Interesting Q someone asked: quantum electrodynamics has an obvious classical theory it corresponds to, but quantum chromodynamics and electroweak theory don’t—why’s that? Possible answer: there’s electromagnetism in daily life, but no strong or electroweak force
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Replying to @litgenstein
There definitely is a classical Lagrangian for those theories. It’s just that quantum effects are very important in the presence of confinement and SSB, not so much when everything’s perturbative.
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Replying to @seanmcarroll
Good stuff! Do you think the classical lagrangian is unique? Or that it depends on how you take the classical limit?
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Replying to @litgenstein @seanmcarroll
In non perturbative QFTs there actually exist observables for which we don't have Lagrangians .. Do you mean unique modulo gauge symmetries & conserved global symmetries? Even the Dirac equation gives the same physics depending on if you use +-m
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Replying to @InertialObservr @seanmcarroll
The uniqueness of the lagrangian, wholesale, is what I had in mind! Weinberg and friends made arguments for example about the uniqueness of GR’s lagrangian and that for other fields (conditional of course on the exclusion of things like higher derivatives)
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Yea, i think it's a super interesting question. The restriction to being gauge symmetric and Renormalizable + Poincare invariant really restricts what you can write down.
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