When did particles become “indistinguishable”? Einstein, Schrödinger, Heisenberg, Dirac, and the interpretive flexibility of mathematical-theoretical apparatus in the emergence of quantum statistics

Quantum physics has changed the concept of particles profoundly. This change—commonly summarized with the formula, “particles have become indistinguishable”—did not stem from a deliberate theoretical choice or a single experimental finding. It was, rather, the result of prolonged efforts to arrive at a unified interpretation of the mathematical-theoretical apparatus of the quantum statistics and the quantum mechanics of multi-particle systems. Several interpretations were sketched in the 1920s and 1930s, as the possibilities, limits, and conditions of the quantum theoretical technologies were probed on different physical systems in different contexts of theoretical and experimental practice. This paper examines the first formulation and interpretation of quantum statistics by Albert Einstein, and relates it to the early interpretative efforts of Edwin Schrödinger, Werner Heisenberg, and Paul Dirac, showing how professional cultures and interactions shaped the diverse views of these pioneers.