BRIAN GREENE: Good. So, Birgitta, if you can just take us through a particle experiment to build up our intuition. So let’s say we carry out the experiment that David described, but we don’t start with photons or electrons, we start with pellets – bullets or something. So I think we have a little animation that you could take us through. So what would we expect to happen in this experiment?
BIRGITTA WHALEY: Well so you have the source of the pellets here in front of us, spitting out the pellets and some of them go through the holes, and the ones that go through the holes basically travel rectilinear, straight ahead, as we might expect from our classical intuition. And we get two bands at the back, indicating the pellets that went through the right slit, on the right, and the left band is the pellets that went through the left slit.
BRIAN GREENE: Now, if I was – that’s completely intuitive right? So this is the stuff that our forebears would have known even on the Savannah. Now, if we took the size of the pellets and we dialed them down to a very small size, before going to your quantum intuition that you have, what would you expect naively to happen if you simply dialed down the size? Would you expect there to be any different if you were – this is a leading question, by the way.
So, just follow me here. The answer is…would you expect anything different? Would you expect anything different? Good. Good. So there we—
BIRGITTA WHALEY: Naively not.
BRIAN GREENE: Naively not! Exactly right. So here’s what you would naively expect would happen. Again, you got the particles going through the two slits. So Mark, tell us what actually does happen – not that I don’t think Birgitta could, just to give us all a little airtime.
MARK VAN RAAMSDONK: So it’s of course, while the place where you would least expect to see something on that screen is exactly behind that big barrier that’s in the middle. And somehow, when you actually do the experiment, you see that actually, that’s where most of the particles end up. So, it’s always exactly the opposite. And then you get this weird pattern with other bands going out. And so you initially would stare at it and shake your head and wonder what you’d actually have.
BRIAN GREENE: So we’ll analyze what that means in just a moment. But I, you know, we often, I don’t know, probably most everyone in this audience has seen a still image or animation like this in the discussion of quantum mechanics. And I thought it would be kind of nice to show you that it, that this actually happens. It’s not just an animation that an artist does.
So we’re going to actually do the Double Slit Experiment, for real, right now. And to do that, I’m going to invite a friend of mine from Princeton University. Omalon, can you wheel out, if you would, the Double Slit Experiment?
All right, so what we have here is a laser on this far side. So this is our source. So actually we’re doing this in some sense opposite to the orientation that we saw in the animation. And we’re going to fire this laser, which is photons, in essence. And the photons are going to go through a barrier that has two openings in it – it’s harder to see that of course mechanically, but trust me there’s a barrier with two openings.
And we’re going to take a look at the data that falls on a detector screen, which in the modern age is a more complicated and somewhat finicky piece of equipment. So we’re all sitting here, on shpilkes, if you speak any Yiddish, you know exactly what I’m talking about right there. But hopefully this will work out.
So, Omalon, why don’t we just actually see ambient noise. Can we see a little bit of that first? Can we switch over to the input to the screen?
All right so this is the output from that device. And now, if we actually turn on the laser and allow us to collect all the photons that land. There, they’re building up. And there you see what actually happens. So this is the result of this very device here. And you see it.
You can see on the very far left, we see some of the photons are landing. Then we get a dark region in between. Then a bright, a dark, a bright, a dark, a bright, a dark, and a bright and a dark…even though this device over here really is a barrier that has only 2 slits in it. So the animation that we showed you actually does hold true in real experiments. And that then forces us to come to grips with it, to try to understand what in the heck is actually going on. OK. So, thank you Omalon.
So there we have it. We have this situation in which we expected to get two bands and we got more. What does that tell us? Where do we go from there?
MARK VAN RAAMSDONK: That there’s an existing bit of mathematics that comes up with exactly that same pattern. But it has nothing to do with particles. It’s the mathematics that you use to describe waves, water waves, or other kinds of waves.
BRIAN GREENE: Yeah. So can we see the animation that has a single? So this is a warm-up to the problem, where we have water going through a single opening. Just tell us what we see happening here.