The paper reconstructs Karl Popper’s little-studied early engagement with quantum mechanics between 1934 and 1936.

It shows that, long before his mature philosophy of science, Popper sought to “beat” Heisenberg’s uncertainty relations by designing a thought experiment based on particle collisions. He believed one could reconstruct an electron’s past trajectory and, using momentum conservation, predict another particle’s position and momentum with arbitrary precision—thus separating “preparation” from “measurement.” Drawing on unpublished correspondence with Weisskopf, Heisenberg, von Weizsäcker, and Einstein, the study reveals how Popper misunderstood the very link between these notions (the “coupling hypothesis”) that defines quantum mechanics itself. The author argues that Popper’s youthful “gross mistake” was neither the origin of the 1935 EPR paper nor an anticipation of later ideas about preparation and measurement, but rather a misreading of the 1931 Einstein–Tolman–Podolsky setup—an instructive episode illustrating how Popper’s realism collided with the conceptual foundations of quantum theory.

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