Physics 2000 The Atomic Lab Interference Experiments

Electron Interference

We've seen what happens when we shine light through two slits, or when water waves do something similar. But what do you think happens when solid objects go through the slits? I don't get it. What do you mean by solid objects? Like rocks? Solid things don't travel in waves, do they?

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.

Right! The bullets don't interfere with each other like waves do... Wait a second! But they might! Two bullets, one from each hole, might bounce into each other and knock each other all over the place. That's a kind of interference, right?


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 once.

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"...

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