Chapter 11 EXTRA: Dr. Ralph describes the 2slit 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 saidUnfortunately, 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 whitean
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 thatif 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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