This paper will not present a case study of the historical development of a virtual entity. Rather, I will develop an outlook on virtual entities in the sciences and propose a corresponding method for studying them (historically). In essence, my presentation can be considered a synthesis of different observations from the history and philosophy of science and has its roots in my dissertational research on the development of the virtual particle. Starting with a reflection on the role of presentism for (...) the study of concept formation and development processes, I will show, through the example of the virtual particle, how current debates and interpretations can inform our access to a historical reconstruction. Following these reflections, I will argue for a pragmatist account of concepts as tools for the scientific practitioners. According to the approach presented in my article, concepts perform their functions through representations, and I will lay special focus on verbal representations and their different functions within scientific reasoning. In conclusion, I will frame the outcome of my discussion in terms of a proposal that might, through further research, enrich our understanding of virtual entities in the sciences. (shrink)

In 1927, Paul Dirac first explicitly introduced the idea that electrodynamical processes can be evaluated by decomposing them into virtual (modern terminology), energy non-conserving subprocesses. This mode of reasoning structured a lot of the perturbative evaluations of quantum electrodynamics during the 1930s. Although the physical picture connected to Feynman diagrams is no longer based on energy non-conserving transitions but on off-shell particles, emission and absorption subprocesses still remain their fundamental constituents. This article will access the introduction and the initial reception (...) of this picture of subsequent transitions (PST) by conceiving of concepts, models, and their representations as tools for the practitioners. I will argue for a multi-factorial explanation of Dirac’s initial, verbally explicit introduction: the mathematical representation he had developed was highly suggestive and already partly conceptualized; Dirac was philosophical flexible enough to talk about transitions when no actual transitions, according to the general interpretation of quantum mechanics of the time, occurred; and, importantly, Dirac eventually used the verbal exposition in the same paper in which he introduced it. The direct impact of PST on the conception of quantum electrodynamical processes will be exemplified by its reflection in diagrammatical representations. The study of the diverging ontological commitments towards PST immediately after its introduction opens up the prehistory of a philosophical debate that stretches out into the present: the dispute about the representational and ontological status of the physical picture connected to the evaluation of the perturbative series of QED and QFT. (shrink)