MARK VAN RAAMSDONK: That’s right, so you’ve got sort of a water wave, a wave front coming along, and then that slit acts as a bit of a source for this rippling wave going out in a circular pattern. And you see it’s most wavy at the place behind the slit on the wall. That’s indicated by the brightness there.
BRIAN GREENE: Yeah. And then if we go on to a more relevant version for the actual Double Slit Experiment…
MARK VAN RAAMSDONK: Yeah, so now we’ve got that same wave front. But now there’s two slits, and it’s like there’s two different sources of waves, like if you threw two different pebbles in the pond at the same time. And then what happens is they’re both, you’re both creating waviness. But some places on the screen, the wave from one is doing this, and the wave from the other is doing this, and they kind of cancel out.
But right there in the middle, what’s happening is that the wave from the one slit is going up right when the wave from the other slit is going up, and then they do this, and then you get a big wave and that’s the bright part. But, if you work out the mathematics, then the places that have the big waves are exactly these bright ones, and that’s just like we saw in the Double Slit for the particles.
BRIAN GREENE: Right. So as Gerard was saying, as Mark was saying, we now have a strange confluence of two things: the data that comes out of the Double Slit Experiment when done with particles, and something that seems to have nothing to do with it, where we just have waves going through a barrier with two openings.
So, the conclusion then is that there is some weird connection between particles and waves, that’s where that connection comes from. And, let’s push that further, so…
DAVID WALLACE: Yeah. I mean let’s just drive home how weird it should be that there is any kind of connection here. So imagine I do the Two Slit Experiment. I cover up one of the slits. The effect completely goes away. I get a bit of a spreading out of the particles, but I don’t get that interference. I don’t get those bands.
BRIAN GREENE: Much as we saw with water going through a single opening.
DAVID WALLACE: Exactly, much as we see with water going through the single slit, and much as we see with your classical intuition about particles. And I cover up the other slit, exactly the same result. It’s only if I have both slits open at the same time that the effect happens.
So it seems to be, for all the whirls, if somehow something’s going through the first slit, and something else is going through the other slit, and between them they’re interacting to create this strange effect. And that’s why it matters so much, that I can do this experiment with one particle at a time.
If this was just a massive light going through, no surprise. The sunlight’s going through the left slit, the sunlight’s going through the right slit. The left-hand light, the right-hand light interferes. But I can set this stuff up so that only one photon goes through every hour and a half, I still see the effect. It doesn’t go down in its likeness.
BRIAN GREENE: Yeah, can we see that? I think we have that–
DAVID WALLACE: And then you might be thinking, well maybe each individual particle breaks in half, and half of the particle goes through one slit, and half of the particle goes through another slit. But again, then you’d think you could — look, then you’d think you’d be seeing half-strength detections. But that’s not what you see.
Whenever you look, each time you send a particle through, if you look where it is, you see the particle in one place and one place only. So trying to reconcile those two accounts of what’s going on makes your mind hurt.
BRIAN GREENE: Yeah, exactly. So we’re forced into, as David was saying, not just thinking that a large collection of particles behaves like a wave, which maybe would not be that surprising because water waves are made of H2O molecules, particles, and therefore they’re kind of wavy, but each individual particle somehow has a wave-like quality.
And historically, people struggled to figure out what wave, what kind of wave, what is it made of and what does it represent if you have a wave associated with a particle. A wave is spread out, a particle is at a point. And it was Max Born in the 1920’s who came up with the strange idea of what these waves are. So, Birgitta, what are these waves telling us about?
BIRGITTA WHALEY: Well the waves, what we see is the probability which, the square of the wave or the modulus of the wave, but—
BRIAN GREENE: So here’s a wave behind you. So you said, “probability,” in essence—
BIRGITTA WHALEY: Yes. This is an amplitude, this is an amplitude which will give us a probability. If we take this amplitude and look anywhere here with some measuring device, we will find with some distinct probability, after measuring many times, we’ll find that there’s a definite probability of the particle being there, just as in the double slit. After sending many particles through, we found with a certain probability that they would all appear on the left, or all on the right.