Planck and the genesis of the quantum

During the first quarter-century of quantum theory, it developed by addressing a considerable range of topics – atomic physics, especially atomic spectroscopy; interactions involving fundamental particles – electrons, protons and so on, and also electromagnetic radiation; and the thermal capacities of solids and gases, where deviations from classical physics seem, at least in retrospect, rather straightforward.

Yet the first intimation of quantum ideas, which came to Max Planck in 1900, appeared in rather a recondite area of physics, where there would be no satisfactory classical theory for another five years, and where considerations were very much complicated by the fact that statistical methods were required. This was the area of black-body radiation or cavity radiation [32].

All surfaces emit energy in the form of electromagnetic radiation. This emission is of so-called thermal radiation, the word ‘thermal’ emphasising that the amount of energy radiated, and its frequency distribution, depend strongly on temperature. In 1879, Josef Stefan deduced from experiment that the total amount of radiation is proportional to T4 (T being an absolute temperature, of course), and Boltzmann confirmed this theoretically using thermodynamics.

The nature of the surface is also very influential and, to explain this, it is helpful to start with the absorption of energy. Different surfaces absorb different fractions of the energy that is incident on them. In particular, black surfaces absorb more than white ones.