Session 8 continued with some further comments on the quantization of gravity. To- ward the end of the conference (in the final Session 9), John Wheeler mentioned that
there exists another proposal that there is one "universal wave function". This function has already been discussed by Everett, and it might be easier to look for this "universal wave function" than to look for all the propagators.
We now understand that, for most systems, separate propagators would not even exist because of their unavoidable entanglement (which locally leads to decoherence).
Feynman said that the concept of a "universal wave function" has serious conceptual difficulties. This is so since this function must contain amplitudes for all possible worlds depending on all quantum-mechanical possibilities in the past and thus one is forced to believe in the equal reality [sic!] of an infinity of possible worlds.
Well said – although we may restrict ourselves in practice to an individual autonomous branch of this wave function ("our world"). The total number of such branches must indeed be extremely large, but it need not be infinite, since coherence lengths for continuous variables never vanish exactly. Reality is conceptually difficult and complex, and we should not be sur- prised that it seems to go far beyond what we will ever be able to observe. But Feynman is not ready to draw this ultimate conclusion from the superposition principle that he always de- fended during this discussion. Why should a superposition not be maintained even when it includes an observer? Why “is” there not an amplitude for me (and you) observing this and an amplitude for me (and you) observing that in a quantum measurement – just as it would be required by the Schrödinger equation for a gravitational field?