The curious concept that almost nobody seemed to care about at first

Short-lived, unobservable, and not subject to the usual rules of conservation of energy and momentum, virtual particles—an integral part of the conceptual framework of quantum field theory (QFT)—exhibit a number of curious characteristics which, in recent decades, have in part fueled important discussions about their ontological status. Central to these debates is Richard Feynman's diagrammatic technique for QFT calculations, which provided in the late 1940s the first systematized and generalized description of the concept of virtual particles. At the time, however, the curious characteristics and the ontology of the latter were the subject of little, if any, debate. This article explores how the concept of virtual particles gradually became subject to interpretative scrutiny in the post-war period. It examines the weight of various aspects of pre-Feynman developments which once guaranteed a firmer phenomenological anchoring of the scientific practices associated with the virtual particle concept. Subsequently, it shows how the questioning of this concept did not result from a simple assessment of its curious characteristics but was part of a wider critique of the new quantum electrodynamics and Feynman's methods.