Chapter 11 EXTRA: Dr. Ralph describes the 2-slit experiment in detail

...... "Richard Feynman said that all of quantum mechanics can be understood by understanding the two slit experiment." Dr. Ralph threw a quick glance over his shoulder at the diagram, and then added, "But Feynman also said--Unfortunately, no one understands the two slit experiment."
...... He stood aside, to provide the boys an unobstructed view of the posterboard. "Okay, then." He described the illustration, then took out a pen and used it as a pointer. "Suppose the electron gun shoots an electron at the box every few milliseconds or so. Most of the time, the electron will just hit the front of the box and get absorbed. But sometimes an electron will go through and register as a dot on the photographic plate." He gazed out at his class. "Now we go away for a few days and then come back and look at the photographic plate. What would we expect to see?"
...... Dr. Ralph picked up one of the smaller posterboards and slapped it in place of the photographic plate. Directly in back of each slit, the strip showed a dark blotch, black in the center and fading to gray at the edges.

[The Two Slit Experiment: What you might expect to see --WRONG!]
...... "This is what you might expect. Two dark patches, a doubling of what we saw in the one slit case." He ripped away the strip. "But you'd be wrong!" He put up the second posterboard strip in place of the first. It showed alternating bands of grey and white--an interference pattern.

[The Two Slit Experiment: What actually happens!]
...... "This is what actually happens. It's as if a lot of electrons, acting like waves, are going through the slits at the same time and interfer with each other. But." He flourished his pen for emphasis. "But we're sending the electrons through one at a time. How can there be interference, then?" He waited a few seconds and went on. "Each electron has to be interfering with itself. It's as if each electron is simultaneously going through both slits."
...... He paused. "Think about it."
...... After a few seconds, a boy asked "You said it's as if the electron goes through both slits. Does it really go through both slits?"
...... "I don't know." Dr. Ralph shrugged. "I don't even know if the question makes sense." He tapped the posterboard. "All I can do is show you the results of the experiment. Somehow, we've got to make sense out of it."
...... "Can't we just look and see which hole the electron went through?" asked another boy.
...... "Sort of," said Dr. Ralph. "We can put a charge detector at one of the slits to record if an electron goes through. But,"--Dr. Ralph held up the first strip, showing just the two blotches--"if we do that--if we measure which slit the electron actually goes through, we get the pattern we might have expected in the first place."
...... "Huh?" said a kid.
...... "Strange, isn't it," said Dr. Ralph. "By checking which slit the electron goes through, we force it to make up its mind and go through one of them. If we don't check, it goes through both."
...... "That's weird," said a kid. "That's really weird."
...... "Yes, it is," said Dr. Ralph. -

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