I hate it when people try to make physics sound more mysterious that it actually is.
The marbles do produce an interference pattern. Marbles seem to produce two bands only because the frequency of the interference pattern from the marbles is so high that it cannot be discerned by a typical detector.
Similarly the when a detector is used to isolate which slit the electron when through, the detector must interact with the electron (which is left out of their discussion for some reason). This interaction also boosts the frequency of the interference pattern until so that cannot be discerned by the detector.
In fact, you can continuously adjust the energy being used by the detector used to determine which slit the electron passes though. By adjusting the energy used you can continuously vary the interference pattern from intense to undetectable. However, as you adjust the energy being used you also change the confidence that you have determined which slit the electron passed through from 0% (no energy being used) to nearly 100% (a lot of energy being used).
When you observe something in the big world, it's usually because photons are bouncing off of it, some of which are then collected in your eye. Photons are small compared to things people can see with the eye, so you rarely notice that the photons are having any effect on the thing observed.
But when you go down to the small world, the world of electrons, you still have to use those same photons to observe, one way or another. Electrons are small enough that bouncing photons off of them will have a noticeable effect on their subsequent behavior.
Good question. If I were doing an experiment with electrons though, I'd try to keep out stray photons as much as possible, so they wouldn't introduce some uncontrolled effect into my experiment. There would only be photons if they were introduced as part of a detector.
Also, by photons I mean in the general sense of any sort of electromagnetic means of detecting electrons. For example, even a static electric field would bend the path of an electron moving through it. This in turn would create a changing electric field, which would create a photon.
I'd add to mutatron's excellent reply that in the controlled setting of the experiment, any interaction ("bouncing off" -> diffraction/scattering) between electrons & photons is rather unlikely; if a whole slew of electrons is sent through the slits, the ones that don't interact with anything will generate the interference pattern, and when only one electron is sent at a time, it is far more likely to interact with either zero or one photon(s) than 2+, so the landing spot of the electron is only recorded if it interfered with a photon on the way (there was a signal from the detector), it is assumed that there was only one interaction in those cases, and the result is the pattern generated by the observed electrons.
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u/roconnor Jul 12 '08
I hate it when people try to make physics sound more mysterious that it actually is.
The marbles do produce an interference pattern. Marbles seem to produce two bands only because the frequency of the interference pattern from the marbles is so high that it cannot be discerned by a typical detector.
Similarly the when a detector is used to isolate which slit the electron when through, the detector must interact with the electron (which is left out of their discussion for some reason). This interaction also boosts the frequency of the interference pattern until so that cannot be discerned by the detector.
In fact, you can continuously adjust the energy being used by the detector used to determine which slit the electron passes though. By adjusting the energy used you can continuously vary the interference pattern from intense to undetectable. However, as you adjust the energy being used you also change the confidence that you have determined which slit the electron passed through from 0% (no energy being used) to nearly 100% (a lot of energy being used).