By George Musser | February 21, 2013 | For physicists trying to make sense of quantum mechanics, Albert Einsteins thinking remains highly relevant. This guy saw more deeply and more quickly into the problems that plague us today, one quantum physicist told me. The latest volume of theCollected Papers of Albert Einstein, which contains Einsteins publications, draft papers, letters, and scribblings from January 1922 through March 1923, shows that his deep concerns with the quantum predated his well-known duels with Niels Bohr and played a major role in shaping the emerging theory. Many of these writings have not been made public before. Ive invited Tilman Sauer of Caltech, a senior editor at the Einstein Papers Project, to describe some of the new things that have turned up. George MusserAlbert Einstein and Niels Bohr famously clashed in the late 1920s and early 1930s over the interpretation of quantum mechanics and the phenomenon we now call quantum entanglement. However, Einstein had already been probing quantum mechanics for many years, both theoretically and experimentally. He proposed not only thought experiments but also real experiments that, when conducted, were crucial in shaping physicists understanding of the emerging theory. Details of Einsteins ideas and new insights into his involvement with quantum theory come to light in the course of our work on his papers.Bohr had postulated that electrons move around atomic nuclei only in certain stationary orbits, and that they send off radiation only when they fell from one such orbit to another one of lower energy. Equating the energy of the emitted light to the energy difference of the two orbits, he was able to explain the spectrum of hydrogen. It was a spectacular breakthrough. But despite their empirical success, Bohrs postulates challenged basic tenets of classical electrodynamics. Everybody was puzzled. Bohr had opened the door to a world of mystery and surprise.Einstein, too, was challenged and provoked by Bohrs bold theory. Himself a founding father of the early quantum theory and author of the light quantum hypothesis, he sensed that here was a problem that could not be reconciled with classical concepts and imperatively required new ideas and approaches.In late 1921, Einstein devised what he thought would be a crucial experiment to determine the nature of the light emission process. When electrons move from one orbit to the next, is light emitted by atoms sent out instantaneously, as a quantum, or gradually, as a continuous wave? He proposed studying the process using so-called canal rays, which are particles accelerated by a voltage in a glass tube; they stream through pores (canals) in the cathode of the tube. After passing through the cathode, they may reneutralize or not, but in any case can send out light (by Bohrs mechanism of electrons jumping to a lower energy orbit). The light is shifted in frequency because of the Doppler effect, indicating that the light-emitting particles are moving fast.If you send light produced in this way through a dispersive medium, Einstein predicted, the wave fronts should be deflected if the emission process were classical. When Hans Geiger and Walther