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could cut back the rate at which electrons were being fired at a screen to less than one per minute. Hence, there is no obvious way in which successive electrons fired at the screen could interfere with one another. But left for a long period of time, this system also produced a barred interference pattern!!
The startling conclusion!! With both slits open single electrons were going through both slits at the same time--and interfering with themselves.
This conclusion has been confirmed over and over again, not only for electrons but also for photons of light and even for atoms. For light, it is also confirmed when, instead of slits, a beam splitter is used.
One form of beam splitter is actually a mirror with holes so that any photon impinging on its surface has a fifty percent chance of passing through a hole or being reflected by the mirror. Given these alternative pathways in experiments designed to show "which way", the single photon will take both paths.
As more and more of this type of work was performed, many experimenters gained the impression that their own minds were part of the system under study. And so the idea arose that the particle/wave under study remained in a "superposed" state in which it retained both particle and wave properties until forced to reveal one of the alternatives to a conscious observer. If the experimenter set up to distinguish a wave, that was what was seen, but if set up to detect a particle, that, too, was what was seen. This concept was much ridiculed as God-playing by many materialists.
This state of superpositioning was found to be general, and not confined to just wave or particle phenomena. Electrons can be separated into classes that can be considered to have up or down and left or right spin2, and light can be vertically or horizontally polarized. These states can also exist in "twin" pairs, so that if one of a pair of "entwined" electrons is forced to reveal a spin state, its twin must take the opposite state.
One of the greatest of all time among physicists, Albert Einstein, always hated the notion that, at the quantum level, probability rather than certainty holds center stage. Einstein believed firmly in the cause-effect deterministic relationships of classical physics that appear to be so reliable in the macro-world. For him, probability at the quantum level was a measure of incompleteness of the theory. When its deficiencies were corrected, Einstein believed the probabilities would disappear. So firmly held was this belief that he spent a considerable period of his life in devising thought experiments to disprove quantum theory.
One of the thought experiments devised by Einstein and two associates, Podolsky and Rosen, saw the superpositioning proposal for "intertwined" or "correlated" electrons as a means to discredit the theory. If one of a pair of electrons was shown to have say, "up" spin then, automatically, its super-positioned partner must display "down" spin, regardless of whether they were at opposite sides of the universe. Classical physics requires some kind of force operating between the two particles for this to occur and relativity theory requires that no signal should be transmitted at speeds greater than the speed of light. For many years, Einstein appeared to be right. But eventually improved technology provided the means by which the Einstein, Podolsky, Rosen (E-P-R) proposal could be tested experimentally.
Although preceded by several experiments of a statistical nature that came out in favor of quantum theory, one devised by Alaine Aspect and associates in France was the first to produce truly convincing results. It was performed with correlated photons, twin photons that are emitted in opposite directions from radioactive calcium.
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