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Bohr's Grand Battle

Michael D. Gordin

Uncertainty: Einstein, Heisenberg, Bohr, and the Struggle for the Soul of Science. David Lindley. x + 257 pp. Doubleday, 2007. $26.

Faust in Copenhagen: A Struggle for the Soul of Physics. Gino Segrè. x + 310 pp. Viking, 2007. $25.95.

From%20Faust%20in%20CopenhagenClick to Enlarge ImageIn the epic of 20th-century physics, quantum mechanics isn't just a central story—it is the central story. The narrative of the development of the microphysical contains just about everything one could desire for a popular-science book: complicated science that demands guided explanation; dramatic concepts that overturn almost all of our intuitions; the trauma of interwar Europe; and a cast of household names in the physics world—Niels Bohr, Werner Heisenberg and especially Albert Einstein.

It is this last figure who gives the plotline most of its punch, for Einstein, with his paper on the quantum nature of light in 1905, was one of the founders of this revolution in physics. But by the late 1920s he had turned into its arch-critic. As David Lindley puts it in his new book, Uncertainty,

Here is the root of a problem that was to plague Einstein for the rest of his life. He believed in the reality of light quanta sooner than anyone else, but he rebelled more strenuously than anyone else against the implication that light quanta inevitably brought spontaneity and probability into physics.

How do we make sense of Einstein's visceral opposition to quantum mechanics, and especially quantum indeterminism? How could the greatest figure of modern physics be so colossally wrong about something so important?

There are indeed many ironies in the history of quantum mechanics. One of the central ones is not so much what happened to Einstein the historical figure but what happens to the reader who consumes one of these books ravenous for a tale about the great hero—for Einstein is simply not the central figure of this most central story in modern physics. That role goes unequivocally to Niels Bohr. Both Lindley's book and Gino Segrè's recently released Faust in Copenhagen place Bohr in the limelight, and understandably so. Einstein might lure the reader in, but quantum mechanics is simply unintelligible without Bohr's concepts (the atom, complementarity) and his social position (as the great sage of Copenhagen).

Both of these books thus emphasize not just Bohr, but Bohr as the center of a grand battle, a "struggle for the soul" of physics and science. Isn't this phrasing a little hyperbolic? That depends on how you understand the story, and understanding it requires both a grasp of the physics and a sensitivity to history.

Quantum mechanics is difficult to comprehend simply on the level of philosophy, imbued as it is with indeterminism and discontinuity, but it is even harder to explain to the lay reader, because the theory of quantum phenomena is shot through with differential equations, eigenvalues, imaginary numbers, commutation relations, matrices and other mathematical adornments that have frightened off many an intrepid seeker. The challenge for any book that attempts to present this material to a general audience is to explain the science clearly while navigating a path between the Scylla of technical equations and the Charybdis of oversimplification and banality.

David Lindley's Uncertainty is far and away the best popular account of the development of quantum mechanics I have encountered. Lindley, who has a doctorate in astrophysics, has been an editor at Nature, Science and Science News and is the author of several books about science for a general audience. In Uncertainty, he admirably steers the reader through the entire story with a careful attention to physics, biography and language. (Rather than relying on calcified mythological accounts of yesteryear, he has consulted the mostly German sources in the original and made his own translations—something that is rarely done these days.) In clear prose, using no photographs or illustrations, Lindley explicates the course of the development of quantum mechanics, placing the story in the context of an original argument. Throughout, he draws on much recent work in the history of science about the contexts of atomism, the development of the Copenhagen Interpretation of quantum mechanics and the personalities of the major actors. When a popular myth about the interactions among the principal characters has been debunked by meticulous research, Lindley says so. He does not perpetuate romanticized errors that serve to amplify the conflicts between Bohr and Einstein without illuminating them.

For Lindley, the story of quantum mechanics is the story of how indeterminism became a central tenet of modern physics. To this end, he argues for a connection between two developments: the adoption of statistical methods for understanding atoms (which was a way of arguing that in practice we cannot ascertain what a particular particle is going to do in a system composed of large numbers of atoms) and quantum mechanics (which asserted that such determinations are in principle impossible for individual quantum movements—which radioactive atom will decay, which electron will jump to which orbit—although probabilistic predictions could be made for the ensemble as a whole).

In the end, Lindley's argument falls slightly short. The fundamental difference lies between the use of probabilities as a convenient tool—a development central to 19th-century physics and one that Einstein happened to pioneer in his 1905 study of Brownian motion—and a more absolute point that not only are individual particle paths unknown(the classical view), but they are inherently unknowable (the Bohr-devised claim for quantum mechanics). Einstein continued to argue against this latter extrapolation for the rest of his life, most famously in two showdowns at the international Solvay Conferences for physics in 1927 and 1930, and then in the 1935 paper he coauthored with Boris Podolsky and Nathan Rosen, which has been enshrined as the "EPR [Einstein-Podolsky-Rosen] paradox."

Lindley thus connects two very different positions about what "uncertainty" means. Some individuals, particularly Heisenberg and his mentor Bohr, did link those two positions together, hoping to build consensus around the abandonment of one of the central goals of physics since Aristotle: causal explanation. But for participants (including Heisenberg and Bohr), the two uncertainties were deeply distinct beyond the level of rhetoric. The uncertainty of statistical mechanics was in no sense explained by quantum mechanics—the two kinds of indeterminacy were solutions to different kinds of questions. Nonetheless, a reader wanting to know about either could do no better than to turn to this stimulating volume.

