BRIAN GREENE: Good. Birgitta?
BIRGITTA WHALEY: I think it’s here to stay. There may be extensions, modifications, there may be something more complete, but this will still be part of it, in my view.
BRIAN GREENE: OK, Mark.
MARK VAN RAAMSDONK: Yeah, so there’s a frontier in quantum mechanics that I work in, and this is the frontier. It’s like the wild west of theoretical physics, where we’re trying to combine quantum mechanics and gravity, and we need to do that to understand black holes and hopefully eventually understand the big bang. And there’s a lot to do, and we don’t know if we’re going to have to modify quantum mechanics, or it will all be the same quantum mechanics all the way down.
BRIAN GREENE: Now, Gerard, you have unusual views.
GERARD ‘T HOOFT: Well yes, I could spend the rest of the evening explaining them. But, to my mind, quantum mechanics is a tool, a very important mathematical tool, to calculate what happens if you have some underlying equations. And telling us how particles and other small things behave. We know the answer to that question– the answer is quantum mechanics. But we don’t know the question, that’s still something we’re trying to figure out.
BRIAN GREENE: Good. So, sort of a jeopardy issue, if you know the American reference. All right, so just a quick overview. We’re going to start with some of the basics of quantum mechanics just to sort of make sure that all of us are more or less on the same page. We’ll then turn to a section on something called the “quantum measurement problem,” something weird, “quantum entanglement” as in the title of the program.
We’ll then turn to issues of black holes, spacetime, and quantum computation, which will take us right through to the end.
All right, so just to get to the basics of quantum mechanics. The story, of course, began more or less in the way that I started. We understood the world using classical physics in the early days, way back to the 1600s.
And then something happened in the early part of the 20th century, where people like – we started with Newton, of course, then we moved on to people like Max Planck, Albert Einstein. What drove the initial move into quantum physics? David?
DAVID WALLACE: I think it was really just pushing really hard at classical mechanics as it went down into the scale of atoms and the structure of atoms, and just finding that that structure snapped and broke. That trying to use classical mechanics to understand how hot things got or how electrons went around atoms without collapsing into the nucleus.
In all those places, we had a series of hints that something was amiss in our classical physics. And it took, I guess, most of 30 years for those hints to coalesce into a coherent theory. But that coherent theory then became not really just a single physical theory, but the language for writing physical theory, be it theories of particles or fields, maybe someday even gravity. And that language was more or less sort of solid by, I guess about 1930.
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.