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Observing the double slit experiment

  1. Aug 30, 2010 #1
    In this experiment wave particle duality is shown. Without observation the wave function is used. With observation the particle function is used.

    My question is what is the definition of observation? Specifically in the case of an electron gun is the observer a conductive piece of material in and around the electron path?
  2. jcsd
  3. Aug 30, 2010 #2
    "Observation" is not a very good concept. "Detection" is better (presence of human "observers" is irrelevant for the detection to take place). You have a detection when you have a detector (or several of them) coupled to your quantum system.
    Last edited: Aug 30, 2010
  4. Aug 30, 2010 #3
    Ok, anyone got some examples of detectors that influence the system?
  5. Aug 30, 2010 #4
    Photographic plate, Geiger counter, cloud chamber, fluorescent screen. Eye of a fly.
  6. Aug 30, 2010 #5
    I should add that in "Bohmian mechanics" the philosophy is different. But I do not think you want to know about Bohmian mechanics - it is a separate subject.
  7. Aug 30, 2010 #6

    How about ear?
  8. Aug 30, 2010 #7
    Ear is not that sensitive to individual quanta. Not a good detector for quantum events.
  9. Aug 30, 2010 #8

    Detection clicks are audiable. But my focus was on the "fly's ear" not just ear, or just 'eye'.
  10. Aug 30, 2010 #9
    Without observation, what is the wave function used for?? What is the "particle function"??
    A quantum experiment must include a measurement result that is obtained when the particle is detected. I think this is what you mean by observation; observation is the measurement result. This result is a necessary part of the experiment. (Bohr) Without a result (observation) there is no experiment to discuss. The wavefunction is used to calculate the probability of obtaining that result. Without observation, there is no wave function.
    Neither the theory nor the experiment describe what the particle is doing before it is detected. Quantum mechanics predicts only the possible results of a measurement and the statistical distribution of those results. We have no idea what the particle is doing before detection. (Wheeler)
    The particle is always detected as a particle. It is the statistical distribution of the results that we identify as an interference pattern, which is characteristic of classical waves. Thus, we see "a particle" when the individual particle hits a detection screen. We see an "interference pattern" (caused ?? by "a wave") when a large number of particles have hit the screen one at a time.
    An observation is a measurement result that gives a value for a specified observable, such as energy, momentum, spin, etc. The particle detector is part of the measuring apparatus, which is the observer. As noted by others, the experiment does not require a human observer.
    What are you trying to measure with " a conductive piece of material in and around the electron path?"
  11. Aug 31, 2010 #10
    Ear is not necessary. Irreversible recording of whatever kind is.
  12. Aug 31, 2010 #11
    That depends on which "theory". If your theory does not bother - then it does not describe. If it bothers - then it describes. Some people do not have an idea and do not care, some other people have an idea (right or wrong) and describe. Some theories bother only with the statistics, some with description of individual detection processes.
  13. Aug 31, 2010 #12

    If there is no "human observer", how does one acquire knowledge of the experimental results?
  14. Aug 31, 2010 #13
    Acquisition of knowledge is a separate branch of science. It is not needed if you want to discuss the behavior of detectors, interference, double slit experiments etc. For this you need physics, dynamics, evolution equation, a little bit of probability theory.
  15. Aug 31, 2010 #14
    But in order for such a discussion to take place, doesn't there at some point have to be "human observation"?
  16. Aug 31, 2010 #15
    Are you interested in human beings or are you interested in physics. Human beings, their psychology, neurobiology, etc. is a different branch. Of course quantum processes, in particular detection processes take also place in neural activities. But that does not influence the laws physics.
  17. Aug 31, 2010 #16
    But you agree that "human observation" is required for completion of the exercise?
  18. Aug 31, 2010 #17
    It depends on the exercise. Rats also exercise, frogs do, and birds exercise as well. Perahps the whole Nature exercises, one way or another. Physics is about making the patterns. Recognition of patterns is another thing.
  19. Aug 31, 2010 #18
    Quoting from J. A. Wheeler ("Geons, Black Holes & Quantum Foam", p. 343):

    "I wanted to emphasize in this talk that the essential feature of act of 'measurement' is amplification from the quantum thing observed to the classical thing doing the observing, which need have nothing to do with human intervention or human consciousness"

    Notice that "measurement" is in quotation marks. Means: be careful even with this concept.
  20. Aug 31, 2010 #19
    My question wasn't to imply observation was necessarily an act of measurement, merely that "human observation" was a conclusive requirement of experimentation.
  21. Aug 31, 2010 #20
    Human observation may be right or wrong. Some people see things that do not exist. Some see wrong colors. You would have to define the terms you are using. But this will take you out of the domain of physics. These forums are called "physicsforums", and this part of the forums is called "Quantum physics". You need to distinguish physics from philosophy, psychology etc.

    You do need to know anything about philosophy and psychology to study and analyze physics phenomena. Just learn physics and mathematics and how to apply them in real world.
  22. Aug 31, 2010 #21
    I disagree that this is not the domain of physics, indeed, i would have thought it's importance to quantum physics somewhat a mute point. But i'm willing to stop posting on the subject if others wish it so.

    My definition of "human observation" would be - An intelligible interpretation (whether correct or otherwise) of information.

    Isn't the above an inescapable inevitability (if one desires to form and communicate a conclusion) of experimentation?
  23. Aug 31, 2010 #22
    Well, you can post, why not? Nobody seems to be disagreeing with you so far - except of me.
  24. Aug 31, 2010 #23
    You are confusing theory with interpretation. I am suggesting that we take quantum mechanics at face value without any interpretation.
    All the different interpretations try to explain, "what is really happening". People, including Einstein, refuse to accept the purely statistical nature of quantum events. They believe that the quantum particle must be doing something before it is detected and a complete theory would describe that something. But every attempt to date that assumes a classical-like existence for the particle gives results that do not agree with actual experiments. This is why real experiments do not agree with the EPR experiment and Bell's theorem. (I think a review of delayed choice experiments would also be helpful.)
  25. Aug 31, 2010 #24
    "People, including Einstein, refuse to accept the purely statistical nature of quantum events. "

    Quantum theory is not purely statistical. Schrodinger equation is deterministic.

    Theory and interpretation are always mixed. What's wrong with it?
  26. Aug 31, 2010 #25
    That is why the Schrodinger equation is semi-classical. More intelligent species will make no interpretations, but accept the theory and think abstractly.
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