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Observation and the Double Slit Expt

  1. Jun 20, 2008 #1
    So I'm embarking through Brian Greene's magnificent book The Fabric of The Cosmos and in the quantum section he goes through the whole idea about how observation of a particle leads to collapse of the wavefunction into a definite location. He also uses the double slit experiment for photons and electrons and using the resultant interference pattern to explain the wave-like nature. My question is...why is the double slit experiment not considered a form of observation? It seems that other tools such as the photon splitter can potentially act as detectors but yet a double-slit and screen do not act as "observers"? Why is that? This has been a problem I've had when reading anything about quantum and I'm yet to find a substantial answer.
  2. jcsd
  3. Jun 20, 2008 #2
    When a photon hits the screen the observer knows the the photons position. This is a measurment. How can a photon splitted act as a detector?

  4. Jun 20, 2008 #3
    It can't, that's a mistake on my part...it's the down-converter that can potentially indicate the path taken by the photon. Sorry.

    That's besides my main concern though, I don't see how a double split experiment doesn't constitute observation of the photon?
  5. Jun 21, 2008 #4
    How do you think the "observer effect" work?
  6. Jun 21, 2008 #5


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    You asked why the experiment isn't an observation, and you got the answer that it is. You seemed to agree, but then you dismissed it as irrelevant and asked the same question again. :confused:

    I assume that what you really want to know is why the interaction between the photon and the screen is a measurement, but the interaction between the photon and the slits isn't. You may not find the answer completely satisfactory, but it goes something like this: The interaction between the photon and the slits doesn't change the state of the matter except at a microscopic level, where superpositions can exist for some time. The interaction between the photon and the screen changes the state of the matter into a superposition of macroscopically distinguishable states, and such superpositions are quickly destroyed by decoherence.
  7. Jun 22, 2008 #6
    I think what happened was that we were talking about two different experiments, it's ok though.

    And what you said makes a lot of sense. You're right, it's not entirely satisfactory but it does answer my question better than any other source has. Thank you.
  8. Jun 23, 2008 #7
    The double-slit is a filter. The screen is a detector. In quantum experiments where only one electron or photon at a time is recorded on the screen there isn't any way to know the qualitative nature (particle or wave, or particle and wave, or whatever) of what it was that was emitted and filtered that corresponds to each individual detection.

    The mystery (and an example of the wave-particle complementary -- ie. either one by itself isn't sufficient -- models) is that the individual detections are pointlike, but if you accumulate enough of them on the screen (tens of thousands), then a wavelike interference pattern can be seen.

    The reason that the double-slit itself isn't a detector is simply because no changes in the double-slit itself can be observed and recorded. It's the same situation with polarization filters and photomultipliers. Something that corresponds with a photon detection has been emitted, was incident on, and has interacted with the polarizer itself, but the polarizer itself remains unchanged as far as is known, and so can't be used as a detector.
  9. Jun 24, 2008 #8


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    The double slit provides data for a measurement: of the photons emitted from the source, only some actually get through the slits. So, the screen provides a measure of "slit efficiency", the % of the source current that gets through the slits. Thus we can think of the two slit experiment as one that actually provides two measurements, one of which is really of little interest. The experiment deals with the conditional probability that a photon has gone through the slits; no get through, no diffraction.

    There is a long history of binary outcome experiments -- the Stern Gherlach experiment was done with a magnetic filter; and the measurements led to the concept of spin. Measurements of filter performance by means of measurements of in and out voltage are commonplace in electronics.

    The real issue is: what data does a measurement provide, and what can you learn from that data?
    Reilly Atkinson
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