On the occasion of the sixtieth anniversary of Louis de Broglie's thesis, the concept of phase waves, is re-examined. After a brief historical survey a modern conceptual presentation of these waves is described via phase invariance. In this respect de Broglie's hypothesis appears not to be self-consistent. Phase waves in galilean relativity are briefly discussed. As an interesting pedagogical device, inverse phase waves in two-dimensional special relativity are presented. Finally, some remarks are added concerning the development of concepts and theories.
The appearance of single-particle wave functions as field operators or arguments of a field functional in quantum field theory is often regarded as a second quantization, since these wave functions were themselves obtained by quantizing mass points (“particles”). I argue here that this language, though reflecting the historical development, is misleading. Spatial fields must be regarded as the true and only physical variables (in quantum theory becoming the arguments of a fundamental wave functional). This includes spinor and other fields which never become classical. Apparent particles are no more than the result of decoherence in the measuring device or by means of other environmental scattering processes, without playing any fundamental conceptual rôle in the theory or its interpretation. A remark on gauge fields is added.