Bothe conducted the experiment, using carbon disulfide gas as the medium, they saw no such deflection. Einstein took this as evidence that light is emitted as a particle rather than as a wave. But as the new volume of Einsteins Collected Papers makes clear, Einstein had made a mistake. As was pointed out to him by his friend Paul Ehrenfest (shown in the photo above with his son and Einstein), his analysis was flawed. He had not distinguished correctly between group velocity and phase velocity. When you correct for this, neither of the two theoretical alternatives results in a deflection. Retracting his original manuscript, Einstein graciously published an analysis of classical wave propagation through dispersive media, so others would not run into the same trap.But he did not let up. Again and again, he devised experiments designed to shed light on the choice between quantum and classical concepts. Some ideas were short-lived, others were being put to the test. Ehrenfest was fascinated. He dreamed of locking Einstein and Bohr together in a room to fight it out, thus anticipating the famous debates between the two about the Copenhagen interpretation a few years later at the 1927 Solvay conference.Indeed, in the early 1920, Einstein examined most matters in light of the quantum question. Few others probed as deeply as he did. Take superconductivity. The mysterious complete and sudden loss of electric resistivity at liquid helium temperatures had first been observed only some 10 years earlier in Leyden for the case of mercury, and until 1923 it was only in Leyden that Kamerlingh Onnes had the facilities to produce the phenomenon. Einstein hypothesized that the charge current in superconductors is produced by electrons moving on chains of Bohr orbits without emitting radiation. In superconducting metals, atoms are aligned in such a way that their orbits tangentially osculate each other, thus allowing electrons to pass smoothly from one atoms orbit to the next. If that were so, he reasoned, interfaces between different metals should not be superconducting. It was a clever idea. But when Onnes performed the experiment and found superconductivity in a ring consisting of alternating pieces of lead and tin, Einstein sighed: Yet another glimmer of hope for understanding is dashed.In Frankfurt at the same time, Otto Stern and Walther Gerlach were sending silver atoms through a strong, inhomogeneous magnetic field in order to see whether they carried a magnetic momentum and, if so, whether that momentum was quantized in space, as quantum theory postulated. They found that the beam of silver atoms split into two rays, corresponding to two different possibilities of aligning their magnetic momentum in the magnetic field. When Einstein and Ehrenfest discussed the experiment during one of Einsteins visits to Leyden, they immediately realized that the Stern-Gerlach experiment cannot be explained classically. They did a little calculation (the photo at left shows an excerpt from Ehrenfests notes) estimating how long it would take a silver atom to align in a magnetic field by classical emission of radiation through Larmor rotation. They found that this would take 100 years, as