My story starts with a duck pond and with this man, Thomas Young,
because about 200 years ago, he was sitting by a duck pond watching some ducks.
And I have here a pair of ducks.
We're going to have a look at what Thomas Young saw.
Just wind up my ducks.
What he saw, as the ducks swam around,
was a pattern appear in the water, which
you can see down here.
Now the pattern was caused by ripples, on the water,
leaving the ducks, and interfering with each other.
You can see it in a more controlled way.
Here we have a motor that's actually
going to cause ripples.
And it's going to pass through two slits.
And these slits are going to produce ripples which go out
into the water and actually interfere, very much
like with the two ducks.
You can see a pattern emerging.
You can see, here, along this line,
two ripples are adding up.
So you're getting bright and dark lines.
But here doesn't look like anything's happening.
That's because the waves, the ripples,
have actually cancelled each other out.
Now, this made him pretty happy.
First of all because he liked ducks, but second of all,
because it reminded him of an experiment
he'd done a couple of years earlier with light.
I'm going to show you now what he
saw with his light experiment.
Here we have something very similar to the pond and the two ducks
and actually very similar to that tank with water
and the two slits.
Because here we have something that produces light.
And the light passes through two slits, actually very close
together, because light has a very short wavelength,
a very small wave.
So you can't actually see these two slits,
but it's the same idea.
And then, the light passes on and hits this screen.
You can actually see a pattern that's
quite similar to what we saw on the tank with the water.
And there are regions of bright and dark and bright
and dark and bright and dark.
So this set him thinking, well, if I saw the same pattern as I
saw on water and I know that waves travel on water, that
must mean that light is a wave.
After all, particles wouldn't do this.
Let's have a look at what particles would do.
For this, I need a volunteer with a lot of puff.
Could you come down?
Your name is?
Jay.
Could you stand here.
Now, what you're going to do is you're going
to blow through this tube.
And here is sand.
Now, sand consists of grains, particles, lots of them.
And they're all packed in here.
And as Jay blows, he's actually going
to blow sand through this box, up against these two slits.
And the sand can pass through these two slits,
very much like two slits we had with light.
And on the back here, we've got a screen,
which is going to collect the particles of sand
as they go through the slits.
So huff and a puff and blow through the tube Jay.
Keep going.
Great.
You can do it a little bit less.
That's it.
That's great.
Thanks very much.
Go back to your seat.
I'm just going to show you what's on the screen.
And this is it.
It's not really a surprise.
There are two stripes, very much like the two slits.
Because of course, sand consists of grains,
like little particles.
And they either go through one slit
or they go through the other slit.
And that's what you're seeing.
These particles have gone through one slit.
These have gone through the other slit.
So this is what sand does.
And this is what particles do, they produce this pattern.
And it's unlike the pattern that you get with waves.
Now, we're going to try something a little different.
We saw the pattern that light produced.
It's nothing like the sand.
But here, we've got, essentially,
the same piece of equipment as before,
but we're to turn the brightness of the light down.
Here we have the source of light over here.
And it's passing through two slits.
We've had to cover this up, because we don't want any light
to get in.
The light passes through the two slits,
goes over to the other side, instead of a screen over here,
we've actually got a camera, because this detects
really low levels of light.
I'm going to see what we see.
Actually, up there on the screen,
you can see what the camera's picking up.
The first surprise is it's picking up dots, blips.
This is light travelling through this.
But it's arriving as blips, like the sand, as particles.
So that's amazing.
But have a look at this.
If we collect up all these blips, collected over a time
period and have a look at what the pattern is,
we see something quite remarkable.
The pattern you see is the pattern that waves make.
But the light arrives in blips, in packets, as small particles,
which physicists call photons.
So this is incredible, because the light's travelling,
making a pattern as though it were going through two slits.
Because remember, to make that pattern, that wave pattern,
there've got to be two sources, two slits.
The light's got to go through both slits.
And yet it's arriving as a blip, as a particle, like the sand.
This is the essence of the weirdness of the spookiness
in the quantum world.