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Extent of indeterminacy in the double-slit experiment

  1. Jan 19, 2012 #1
    So, I recently read "Divining Light" by Ted Kosmatka. In it, he proposes a thought experiment (acted out in full by his characters) in which the double-slit experiment with electrons is performed. The detectors at each slit are wired to a computer, and do not click or turn on a light, merely tell the computer which detector was triggered. The computer stores this information in a file for the physicist to open upon completion of the experiment. Likewise, the results of the detector screen at the end of the experiment are sent to the computer and stored in a file. The idea, then, is that if the detectors at the slits are turned on (the case in which we would expect the wavefunction to collapse), but the results are erased before being observed, then the detectors remain part of the indeterminate system. Because it is impossible to know which detector was triggered, there still exists a superposition of states in which either could have been triggered, and therefore the interference pattern is preserved. Predictably, I am skeptical. Because this would imply some sort of requirement of consciousness to collapse the wavefunction. However, I cannot find anything to support or disprove this. I contacted the author, and he admitted that he also has no idea if it's true or not; it was only a thought experiment. Can anyone clearly explain why it is or is not true, and give evidence/citations to back up your assertions? I don't want to answer a thought experiment with another thought experiment.
  2. jcsd
  3. Jan 19, 2012 #2
    The result from the screen would be just the two stripes. The wavefunction would have collapsed since it isn't the act of checking you're measurement that makes the collapse happen. It's the interaction with the measurement apparatus. When the electron interacts with the detector it has to 'take a stand' on which slit it is going through.

    I hate to actually still reply with a thought experiment because I don't know of any record of such an experiment.

    But suppose we don't erase the file but first send it somewhere very far away than at about the same moment both files are checked. Then if the checking of the file would make the wavefunction have collapsed there would have to be a correlation with the file about the stripes on the screen (You may have noticed this is a rehash of the EPR paradox).

    The information about the collapse would have to be transmitted to the file on the existence or non-existence of an interference pattern. But this means we have information going faster than light. Now the difference with the EPR paradox here is that the file of the detector holds the same information every time. When we check the screen file it cannot suddenly come out interference because that would mean the detection file can no longer exist and would have to suddenly be erased or something!

    Of course you will notice this isn't really rigorous since there is still the ambiguity of which slit it goes through. So maybe I shouldn't have mentioned this reasoning. Also you could argue maybe the detector was broken and no measurement was made at all!

    I actually hope someone can come up with an example of just such an experiment. i am fairly certain though that it is the act of measurement that collapses the wave function not the act of observing the measurement itsself.

    David Bohm in his intro to quantum mechanics does drive this point home at some point in the book although I can't say exactly which page. Maybe by being a bit more specific about the kin of detector you can predict (using the machinery of quantum mechanics) what happens exactly.
  4. Jan 19, 2012 #3
    John Wheeler said that consciousness has nothing to do with the results obtained in a quantum experiment. Most of us do not believe that we have the mental capacity to kill cats simply by looking at them, or that we can change the result of an experiment by choosing to ignore certain information.

    If you can, in principle, determine which slit the electron went through, then there is no interference, even though you never bother to get that information. Even though the experimenter doesn’t know which way the electron went, the ‘which way’ experimental apparatus is different from the experimental configuration that exhibits interference. Two different experiments with two different results! It is the different ‘which way’ experimental configuration that determines the no-interference results, not the human observer.

    Many actual experiments have been done to confirm this. A quick internet search will provide you with evidence you seek.

    Best wishes
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