Max Planck opposed Einstein's photon idea from the beginning, and remained a champion of the unquantized Maxwell field throughout his life. For example, in 1907 in a letter to Einstein, he said I am not seeking the meaning of the quantum of action (light quantum) in the vacuum but rather in places where emission and absorption occur, and I assume that what happens in the vacuum is rigorously described by Maxwell's equations. It should be noted that nowadays it is customary to refer to "light quanta" as "photons".
In 1911 Planck introduced the hypothesis of the zeropoint electromagnetic field in an effort to avoid Einstein's ideas about discontinuity in the emission and absorption processes. His ideas were published in successive editions of The Theory of Heat Radiation, which came out in an English version, published by Dover, in 1959. They were rediscovered by various people, including myself, in the 1950s and 1960s. We tried to explain all quantum effects with this zeropoint field, and called the new discipline Stochastic Electrodynamics (SED)
While achieving some notable successes, for example with the Casimir effect, SED must overall be judged a failure. (Of the two recent textbooks we cite in the reference list below one agrees and the other disagrees with this evaluation.) It is now clear to me that the reason for this failure is that we did not take into account the extended structure of the electron, which is what gives it its spin for example (see Quantum Electrodynamics ). Nevertheless, we have been able to show that the phenomenon of Parametric Down Conversion is, like the Casimir effect, explainable using only the zeropoint electromagnetic field. Note that recently our work in this area has enabled us to make a direct challenge to the established Photon theory of this phenomenon, with a fairly easy and striking new experimental prediction. For details go to the above page.References