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"The foundational aspects of physics during the first half of the twentieth century have been principally concerned with the characterization of the 'elementary' constituents of matter and the elucidation of the nature of the space-time framework in which their interactions take place. The discovery  of the electron by [Joseph John] Thomson, the precise characterization of its charge by Millikan, the demonstration of the nuclear atom by Rutherford, the photon hypothesis of Planck and Einstein, and Bohr's explanation of the spectrum of hydrogen were some of the landmarks of that history. These early efforts culminated in the mid-twenties with the formulation of quantum mechanics by Heisenberg, Dirac, and Schrödinger (Kuhn 1978; Heilbron 1975; Segrè 1980; Pais 1986; Mehra and Rechenberg 1982-1988). Our brackets to avoid potential confusion. Lord Kelvin's name was William Thomson.
"The revolutionary achievements in the period from 1925 to 1927 stemmed from the confluence of a theoretical understanding (the description of the dynamics of microscopic particles by quantum mechanics), and the apperception of an approximately stable ontology (electrons and nuclei). Approximately stable meant that these particles (electrons and nuclei), the building blocks of the entities (atoms, molecules, simple solids) that populated the domain that was being carved out, could be treated as ahistoric objects (whose physical characteristics were seemingly independent of their mode of production and whose lifetimes could be considered as essentially infinite). These entities could be assumed to be 'elementary' pointlike objects that were specified by their mass, spin, and statistics (whether bosons or fermions), and by electromagnetic properties such as their charge and magnetic moment." (Our bold.)
Paragraphs 2 & 3 from Silvan S. Schweber's