This article discusses the meaning of the notion of virtuality in modern physics. To this end, it develops considerations on the introduction and establishment in nuclear physics of two independent concepts at the turn of the 1920s and 1930s: that of the virtual state, used in the context of neutron scattering studies, and that of the virtual transition, useful for the theoretical understanding of strong nuclear forces, which forms the basis of what are now called virtual particles. Their comparative analysis (...) highlights the theoretical nature of virtual entities and processes in modern physics. It also shows how the virtual has been associated with various purely physical attributes, leading to a form of polysemy of the term, from the beginning of the application of these concepts. (shrink)

In order to formulate a fundamental quantum field theory of nuclear forces that explains their strength, range, and exchange character, while at the same time accounting for the weak β-decay interaction, Hideki Yukawa introduced a new kind of quantum field. In contrast to the real field of quantum electrodynamics , which he took as his model, Yukawa's U-field was complex, and in contrast to the neutral massless photon of QED, the U-field's ‘heavy’ quanta were charged, carrying the electronic charge . (...) The theory was proposed in November 1934 and published a few months later; however, its advantages were ignored, and for more than two years it went unnoticed, probably because there was no direct experimental evidence for the existence of U-quanta. (shrink)