Quantization, String Theory

[alanforr]

@lmf

Goal: Generally, I want to determine to what extent the ideas about physics prevalent in the string theory community (which I will define as anyone who regularly posts their papers on the hep-th arxiv) are bad. More specifically, I want to determine whether or not the centrality of quantization to the way that these physicists think about QFT (quantum field theory) is fundamentally in conflict with MWI (many worlds interpretation) or the lessons of quantum mechanics, as David Deutsch and Alan seem to think.

The quote I gave initially was [1, 2 Chapter 18]:

There’s a related and even worse spanner in the works of elementary particle physics: particles (or fields, strings, or whatever) are supposed to be fully quantum-mechanical entities. But the people who work on them only ever construct classical, single-universe theories. Why? Because they think that the quantum part of the theory necessarily has to be trivial. It is assumed that in order to discover the true quantum-dynamical equations of the world, you have to enact a certain ritual. First you have to invent a theory that you know to be false, using a traditional formalism and laws that were refuted a century ago. Then you subject this theory to a formal process known as quantization (which for these purposes includes renormalization). And that’s supposed to be your quantum theory: a classical ghost in a tacked-on quantum shell.

@lmf

The paper is quite technical, and it’s using a formalism that I’m not familiar with. A specific problem I have is that I don’t know of any examples of quantum theories that can be derived by using this “quantum principle of stationary action” (I’ve only encountered the classical principle of stationary action before). Deutsch doesn’t give any concrete examples in the paper. He cites some other papers which I assume do have more examples, and maybe you (Alan) could recommend one of them to me? (Keep in mind that there’s no references section in the paper you linked, so DD’s citations might be ambiguous; I haven’t checked).

One of the papers cited is this Quantum theory of gravity. II. The manifestly covariant theory from by DeWitt, see especially Section 2. Another paper cited is The commutation laws of relativistic field theory by Peierls.

DeWitt 1965 is a book called “Dynamical Theory of Groups and Fields” by Bryce S DeWitt.

@lmf

You (Alan) showed this paper to me earlier because (I presume) you think that if I had a correct understanding of what Deutsch meant by “fully quantum mechanical entity,” I would see that the criticisms of Deutsch’s view of quantization that I made here are invalid. Where in this paper do you think DD defines what a fully quantum mechanical entity is? I don’t think he does, but it’s not a searchable PDF so I’m not sure. Alternatively, could you maybe define the term yourself?

Section 1 and the first four paragraphs of Section 2 explain the problem that DD is addressing. The way I would state the problem is that a fully quantum mechanical theory would not include any quantities that are supposed to be measurable that are not represented by quantum observables, since all such quantities are actually quantum mechanical.