An exact approach to the oscillator radiation process in an arbitrarily large cavity

Starting from a solution of the problem of a mechanical oscillator coupled to a scalar field inside a reflecting sphere of radius R, we study the behaviour of the system in free space as the limit of an arbitrarily large radius in the confined solution. From a mathematical point of view we show that this way of addressing the problem is not equivalent to considering the system a priori embedded in infinite space. In particular, the matrix elements of the transformation turning the system to the principal axis do not tend to distributions in the limit of an arbitrarily large sphere as should be the case if the two procedures were mathematically equivalent. Also, we introduce 'dressed' coordinates which allow an exact description of the oscillator radiation process. Expanding in powers of the coupling constant, we recover from our exact expressions the well known decay formulae from perturbation theory.