The excerpt below is from the excellent biography Climbing the Mountain by Mehra and Milton. Milton was one of Schwinger's last Harvard grad students, eventually a professor at the University of Oklahoma. Schwinger's view is the one shared by all reasonable physicists: quantum mechanics must apply to the measuring device as well as that which is measured. Once this assumption is made (as Hawking and others have noted): many worlds follows trivially.
(p.369) Schwinger: "To me, the formalism of quantum mechanics is not just mathematics; rather it is a symbolic account of the realities of atomic measurements. That being so, no independent quantum theory of measurement is required -- it is part and parcel of the formalism.Similar views have been expressed by Feynman and Gell-Mann and by Steve Weinberg. Interestingly, this chapter in the biography seems to describe (in slightly odd language) some Schwinger work on decoherence, analyzing a collaborator's claim that Stern-Gerlach beams could be recombined coherently.
[ ... recapitulates usual von Neumann formulation: unitary evolution of wavefunction under "normal" circumstances; non-unitary collapse due to measurement ... discusses paper hypothesizing stochastic (dynamical) wavefunction collapse ... ]
In my opinion, this is a desperate attempt to solve a non-existent problem, one that flows from a false premise, namely the vN dichotomization of quantum mechanics. Surely physicists can agree that a microscopic measurement is a physical process, to be described as would any physical process, that is distinguished only by the effective irreversibility produced by amplification to the macroscopic level. ..."
See also my paper On the origin of probability in quantum mechanics.
Schwinger's precocity, explored in the biography in far greater detail than I had seen before, is overwhelming. At age 17 or so he had read everything there was to read about quantum mechanics, early field theory, nuclear and atomic physics. For example, he had read and understood Dirac's papers, had invented the interaction picture basis, had already read the Einstein, Podolsky, Rosen paper and explained it to Rabi when they first met. He met Bethe and they discussed a problem in quantum scattering (Schwinger had improved Bethe's well-known result and noticed an error that no other theorist had). Bethe later wrote that the 17 year old Schwinger's grasp of quantum electrodynamics was at least as good as his own.
Feyerabend on the giants: "... The younger generation of physicists, the Feynmans, the Schwingers, etc., may be very bright; they may be more intelligent than their predecessors, than Bohr, Einstein, Schrodinger, Boltzmann, Mach and so on. ..."Schwinger survived both Feynman and Tomonaga, with whom he shared the Nobel prize for quantum electrodynamics. He began his eulogy for Feynman: "I am the last of the triumvirate ..."