Quantum field theory is the basic tool to understand the physics of the elementary constituents of matter. … It is both a very powerful and a very precise framework: using it we can describe physical processes in a range of energies going from the few millions electrovolts typical of nuclear physics to the thousands of billions of the Large Hadron Collider (LHC). And all this with astonishing precision.

Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory.

There is a hope, which I nurse but I don't see being realized, that eventually we'll find that quantum mechanics as we know it now is just an approximation ...

String theory has a close relationship to quantum field theory — indeed in some sense it encompasses all interesting quantum field theories.

The mathematical framework of quantum theory has passed countless successful tests and is now universally accepted as a consistent and accurate description of all atomic phenomena. The verbal interpretation, on the other hand – i.e., the metaphysics of quantum theory – is on far less solid ground. In fact, in more than forty years physicists have not been able to provide a clear metaphysical model.

My own conclusion is that today there is no interpretation of quantum mechanics that does not have serious flaws. This view is not universally shared. Indeed, many physicists are satisfied with their own interpretation of quantum mechanics. But different physicists are satisfied with different interpretations. In my view, we ought to take seriously the possibility of finding some more satisfactory other theory, to which quantum mechanics is only a good approximation.

Well, we haven’t learnt yet to live together peacefully... But I don’t know what progress really means. Anyway, I think we need to have faith in the fact that there is more out there to be explained. Even the paradigms that we now have, including subjective value theory, for example, are only provisional. Some physicist might believe that ultimately, we will be able to explain everything. To me, that is utterly stupid, just like saying that an atheist is equally dogmatic as a Texas Baptist. It seems to me that, if you accept evolution, you can still not expect your dog to get up and start talking German. And that’s because your dog is not genetically programmed to do that. We are human animals, and we are equally bound. There are whole realms of discourse out there that we cannot reach, by definition. There are always going to be limits beyond which we cannot go. Knowing that they are there, you can always hope to move a little closer – but that’s all.

When you look at a vacuum in a quantum theory of fields, it isn't exactly nothing.

Surely, after 62 years, we should have an exact formulation of some serious part of quantum mechanics? By 'exact' I do not of course mean 'exactly true'. I mean only that the theory should be fully formulated in mathematical terms, with nothing left to the discretion of the theoretical physicist . . . until workable approximations are needed in applications. By 'serious' I mean that some substantial fragment of physics should be covered. Nonrelativistic 'particle' quantum mechanics, perhaps with the inclusion of the electromagnetic field and a cut-off interaction, is serious enough.

I expect that mathematicians have classified such s. Certainly they have been much used by physicists. But is there not something to be said for the approach of Euclid? Even now that we know that is (in some sense) not quite true? Is it not good to know what follows from what, even if it is not necessarily FAPP? Suppose for example that quantum mechanics were found to resist precise formulation. Suppose that when formulation beyond FAPP was attempted, we find an unmovable finger obstinately pointing outside the subject, to the mind of the observer, to the Hindu scriptures, to God, or even only Gravitation? Would that not be very, very interesting?

Deterministic underlying theories for QM are possible. They are still difficult to construct, but simple "toy models" are possible. These models are not good enough to replace today's existing quantum theories.

So what happens to the effective field theories of electroweak, strong, and gravitational interactions at energies of order 10¹⁵–10¹⁸ GeV? I know of only two plausible alternatives. One possibility is that the theory remains a quantum field theory, but one in which the finite or infinite number of renormalized couplings do not run off to infinity with increasing energy, but hit a fixed point of the renormalizable group equations. ... The other possibility, which I have to admit is a priori more likely, is that at very high energy we will run into really new physics, not describable in terms of a quantum field theory. I think that by far the most likely possibility is that this will be something like a string theory.

Nowadays, however, we are much more aware of the fact that the best proof in the world is worth no more than its premises: every scientific theory is transitory and provisional, in wait for a better one, and accepted only as long as the experimental results conform to its predictions.

I thought... I had a pretty good idea for what the structure of this... theory that's underneath space and time and so on might be like. ...I thought, "Gosh, in my lifetime... we might be able to figure out what happens in the first 10^-100 seconds of the universe. ...It's pretty far from anything that we can see today and it would be ...hard to test for what's right ...To my huge surprise, although it should have been obvious, ...we managed to get unbelievably much further than that. ...It turns out that even though there's this ...bed of computational irreducibility that ...all these simple rules run into, ...there are ...certain pieces of computational reducibility that ...generically occur for large classes of these rules, and... the big pieces of computational reducibility are ...the pillars of 20th century physics. That's the amazing thing, that general relativity and quantum field theory... turn out to be precisely the stuff you can say. There's a lot you can't say... at this... irreducible level where you.. don't... know what's going to happen. You have to run it [and] you can't run it within our universe... The things you can say turn out to be, very beautifully, exactly the structure that was found in 20th century physics...