BRIAN GREENE: Yeah and it’s actually quite remarkable that it only took that number of years to develop a radically new way of thinking about things. And Richard Feynman, who is of course a hero of all of us, also known to the public, famously said that there was one experiment – we can go through the whole history of everything you described, with the ultraviolet catastrophe and photoelectric effect and all these beautiful experiments – but the Double Slit Experiment, luckily for us, in having a relatively brief conversation, allows us to get to the heart of this new idea, where it came from.
This actually is the paper on, in some sense, the Double Slit Experiment. The first version, Davisson and Germer. And I’ll draw your attention to one thing. You see the word “accident”? And this is just a footnote.
But, in the old days, people would actually describe the blind alleys that they went down in a scientific paper. But as science progressed, we were kind of taught, “no no don’t ever say what went wrong. Only talk about what went right!” But here is an old paper, and indeed this experiment emerged from an accident in the laboratory at Bell Labs.
They were doing a version of this experiment, they turned the intensity up too high, some glass tube shattered, and when they re-did the experiment, unwittingly, they had changed the experiment to something that was actually far more interesting than the experiment they were initially carrying out.
So, just to talk about what this experiment is in modern language, so David again, just, what’s the basic idea of the Double Slit Experiment?
DAVID WALLACE: So you take a source of, well of particles of any kind, but let it be light, for instance. You shine that light as a narrow beam on a screen – it has two gaps in it, and you look at the pattern of light behind the two gaps in the screen — two slits, exactly, yes. So the slits are just literally gaps in a black sheet of paper, in principle. The light’s going through.
If light is a particle, you’d expect one sort of result on the far side of the screen. If light is a wave, you might expect something different as the light coming through one part of the slit interferes with the light going through the other part of the slit. And the weird thing about the quantum two-slit experiment is that it seems, in various ways, to be doing both of those things at the same time.
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.