The question of whether virtual quantum particles exist is considered here in light of previous critical analysis and under the assumption that there are particles in the world as described by quantum field theory. The relationship of the classification of particles to quantum-field-theoretic calculations and the diagrammatic aids that are often used in them is clarified. It is pointed out that the distinction between virtual particles and others and, therefore, judgments regarding their reality have been made on basis of these methods rather than on their physical characteristics. As such, it has obscured the question of their existence. It is here argued that the most influential arguments against the existence of virtual particles but not other particles fail because they either are arguments against the existence of particles in general rather than virtual particles per se, or are dependent on the imposition of classical intuitions on quantum systems, or are simply beside the point. Several reasons are then provided for considering virtual particles real, such as their descriptive, explanatory, and predictive value, and a clearer characterization of virtuality—one in terms of intermediate states—that also applies beyond perturbation theory is provided. It is also pointed out that in the role of force mediators, they serve to preclude action-at-a-distance between interacting particles. For these reasons, it is concluded that virtual particles are as real as other quantum particles. [ 9 ] (Section 1.3) and the comments of Steven Weinberg near the end of the following section). 2. The Real/Virtual Distinction and its Limits The distinction between virtual quanta and other quanta is standardly made in a very specific way: the virtual particles are those that appear as “internal lines” of the Feynman diagrams; and any particle not eventually It is also sometimes, though less often, taken instead as the distinguishing characteristic of a virtual particle that it be “off mass-shell”, that is, that it have a mass not corresponding to the value standardly attributed to the corresponding quantum and, so, be a virtual version of that quantum. This may or may not coincide with its being symbolized by an internal line in perturbation-theoretical diagrams and is also a consequence of electing a particular mathematical method of analysis, that of Feynman himself—this is discussed near the end of Section 5 , below. Alternatively, the mass can be taken to be fixed at its standard value, the corresponding energy-momentum relation— E 2 = p 2 + m 2 , where p is momentum, m is mass, and c = 1 —may be considered not to be satisfied at the diagrammatic vertices, as discussed in greater detail in Section 5 , below, instead of enforcing this relation. (It is because this equation is that of a sphere that particles not satisfying it are considered off shell, that is, off that sphere). It is, therefore, sometimes instead (incorrectly) concluded that virtual particles violate the conservation of energy. The consideration of quanta as off mass-shell, like the perturbation-theoretical calculations in which it arises, is therefore a matter of the choice of method of analysis of interaction and, more importantly, neither of the above alternatives requires the rejection of virtual quanta as quanta, that is, to consider them not to exist in the sense I am arguing here that they do. The most influential argument against the reality of virtual particles, which has come to be known as the “superposition argument”, is that virtual particles do not exist because neither the number nor kinds of virtual particles are determinate during scattering processes as modeled by quantum field theory—cf. The following commentary from Steven Weinberg’s 1977 article “A Search for Unity: Notes for a History of Quantum Field Theory” indicates the significance of the introduction of virtual particles by quantum field theory and addresses this as well as the issue of energy conservation: “It is important to understand that quantum field theory gave rise to a new view not only of particles but also of the forces among them. We can think of two charged particles interacting at a distance not by creating classical electromagnetic fields which act on one another, but by exchanging photons, which continually pass from one particle to the other. Thus, all observed phenomena of particle physics can, within the constraints of such approximations, be related to the quantum fields appearing in theory through experiments involving standard preparation and detection procedures and will involve (virtual quanta of) force-mediating fields. However, for example, analyses of effects involving collections of virtual particles giving rise to mass- and charge-value corrections—in which there are an infinite number of perturbation-theoretical contributions wherein more than one of them contributes significantly—are far less susceptible to such intuitive, quasi-physical misinterpretation but still involve intermediate quantum states including such quanta. The virtual particle mass becomes a variable of integration (cf., [ 34 ]) and the particle is said to be “off mass-shell,” which as mentioned previously is for some an alternative indication of virtuality; any difference of mass of virtual particles from standard values in intermediate states is not physically problematic but only requires caution against incorrectly attributing the free-state values of masses given to particles while they are in those short-lived states. Under this more objective characterization, virtual particles are as real as all others. One may note also that one method of non-perturbative field theory, lattice quantum chromodynamics, treats virtual particles differently from others, as “links” between sites of a discrete space-time lattice at which interacting particles are located, (in the limit taken) infinitesimally closely without a finite interval in which to propagate.