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Around the 1950s and 1960s there was a lot of work in particle physics motivated by two issues - finding a workable quantum field model for the weak interaction, and finding a model that unified the weak and electromagnetic interactions.

I can easily see why the former was of interest (EM had a classical field explanation since Maxwell, and QED was a recent and stunning success, so the implication that a quantum field theory model would exist for the weak and EM interactions was clear).

But why was there any belief or expectation (beyond "elegance" and mysticism) to believe that EM and W were the two facets of a single unified interaction? (Other than in the broadest sense that everything is a facet of some grand theory of everything: this seems much more specific).

After all, the only unified interactions at that point were electricity and magnetism, both of which could be observed directly to create effects of the other type, and had been unified long ago. But as far as I'm aware there was no "smoking gun" that did the same and made clear that these were two sides of a unified "electroweak" whole, and at that point no other forces had been unified to suggest this was common. If anything, practical unification dreams had failed - gravity hadn't been reconciled even with QM, let alone with any of the 3 known other forces.

So what led people to such a belief that the 60's gave rise to multiple theoreticians who judged their theories based on whether unification 'worked', or who used that as a guide or outcome?

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  • $\begingroup$ The EM interactions are based on the U(1) symmetry. The weak interactions are based on the SU(1) symmetry. It seems natural to wonder if both can be combined under the SU(1)xU(1) symmetry. Has this been successful? Time will tell. The Higgs mechanism has not been experimentally confirmed yet. Is every photon in the universe a superposition of two bosons? Perhaps, but it makes no intuitive sense. $\endgroup$ – safesphere May 30 '18 at 15:40
  • $\begingroup$ If you don't get an answer here in a few days, you may want to consider deleting it here and posting it on History of Science and Mathematics. $\endgroup$ – Kyle Kanos May 30 '18 at 16:56
  • $\begingroup$ @safesphere - point of information, were those both well known before the 1950/1960s? $\endgroup$ – Stilez May 30 '18 at 18:19
  • $\begingroup$ I'm not a historian, but I recall reading a book on electroweak theory. It was developed by combining the electromagnetic and weak theories, which were based on these symmetries, so I assume both symmetries were well known before the electroweak theory was developed. $\endgroup$ – safesphere May 30 '18 at 18:24
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By pure chance I just found an answer, in the Wikipedia article on weak neutral currents.

It led to a 1958 paper by J. Leite Lopes (Brazil) (and a 40 years 'looking back' paper dated 1999) concerning the reasons why he suspected electroweak to be a single force. I don't fully understand the detail, but it's related to neutrino interactions and the role of weak interaction bosons, and their somewhat analogous nature to photons, and it's stated to be a direct reason/cause why the electroweak model was later developed.

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