In '63…'64 I worked on trying to understand quantum mechanics, and I brought in Felix Villars and for a while some comments... there were some comments by Dick Feynman who was nearby. And we all agreed on a rough understanding of quantum mechanics and the second law of thermodynamics and so on and so on, that was not really very different from what I'd been working on in the last ten or fifteen years. I was not aware, and I don't think Felix was aware either, of the work of Everett when he was a graduate student at Princeton and worked on this, what some people have called 'many worlds' idea, suggested more or less by Wheeler. Apparently Everett was, as we learned at the Massagon [sic] meeting, Everett was an interesting person. He… it wasn't that he was passionately interested in quantum mechanics; he just liked to solve problems, and trying to improve the understanding of quantum mechanics was just one problem that he happened to look at. He spent most of the rest of his life working for the Weapon System Evaluation Group in Washington, WSEG, on military problems. Apparently he didn't care much as long as he could solve some interesting problems! Anyway, I didn't know about Everett's work so we discovered our interpretation independent of Everett. Now maybe Feynman knew about… about Everett's work and when he was commenting maybe he was drawing upon his knowledge of Everett, I have no idea, but… but certainly Felix and I didn't know about it, so we recreated something related to it. Now, as interpreted by some people, Everett's work has two peculiar features: one is that this talk about many worlds and equally… many worlds equally real, which has confused a lot of people, including some very scholarly students of quantum mechanics. What does it mean, 'equally real'? It doesn't really have any useful meaning. What the people mean is that there are many histories of the… many alternative histories of the universe, many alternative course-grained, decoherent histories of the universe, and the theory treats them all on an equal footing, except for their probabilities. Now if that's what you mean by equally real, okay, but that's all it means; that the theory treats them on an equal footing apart from their probabilities. Which one actually happens in our experience, is a different matter and it's determined only probabilistically. Anyway, there's considerable continuity between the thoughts of '63-'64 and the thoughts that, and… and maybe earlier in the ‘60s, and the thoughts that Jim Hartle and I have had more recently, starting around '84-'85.

In the late '80s, Bob Griffiths and Roland Omnes, independently, worked on decoherent histories and they published the idea before Jim and I did. In fact I'd never written anything about quantum mechanics until the very end of the 1980s. But I'd thought a lot about it, and the thoughts, as I say, were sort of continuous. Jim Hartle and I have worked on decoherent histories with a particular point of view. Some people have tried to get the minimum conditions for consistency allowing probabilities to be assigned to course-grained histories, so they get a weaker and weaker form of decoherence. We have been interested in finding a stronger and stronger form of decoherence, trying to appreciate what actually happens in the real world where decoherence is far from minimal. The actual decoherent histories with which one conventionally deals are decoherent in a very strong manner, and it's that strong decoherence that we've tried to describe, approach and so on. But now if you consider a set of decoherent, course-grained, alternative histories of the universe, given the two fundamental principles; the unified theory of all the particles and forces, and the initial condition, initial density matrix or a wave function, then… the… let me see, what was I going to say… if… if you're given all that, there's a question of… of which set of decoherent histories you use. And we like to talk about a realm, which is a set of alternative decoherent histories subject to some condition. Usually it's a kind of maximality condition so that you… if you were to fine-grain further you would start to lose the… the decoherence, something like that; or if you fine-grained further you would start to lose either the decoherence or some other desirable property, and such a system we call a realm.

Quantum mechanics is protean, in the sense that you can keep changing the representation, in fact at every time you can change the representation, and the representation has a huge freedom in change of representation, in change of basis. Besides that, even if you're given the basis at every single time, there's a huge freedom of coarse-graining. So the number of possible realms is gigantic, and yet we seem to use almost always a realm that can be called hydrodynamic. It’s described by ranges of values of operators, which are integrals of conserved or almost conserved densities over small volumes of space, and spaced at small intervals of time. And the volumes of space are chosen to be large enough for some kind of internal equilibrium to occur, but small enough to, well, to allow the realm to be maximal; that would be one way to say it.

Now, what properties this usual quasi-classical realm has, so that everything we know about uses it, is one thing we've tried to understand. Quasi, by quasi-classical we mean that these variables very crudely obey classical equations over considerable intervals of time, interrupted constantly by small fluctuations and occasionally by big branchings, big probabilistic branchings. So we are concerned with what makes a realm quasi-classical; why this particular quasi-classical realm seems to play an important role; is there perhaps some fundamental restriction on the representation of quantum mechanics, or ithe transformation theory is really only approximate? One great virtue of our method is that it allows treating, as Hartle has shown in some papers, it allows treating the general relativistic situation, that is a situation in which gravitation as well as all other fields is… is quantized and there are huge quantum variations in the metric. Under those conditions it's very difficult to define a sequence of time slices so that the usual Schrödinger approach to quantum mechanics can be implemented. But this approach, this sum over histories approach, where the history's coarse-grained enough… coarse-grained enough to be strongly decoherent—that works. And the other one may be impossible actually, may not be possible to formulate quantum mechanics any other way. This may be a slight generalization of quantum mechanics. When I wrote an article for the Feynman memorial issue of Physics Today I mentioned that, that Richard was always upset because he felt he had done mostly mathematical work on theories that had been proposed by other people and he wanted to do a fundamental theory of his own. And maybe it will turn out that the sum over histories work actually is a fundamental generalization of quantum mechanics that's necessary, rather than simply a reformulation of quantum mechanics. And our… our work may tend in that direction.

[q] Do you think–just on a more practical, pedagogical level—that the traditional way of teaching quantum mechanics, like introducing students to quantum mechanics, should evolve into teaching it more from the sum over histories viewpoint?

Well the… not only the sum over histories viewpoint, but the decoherent histories viewpoint. Well, when the point of view is perfected, which may be very soon, I think that's true. The reason is that although the so-called Copenhagen interpretation is perfectly correct for all laboratory physics, laboratory experiments and so on, it's too special otherwise to be fundamental and it sort of strains credulity. It's… it’s not a convincing fundamental presentation, correct though… though it is, and as far as quantum cosmology is concerned it's hopeless. We were just say ing, we were just quoting that old saw: describe the universe and give three examples. Well, to apply the… the Copenhagen interpretation to quantum cosmology, you'd need a physicist outside the universe making repeated experiments, preferably on multiple copies of the universe and so on and so on. It's absurd. Clearly there is a definition to things happening independent of human observers. So I think that as this point of view is perfected it should be included in… in teaching fairly early, so that students aren't convinced that in order to understand quantum mechanics deeply they have to swallow some of this…very… some of these things that are very difficult to believe. But in the end of course, one can use the Copenhagen interpretations.

[q] Among some of the leading practitioners, Witten, Susskind, and so on, there is this vision that 'a revolution has taken place that is bigger than quantum mechanics', quote…

What is the revolution?

[q] Whatever M-theory is, is somehow going to be bigger than quantum mechanics. That it’s going to change even the basics of fundamental physics. Have you been involved in, paid any attention to this?

Well, there is some sense in which the co-ordinate operators for the target space are represented by non-commuting quantities, matrices. Now you can still define commuting quantities which also enter, but there are non-commuting quantities that enter, and so on and so forth. So there're many very interesting features which will surely lead to a lot of re-formulations of... and possible interpretations... of fundamental theory, but I don't know what they mean by ‘bigger than quantum mechanics’; I don't understand that. Could be though that when that theory is clarified and… it will help with the clarification of the interpretation of quantum mechanics. That's conceivable. I don't quite see how, but it might happen.