Natural Order and Science

[Harris]

In the Quantum Field Theory view, actual particles are viewed as being detectable excitations of underlying Quantum Fields. Virtual Particles are also viewed as excitations of the underlying fields, but appear only as forces, not as detectable particles. They are "temporary" in the sense that they appear in calculations, but are not detected as single particles. Thus, in mathematical terms, they never appear as indices to the scattering matrix, which is to say, they never appear as the observable inputs and outputs of the physical process being modelled.

I would like to come back to this statement, in which you seem to trace back the reason why particles and virtual particles have a different "ontological status" (one really exists, the other is an artefact of calculational method) to the fact that one appears as scattering matrix-elements, whereas the others don't. Since you are eager to discuss S-matrix theory, you must be aware that this distinction is problematic because

1. S-Matrix elements are infra-red divergent due to soft emission, and only lead to physically meaningful results if they are combined with virtual corrections in scattering cross-sections. From this it appears to me that real emission of particles alone isn't even defined, and that the distinction between the two classes of particles based on arguments from S-Matrix theory is problematic. What's your perspective?

2. The S-Matrix is a somewhat artificial construct obtained by assuming that ingoing and outgoing particles stop interacting, and then taking the time to +/- infinity and using the resulting asymptotic free states as "external particles". This works great when describing scattering processes in which isolated, massive and stable particles meet and part again, but if you depart from this idealized scenario, things like 1.) happen. Also, if you just look at the unitary time evolution operator without  taking it to asymptotic times, the distinction between what's an S-Matrix index and what isn't surely isn't as clear-cut any more.

Do you disagree? Or do you subscribe to an axiomatic S-Matrix approach to particle physics in which there is no local Lagrangian and no time evolution? Because that worked out so well?

Moderator Notice
Unbelievable. Content of post removed due to plagiarism, only one day after your previous warning.

Continue on this path and it will end terribly for you.

Pandæmonium