Chapter 7 EXTRA: Dr. Ralph demonstrates waves
...... Again, Dr. Ralph gazed up. "Okay.
Now look at the waves as they get to the gap. They go through and then spread
out again--just the way the electrons do." He turned to the table. "Thank
...... Once more he faced his class. "We remember from yesterday, that electrons going through a slit spread out--like a wave. So, in some sense, an electron acts like a wave until it arrives at the film, and then it acts like a particle. Each electron gets absorbed at a particular point on the film--a single electron, not a half or quarter electron. We only see whole electrons."
...... "That's sort of weird," said Kip.
...... Dr. Ralph nodded. "The physicist Louis de Broglie, one of the first who commented on what we call the wave-particle duality, talked about the ghost waves that guide the electron." He touched his finger to the water's surface again.
...... "Are there ghost waves, Dr. Ralph?" said Kip.
...... "I don't know if the question is even the right one. It's a map versus territory question. But ghost waves doesn't seem to be a good map."
...... "Then what is a good map?" said Kip.
...... "I don't know. There doesn't seem to be one." His expression became thoughtful. "I hope at some point, you boys will provide a better way of thinking about quantum mechanics. In fact, that's the reason I've put my research career on hold to teach here." He gave a hint of a shrug and returned his attention to the ripple tank. Sticking his hands into the water, he rearranged the blocks. "Let's see," he said with his back to the class, "how waves behave when there is not one, but two openings." He dried his hands on his pants, then scooped up the cat. "I hope Paradox is still thirsty." He set the cat down at the edge of the tank and again the cat lapped up water, sending waves down the tank.
[Wave Interference in the Ripple-tank]
...... Dr. Ralph took a pen from his shirt pocket
and pointed at the gaps in the tank. Then he gave a little laugh. "I forgot
this pen is also a laser pointer." He aimed the pen and a red dot appeared
on the ceiling. "You see that when the waves get to the gaps, they go through
and make new waves at the other side of the gaps--just the way we saw when there
was only one gap. Except with two gaps, the waves interfere with each other."
Dr. Ralph emphasized the effect with the laser pointer. "Nothing mysterious
here. Just interference--a normal behavior of waves."
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