. This I think may where the issue lies?
  10. Jun 22, 2011 #9
    Well, let's get deeper than that then albroun since we are discussing the eraser experiment. Information in physics is called ''bits'' and they come in various forms, however, field theory has information at it's core because information is what scripts a peice of work. The arena of the conscious mind (and other varieties or forms of awareness which constitute as an observer) is an internal world or sphere of mental projection, made from a series of complex electrical signals which have been formed from two dimensional images into a three-dimensional phenomenon. How the brain percepts this is unclear. But can we seperate the human as though it plays no part? Then no... the world we see and the world out there might be two different things but the subjective and the objective seem to coincide in that they are interconnected when a disturbance is made in the wave function. The observer and the observed must then somehow corrolate when they come into contact with each other, quite possibly just a probability field, or something along those effects. Simply a state on the system is the sum of two states.
  11. Jun 22, 2011 #10
    There are ways for instance, to describe a series of objects rather than one object being observed. We could say that

    [tex]|\Psi>=\sum_i |i>\psi_i +\sum_j |j>\psi_j[/tex]

    We can use a riggid Hilbert Space, and assume:

    [tex]U(|\psi_1> \otimes |in>)[/tex]

    takes the first state and

    [tex]U(|\psi_2> \otimes |in>)[/tex]

    Takes the second state, you can choose an orthonormal basis [tex]{e_i}[/tex] so that:

    [tex]\sum_i|e_i> \otimes |f_{1i}>[/tex]


    [tex]\sum_j|e_j>\otimes |f_{2j}>[/tex]

    Assume for a second that we had to make our terms Unitary because all measurements or observations (as physics clearly states) are made in real time events; then there is a real time evolution associated to subject [tex]|f_{1i}>[/tex] and [tex]|f_{2j}>[/tex].
    Last edited: Jun 22, 2011
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