Note also that because the “quantum field” is operator-valued, calling a field “excited” is short hand to indicate that the state in question is such that energy is added. (As discussed below, a good criterion for holding that individual instances of particles, as understood here, are non-present is that the corresponding field mode has zero probability of having been appropriately excited. The powerful arguments against the reduction of fundamental physics to that of particles in the traditional sense of that notion, rather than fields, are not addressed here see for discussion of their status as a class. Nonetheless, it has been argued that virtual particles do not exist even when the other sort, the so-called ordinary particles, do exist. Note also that as a result, quarks and the gluons that bind them do not correspond to “poles”, unlike other non-virtual particles). Indeed, quarks are widely regarded as fundamental building blocks of matter of which hadrons are (mainly) constituted, are believed to be confined within those composite particles, and have never appeared (at least convincingly) in isolation. (See for a discussion of the sense in which quarks can be observed). For example, in particle physics, the existence of quarks is rarely questioned, at least not vigorously, even though are not so observed. However, being directly observed free is not a necessary condition for existence. The fact that the particles labeled virtual are those not later directly observed in the already very limited sense that other particles are said to be is sometimes in itself taken as a ground for skepticism toward their existence. The particles that are in practice (relatively) directly prepared, detected and thereby observed are, in standard analyses, reducible to fields taken to be free in the initial and the final, large-time limits this is not so for the virtual particles, which are considered to mediate interactions between other particles taking place between these time limits when none of the particles involved can be accurately considered free. The standard distinction between virtual and other particles is made within a particular approach-that of Feynman diagrams-to the calculation of perturbation-theoretical quantities that are only indirectly related to the observed events to which they may, but need not be applied. Even though virtual particles have been and continue to be widely used in descriptions, explanations, and predictions of fundamental physical phenomena at subatomic scales, the reality of the virtual particles as distinguished from free particles has been the subject of an ongoing debate leaning toward their dismissal-cf. The notion of the virtual particle arises in the context of relativistic quantum field theory where elementary particles are considered quanta associated with fields characterized via appropriate Hamiltonian or Lagrangian functions. For these reasons, it is concluded that virtual particles are as real as other quantum particles. It is also pointed out that in the role of force mediators, they serve to preclude action-at-a-distance between interacting particles. 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 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. As such, it has obscured the question of their existence. 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. The relationship of the classification of particles to quantum-field-theoretic calculations and the diagrammatic aids that are often used in them is clarified. 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.
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