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"Feynman 28 "sum-over-histories" a theory of everything?
P.250 (Gleick) "Feynman's path-integral view of nature, his vision of a sum
over histories" was also the principle of least action, the principle of
least time, reborn.  Feynman felt that he had uncovered the deep laws that
gave rise to the centuries-old principles of mechanics and optics... How
does a thrown ball know how to find the particular arc whose path minimizes
action?  How does a ray of light know how to find the path that minimizes
time?  .... These paths .... are special because they dend to be where the
contributions of nearby paths are most closely in phase and most reinforce
one another.  Far from the path of least time - at the distant edge of a
mirror, for example, paths tend to cancel on another out.  Yet light does
take every possible path ... The seemingly irrelevant paths are always
lurking in the background, making their contributions, ready to make their
presence felt in such phenomena as mirages and diffraction gratings.

.... Feynman was - with finality - eliminating the wave viewpoint
altogether.  Waviness was built into the phases carried by amplitudes, like
little clocks. .... The actors were, more clearly than ever, particles ....

p.264  Feynman cared far less than Dyson about his still-patchwork scheme
for renormalizing quantum electrodynamics.  It was his sum-over-histories
theory of physics that claimed his passion.  As Dyson saw it, it was a
grand vision and a unifying vision - too ambitious ...

'It was essential to his view of things that it must be universal.  It must
describe everything that happens in nature.  You could not imagine the sum-
over-histories picture being true for part of nature and untrue for another
part.  You could not imagine it being true for electrons and untrue for
gravity. It was a unifying principle that would either explain everything
or explain nothing.