For physicists trying to make sense of quantum mechanics, Albert Einstein's 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 the Collected Papers of Albert Einstein, which contains Einstein's 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. I've 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 Musser
Albert 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 Einstein's 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, Bohr's 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 Bohr's 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 Bohr's 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 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 Einstein's 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 atom's 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 Einstein's 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 Ehrenfest's 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 compared to the few microseconds that the atoms had available during their time of flight. Confronted with the first example of a genuine quantum measurement process, Einstein and Ehrenfest immediately realized the fundamental puzzle of what later became a textbook example of the wavefunction collapse.
The Einstein Papers Project aims at making accessible Einstein's writings and correspondence in a carefully annotated, scholarly edition. The editorial project builds on the Albert Einstein Archives, an archival collection of his published and unpublished writings and extensive correspondence deposited at the Hebrew University of Jerusalem. A catalogue of the collection is accessible in a database with more than 80,000 records and many facsimiles. The editorial project is organized chronologically. With the latest volume, the published series now covers Einstein's life and work up to his 44th birthday. Who knows what other nuggets of insight will turn up in the volumes to come?
Photos (C) Museum Boerhaave, NL.