1905 – Einstein's annus mirabilis

Up to 1905, Planck's work had made little impression, and he was no further forward in understanding the profound implications of what he had done. As discussed in Chapter 13, he expended a great deal of unsuccessful effort in trying to find a classical interpretation for the ‘quantum of action’ h, which he correctly recognised had fundamental significance for understanding the spectrum of black-body radiation. The next great steps were taken by Albert Einstein, and it is no exaggeration to state that he was the first person to appreciate the full significance of quantisation and the reality of quanta. He showed that this is a fundamental aspect of all physical phenomena, rather than just a ‘formal assumption’ for accounting for the Planck distribution. From 1905 onwards, he never deviated from his belief in the reality of quanta – it was some considerable time before the great figures of the day conceded that Einstein was indeed correct. He came to this conclusion in a series of brilliant papers of dazzling scientific virtuosity.

Einstein completed what we would now call his undergraduate studies in August 1900. Between 1902 and 1904, he wrote three papers on the foundations of Boltzmann's statistical mechanics. Once again, notice how a deep understanding of thermodynamics and statistical physics provided the starting point for the investigation of basic problems in theoretical physics. As was explained in Case Study IV, thermodynamics and statistical physics do not deal with specific physical processes, which might not be particularly well understood; rather, they deal with the overall properties of physical systems and provide general rules about the expected behaviour.