  It is not very well known that the philosopher Karl Popper has been one of
the foremost critics of the orthodox interpretation of quantum physics for
about six decades. This paper reconstructs in detail most of Popper's
activities on foundations of quantum mechanics (FQM) in the period of 1950s and
1960s, when his involvement in the community of quantum physicists became
extensive and quite influential. Thanks to unpublished documents and
correspondence, it is now possible to shed new light on Popper's central -
though neglected - role in this "thought collective" of physicists concerned
with FQM, and on the intellectual relationships that Popper established in this
context with some of the protagonists of the debate over quantum physics (such
as David Bohm, Alfed Land\'e and Henry Margenau, among many others).
Foundations of quantum mechanics represented in those years also the initial
ground for the embittering controversy between Popper and perhaps his most
notable former student, Paul Feyerabend. I present here novel elements to
further understand the origin of their troubled relationship.
