Let's forget about waves for a second and just keep it simple. Dr. Feynman
liked to talk about shooting a machine gun at an iron plate with two slots in
it. If there were a concrete wall behind the iron plate, what kind of pattern
do you think the bullets would make?
Well, I would think bullets would just pile up behind the two slots. I guess
they would bounce off the edges of the holes a little bit, so it wouldn't be
real neat, but mostly they would just be in two areas.
|Let's think about that. For two bullets to bump into each other would mean they left the gun at the same time. Do machine guns work like that?||I hadn't thought about it, but I guess not. No matter how fast the machine gun seems to shoot, it's still just one bullet at a time. So there's no way the bullets could interfere.|
OK. Now we're going to try an experiment. Using our two slits from before,
we're going to use an "electron gun" which shoots a steady stream of
electrons, the same particles that orbit atoms, at a sensitive screen...
|Like a machine gun that shoots really small bullets.||Yes. Each time an electron hits the screen it will make a green dot. Try switching it on...|
Wait a second; it's slowly building up an interference pattern, just like with
light. But that doesn't make sense. Are you sure the electrons aren't
interfering with each other as they go through the slits? Maybe the electron
gun doesn't work like a machine gun, and it shoots a bunch of electrons at
|OK, maybe so. How could we test that?||Maybe we could turn down the electron gun until the electrons were coming out slowly enough for us to be sure it was one at a time.|
Lucky for us it does just that. Use the minus and plus keys on
your keyboard to control the speed of the gun, and slow it down a lot. Then
press your backspace key to clear the screen.
Hey! The interference lines are building up anyway! How can it do that if the
electrons are really like little bullets? What are the electrons interfering
with? This is so strange...
|This is quantum physics.||What does it mean? How do you explain it?||We call it "particle/wave duality"...|