Gino Segrè's Faust in Copenhagen is a very different kind of work, in many instances more perceptive than Lindley's book but simultaneously less satisfying. Both the strengths and the drawbacks of Faust in Copenhagen stem from the profession of the author: He is a theoretical physicist interested specifically in neutrino physics. He peppers his anecdote-rich story of the development of quantum mechanics with helpful insights about the practice of theoretical physics, conceptual elucidations of some thorny issues and moments of vibrant wit. Professional physics is the well from which the book draws its strengths.

At the same time, however, it is the source of the book's real distractions. Some physics concepts are introduced into the book with very little explanation of their significance or justification, so that Segrè's analysis is difficult to follow for someone who has not been exposed to quantum mechanics before. (On the other hand, if one already has had this introduction, Segrè's discussions of issues such as the neutrino or the experiment-theory relation are intriguing.)

Segrè also has the physicist's malady of presenting all physics as Nobel Prize-centered. Each character is introduced with his Nobel Prize, or if he (or she) did not win one, or won it "late," Segrè feels an overwhelming need to explain why. The Nobel Prize intrudes in two ways. First, it interrupts the prose repeatedly, often confusing the reader and distracting from the narrative. An illustration:

By 1930 the physicists who had shaped the revolution's beginnings had already received Nobel Prizes. Planck, Einstein, Bohr, and de Broglie had made the trip to Stockholm. It was now time to celebrate the new generation, but who should go first, Heisenberg or Schrödinger? Heisenberg's matrix mechanics came earlier, but Schrödinger's wave mechanics was still more influential. And what about Pauli, Born, Jordan, and Dirac?

More important from a historical point of view, Segrè retrospectively projects the present-day status of the Nobel Prize, which it did not necessarily hold for each of his participants. (Not that it didn't matter: It was indeed prestigious. But its most important feature in interwar Europe—not mentioned by Segrè—was that it amounted to a lot of money for impoverished German physicists.)

The ostensible origin point for his narrative is the staging of a satire in Copenhagen in 1932. A few years earlier, Bohr had begun to convene disciples for informal discussions on pressing issues in physics. As a tension-reliever, they usually performed a skit, and in 1932, at the centenary of Goethe's death, the gathered physicists provided their own homage to the greatest of German poets and put on a spoof of his masterpiece, Faust.

The story of Faust has often been used to tell the story of modern physics, especially the creation of the atomic bomb. Faust, a scholar, trades his soul to Mephistopheles for knowledge and is redeemed only through the love of a maiden, Gretchen. In the "Blegdamsvej Faust" (as the parody was dubbed in a reference to the street address of Bohr's Copenhagen institute), the Lord was identified with Niels Bohr and was played by Felix Bloch; Mephistopheles was identified with Wolfgang Pauli (who was not present) and was played by Leon Rosenfeld; Faust was Paul Ehrenfest; and Gretchen was identified with the neutrino, whose existence Pauli had recently postulated to solve problems of nuclear beta-decay.

Segrè tells the conventional history of each of these figures and how they came to Copenhagen, as well as the story of other participants at the gathering: Heisenberg, Paul Dirac, Max Delbrück and Lise Meitner. (Other minor characters, such as George Gamow, appear almost as cameos both in the skit and in the book.) The text of the "Copenhagen Faust" that Segrè uses comes from the appendix of George Gamow's 1966 classic, Thirty Years that Shook Physics, and Segrè sprinkles his text with Gamow's illustrations.

But does this juxtaposition of scientists and theater improve our understanding of the transformations in physics in this period? For the most part, the answer, sadly, is "No," because the text of the play is rich in many allusions and in-jokes that are either ignored or left unexplained here.

The key observation in Segrè's book is what makes it worthwhile despite its drawbacks: the importance of 1932. It has long been conventional to punctuate 20th-century physics with two "miracle years": Einstein's solo performance in 1905 and the advances in nuclear physics in 1932. These latter findings—the discoveries of the positron, the neutron, the disintegration of lithium and a few other phenomena—set the conceptual stage for the 1938 discovery of fission and thus for the invention of the atomic bomb. (Faust again!) But 1932 was also the year before the victory of Adolf Hitler in neighboring Germany, and thus the last time some of these participants could join in the illusion of an Edenic European physics. (It also marked the year when Einstein emigrated to the United States from Berlin.) Bohr isn't usually linked to 1932 directly, and Segrè shows us here how intimately the two major revolutions in interwar physics—quantum mechanics and the new atom—can be drawn together by a reading of Faust in Copenhagen.

One final point needs to be made about these two works. What are we as readers to make of the struggle for the soul of physics? In Segrè, it is a clear reference to the Faust legend, but Lindley points out a closer analogue, also fortuitously related to the 1932 conjuncture. In his 1933 Nobel lecture, Erwin Schrödinger claimed that he introduced his wave equation because he wanted to save "the soul of the old system" of mechanics. For what was at stake for physicists in interwar Europe was precisely the essence of what counted as a physical explanation.European physicists agonized over the philosophical foundations of their enterprise—its soul.

When physics became "Americanized" during and after the war, such foundational concerns took a back seat. That focus, which was so important in 1932 Copenhagen, was simply something Einstein couldn't bring across the Atlantic.



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