For Max Planck, taking energy in finite-size packets was only a strange trick that happened to work for the calculation—that is, to reproduce laboratory measurements—but for utterly unclear reasons. Five years later it is Albert Einstein—him again—who comes to understand that Planck’s “packets of energy” are in fact real. This is the subject of the third of the three articles sent to the Annalen der Physik in 1905. And this is the true date of the birth of quantum theory.
