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Nugatory

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The "bar-like pattern" is a diffraction pattern and it varies with the kinetic energy of the particle, so we get that energy measurement for free.If we fire electrons thru the 2 slit we get an interference pattern showing the wave nature of the electron. Now if we cover one of the slits and fire electrons we get a bar like pattern on the screen. If we cover one slit, fire electrons and use a detector we would also get a bar like formation on the screen. Has any one checked to find if the kinetic energy of the electrons in the last 2 exps. were the same or of different values?Please avoid the use of difficult computations. I had enough trouble with long division.

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What is this "quandary" that needs explaining? The point of the double-slit experiment is that the quantum mechanics correctly describes the way electrons interact with slits and classical mechanics does not. That's very surprising if you are assuming (as everyone did before QM was discovered) that electrons will behave like classical particles, but there's no quandary - QM works, classical mechanics doesn't.

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If I do the experiment over and over again, preparing the electron in exactly the same way every time, I will observe a range of electron energies at the screen. This will be true whether one slit is closed or not, whether a detector is present or not. Thus, there is no way of asking whether the electron energies are the same in the various cases - there's always a range of possible energies, and the ranges overlap.

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As for Dr. Al Khalili (whose knowledge I respect), it could be that our apparent difference amounts to nothing. If not - well, this is not a question about real physics, or math, it's just a matter of opinion. If he finds QM mysterious, I have no trouble accepting that it is mysterious - to him.