Misner, Everett, Feynman

In his PhD dissertation, Charles Misner, following a suggestion from his advisor John Wheeler, formulates quantum gravity in terms of the path integral. This article has a very clear explanation for why the Hamiltonian operator in GR is zero.

Rev. Mod. Phys. 29, 497–509 (1957): Feynman Quantization of General Relativity

Of course, in this kind of formulation the "wavefunction of the universe" plays a central role, and the universe is necessarily a closed system. There is no appealing to outside "observers" for help!

Misner was a contemporary of Everett, and played a role in the development of many worlds quantum mechanics. See here for Dieter Zeh's discussion of the 1957 Chapel Hill meeting where Everett's interpretation and quantum gravity were both discussed.

Feynman presents a thought experiment in which a macroscopic mass (source for the gravitational field) is placed in a superposition state. One of the central points is necessarily whether the wavefunction describing the macroscopic system must collapse, and if so exactly when. The discussion sheds some light on Feynman's (early) thoughts on many worlds and his exposure to Everett's ideas, which apparently occurred even before their publication (see below).