In this chapter, interactions between particles are studied and described with the help of the probes introduced in the preceding chapters. It is shown how this leads directly to expressions for the observable reaction rates, without detour through an S-matrix. In the first section the basic ideas are explained for a simple model without IR problems, to wit, a theory of a scalar hermitian field whose stable particles are massive and have spin O. In the following three sections these ideas are generalized to QED. The result is surprising: it does not agree with the customary picture of a particle as a robust object with an unquestionable individuality. It seems that the notion of particles is more subtle than generally believed, in particular that charged particles cannot be defined meaningfully in detachment from an observational situation. This implies that also the notion of reaction cross section takes on a new, intricate meaning. These problems will be discussed in the final Chap. 17.
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