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 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...
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?
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.
We call it "particle/wave duality"...
