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Name: Terry D.
Status: N/A
Age: 50s
Location: N/A
Country: N/A
Date: 4/1/2004


Question:
I am a lay person who likes to study science. I am currently reading Dr. Brian Greens book on the Fabric of the Cosmos. I am fascinated by his description of the double slit experiments that demonstrate the dual nature of electrons and photons. In the quantum mechanic double slit experiments that reduce the electron or photon emission to a very low number and record the accumulated strikes on the detector in the famous interference pattern as would be indicated by the probability wave, are all of the electrons or photons emitted accounted for or is the ratio of emissions to recorded strikes on par with the ratio of the slit area to the illuminated area?


Replies:
All of the electrons / photons can be accounted for. Young's "simple" experiment has been carried out in many variations at high levels of sophistication. Even to the extent that the two slits are far enough apart that light cannot make it from one slit to the other to "tell" the other photon / electron whether the first slit is open or shut. Not withstanding, the results are the same. The Quantum Mechanical particle "knows" whether one or both slits are open. The experiment in principle is so simple, the only conclusion is that our intuition utterly fails us when it comes to Q.M. particles. This is why quantum mechanics (the people, not the subject) have to let "the math" take them where it goes and not try to "explain" the results in classical terms. As unsettling as that is, there does not seem to be any other choice to align the predictions and the experimental results.

Vince Calder


Well, Terry,

You are referring to the experiment wherein someone emits electrons at a pair of side-by-side slits. The electrons that go thru the slits then hit the far wall in a slightly spread-out pattern, which is rippled, weirdly enough, instead of being just one broad bump. Even weirder, the highest point in the rippled pattern is right in line with the blocked spot between the two slits. So how can a straight-line-zinging particle get there? Which slit did it go thru? It looks like one electron split itself and went thru both slits, and then rejoined itself! And it must be one electron, because a single-strike detector ( something which "ticks" like a Geiger counter) was used. The same ripples also show up when you use higher beam currents and average-current detectors, such as a glowing phosphor screen across the wall.

I believe that for most implementations of this experiment, a simple single wide beam of electrons is thrown in the general direction of the slits. So it's true, most of the electrons in the beam miss the slits and hit one of the 3 metal barriers (left right, or between the two slits) and are stopped. By narrowing the beamwidth to just wide enough to paint both slits, and making the slits fairly wide (each width being 1/4 or 1/2 of the separation between slits), one might tune up the efficiency to maybe 10%. But most scientists have been happy to do it with less than 0.1% efficiency; pico-amp current meters and/or electron multipliers to detect single strikes are not too hard to come by.

If one tried to get higher than 10% by making two separate pin-point electron beams aimed only at the slits, I think the interference pattern would stop happening. This is presuming you used two separate filaments to emit the two separate beams. In that case the electrons approaching the left slit are quantum-mechanically distinct from the electrons approaching the right slit, so they usually don't "mix" after they go thru the slits. If you take the beam from one filament and massage it with electric fields to make it one beam with two maxima, then if you were very careful to leave both beams having equal speed and distance, they would still interfere and make a rippled strike-density pattern on the far side. And maybe you would reach >10% efficiency.

This phenomenon depends on blurring across the one emitted electron beam, to make sure that there is "one electron" that might go thru either slit, which slit being unknowable in advance.

Jim Swenson



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