x l i i I N T R O D U C T I O N T O V O L U M E 1 3 earlier, together with Jakob Grommer, he had investigated whether the existence of zero-point energy could be verified for helium.[6] Now, he was working with Grom- mer on the same problem, as related to hydrogen. He was already sure of finding quantum effects, but not of the existence of the zero-point energy (see his exchang- es with Ehrenfest in Docs. 31 and 57). III In spite of all of Einstein’s doubts and unsolved dilemmas with quantum theory, we find several statements by him stressing its importance. He makes it clear in letters and writings that quantum theory, though problematic in its conceptual founda- tions, is absolutely necessary given its empirical successes. Einstein expressed his admiration for Bohr, and for Bohr’s pioneering work, both in formal recommendations, such as the proposal to nominate Bohr as a cor- responding member of the Prussian Academy (Doc. 53), and also in private corre- spondence. He considered Bohr “the greatest contemporary genius in physics” (Doc. 1), whom he dared not disturb with trivial requests. In a letter to Ehrenfest, he expressed his great appreciation for Bohr’s world of ideas (Gedankenwelt): “He is really a brilliant person. It is a stroke of luck that something like him exists at all. I have full confidence in his ideas. The correspondence principle, and the way he is using it, are convincing” (Doc. 107). In more detail he expressed the inevitability of the Planck-Bohr-Rutherford quantum theory in the final passages of his popular article of August 1922, published in the Japanese journal Kaizo, “On the Present Crisis of Theoretical Physics” (Einstein 1922o, Doc. 318). The letters of Max Born to Einstein give us a glimpse of the lively determination to understand and develop quantum theory. In one of these, Born reports on prog- ress with his crystal theory and on his work with Wolfgang Pauli on perturbation theory, which eventually became one of Born’s most celebrated achievements (Doc. 175). In another letter, Born describes his struggle to quantize molecules. This is, probably, one of the first reports on Born’s work in quantum chemistry, which turned out to be of major importance in later years (Doc. 320). IV In addition to his investigations of quantum theory, topics in relativity theory also remained on Einstein’s mind. In the beginning of the year, Erich Marx, the editor of Handbuch der Radiologie, planning to revive Einstein’s 1912 manuscript on special relativity (Vol. 4, Doc. 1),