A Long Journey with Classical Fields

We note the subsidiary rôle, in low-energy physics, of fields in relation to particles, and we make passing reference to the `absorber theory' of radiation.

We note that, in the optical region, fields are not normally measured: it is sufficient to measure the spectral density of the intensity, as a function of polarization. We give an example of a simple calculation to illustrate the significance of this point.

We refer to the famous experiment of Brossel and Bitter, and to the phenomena of radio-frequency resonances in many areas of physics.

We recall the use of highly monochromatic fields in low-frequency resonance experiments, and recall also (using transformation operators) the well-known exact solution of the semiclassical equations which describe magnetic resonance in the customary configuration. We present a different configuration of fields for which an exact solution can also be obtained, and we recall successes of calculations based on these theories.

We address the interpretation of the phenomenon of spontaneous emission, and show first that it can be described through the operation of a stochastic field permeating space (a classical version of the vacuum field), and secondly, how it can be described by a representation of the radiation reaction field whose form is suggested by the time-symmetric, classical absorber theory, and which appears as an operator operating in the Hilbert space of a quantized atom. The first-order Lamb shift and the second-order radiative decay rate are given correctly by this operator, in addition to the Einstein A-coefficient.