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Double slit experiment

  1. Jul 14, 2006 #1
    I have a question thats been buggin me as ive built an interest in what kind of part does consciousness play in quantum mechanics.

    My question is if anyone has conducted the Double slit experiment, in a way that the measuring device(s) at the slits (that determine which slit it goes through) were functioning normally but without recording any data from them, as to ensure that any information was not known about the position of the photons? Does the waveform collapse in this scenario? Does the waveform collapse because of the measuring device(s) interfere some how with the experiment or bc theres an observation been made about the position of the photon?

    Please share if theres any other modifications of the experiment that try to determine "what counts as observation" in the waveform collapse scenario.
  2. jcsd
  3. Jul 14, 2006 #2


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    It is the interference, not whether or not there is an observation.
  4. Jul 14, 2006 #3
    As I understand your question, you are asking what would happen if the measurement device did not record what result was measured. If it is possible, even if only in principle, for an observer to infer which path the particle passes through, then the interference effects vanish.

    In your scenario, it is posssible to record the measurement. Indeed, the point is that a measurement occured. That the device does not record the data is of concern only to the experimentalist.

    As for a reference to published results, perhaps someone else can cite something. Also, this question has been posted in various forms before.

    Try reading the "Brian Greene's Beam Splitter" thread....
  5. Jul 21, 2006 #4
    There are 2 issues in your question...therefore 2 answers:

    1. Technically the wave function "collapse" occurs when an observation by a person is made. (John von Neumann was a big supporter of this.) For example, when someone looks at the pattern on the screen and either sees an interference pattern or gaussian patterns. This is back to the Schrodinger Cat being in a superposition of dead and alive at the same time until someone looks in the box.

    2. However, regarding the measuring devices at the slits, we need to be precise as to what we mean. Let's use Feynman's example of the double slit experiment from The Feynman Lectures on Physics Vol. 3. (Read this book to answer your question better than I am doing here. It's really worth it!) The measuring devices are not detecting the electron, say, (if that's indeed what we're passing through the slit). Instead, what they are detecting are photons from a strong light source located behind the double slit screen. By adjusting the wavelength of the photons, we can in essence make the position measurement of the electrons (by making the wavelength short) or avoid making an accurate position measurement (by making the wavelength long). This in turn changes the pattern on the screen from gaussian to interference patterns, respectively. At this point, we didn't even need to bother checking our photon detectors to see which detector got a photon. It was good enough to bang a short wavelength photon against the electron to destroy the interference pattern.

    Note, though, at the end someone still had to look at the screen to see if there's an interference pattern or not. That's why I stated earlier that this question has 2 issues and this goes back to the first answer.

    Sooo, if your intention was to ask if the wavefn collapses when the detector at the slits goes off...then "no". Because the experiment is not done yet. At this point, the detectors are also in a superposition of states just like the particles. But if your intention was to ask if the wavefn collapses when the pattern appears at the screen, then the answer is...well, we can't ask that :eek: ! We have one more detector and that is your eye (or brain, or conscienceness, or...Oh, just forget that nonsense and let's finish the problem! :wink: )

    So the cool thing is that the whole system can be considered to be in a superposition of states until someone looks at the answer. For that matter, maybe even then it's still in a superposition but now includes the experimenter and remains that way until the experiment gets published. :wink:
  6. Jul 24, 2006 #5
    This is one (not widely supported) possible interpretation of quantum mechanics, but not an interpretation that is generally accepted in the scientific community. Very few scientists believe that consciousness causes wavefunction collapse.

    Best Regards
  7. Jul 24, 2006 #6
    QM basically says that we can either know which way the photon went, or we can observe that photon as part of an interference pattern - but not both. One could set up an experiment to detect which way the photons go, but then place the data into a steel safe and not "look" at the data for several years afterwards. When one does look at the data, one will see information about which way the photons went, but no interference pattern.

    The "consciousness causes collapse" interpretation would have you believe that those photons continued to exist in some superposition of states for the years that the data was stored in the safe, and somehow only collpased into one state or another when someone decided to take the data out and look at it. :rofl:

    As beautiful1 said, take a look at the Brian Green thread to see some really weird quantum effects.

    Best Regards
  8. Aug 4, 2006 #7
    I was thinking about this particular paradox this morning - i.e., if conscious observation is what collapses wave functions, then surely just turning the voltmeters, oscilloscopes to face the wall would abruptly cause the interference pattern to suddenly spring up again, which is just nonsense (though, who knows, the world of QM is weird and wonderful after all).

    I would have thought that the point is that the detector is a macroscopic entity which is going to be forced into one of two very different states depending on whether a photon is detected or not, and it's this that causes the wave function to collapse. Perhaps the detector doesn't have to be macroscopic - just large enough that failure of the photon to interact with it would be statistically significant.

    If this is right, then the fact that you've put a geiger counter into the box is enough to make the cat alive or dead.
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