Virtual processes and virtual particles: real or fictitious?

Abigail Beall

Nicolò Cangiotti, Gianni Arioli, Giovanni Valente

Abstract In this paper we aim to develop an indispensability argument in support of the existence of virtual particles in scattering processes. In order to avoid the Paradox of Infinite Limits, which allegedly poses a challenge to scientific realism, one needs to de-idealize the fictitious systems introduced by the two limiting procedures employed in the perturbation scheme, namely the infinite expansion in Dyson series and the limits for negative and positive infinite times associated with the assumption of free particles. We show that these limits do not introduce essential idealizations, in agreement with scientific realism. What is more, according to our argument, unobservable virtual particles arise as essential approximations and they should be interpreted as propagators of the interaction responsible for subatomic scattering. As such, their existence is based on the use of approximations that matter.

Xu Wang

We apply a recently emerging "virtual detector" method to general free-particle wave packets. We discuss what is actually detected by a virtual detector and give analytical expressions in both the real space and the momentum space. We prove that in the asymptotic limit when the time difference between wave packet creation and detection is large, resembling real detection processes, the obtained differential momentum distribution is equivalent to Fourier transforming the free-particle wave packets. We further consider the situation with the presence of laser fields.

Günter Nimtz, Horst Aichmann

Feynman introduced virtual particles in his diagrams as intermediate states of an interaction process. They represent necessary intermediate states between observable real states. Such virtual particles were introduced to describe the interaction process between an electron and a positron and for much more complicated interaction processes. Other candidates for virtual particles are evanescent modes in optics and in elastic fields. Evanescent modes have a purely imaginary wave number, they represent the mathematical analogy of the tunneling solutions of the Schrödinger equation. Evanescent modes exist in the forbidden frequency bands of a photonic lattice and in undersized wave guides, for instance. The most prominent example for the occurrence of evanescent modes is the frustrated total internal reflection (FTIR) at double prisms. Evanescent modes and tunneling lie outside the bounds of the special theory of relativity. They can cause faster than light (FTL) signal velocities. We present examples of the quantum mechanical behavior of evanescent photons and phonons at a macroscopic scale. The evanescent modes of photons are described by virtual particles as predicted by former QED calculations.