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NATURE|Vol 437|29 September 2005 ESSAY Thinking big Fritz London’s single-minded thinking led him to surpass even Einstein, as he believed correctly that quantum mechanics was right at all scales, including the macroscopic.

Philip W. Anderson immediately begins to realize that the real problem of quantum measurement is not in Fritz London began his career in physics as understanding the simple electron that is one of the originators of quantum theory being measured, but the large and compli- during 1925–27. His training as a philoso- cated apparatus used to measure it. This pher, before taking up physics, no doubt apparatus has all kinds of properties that are enhanced his contribution to the ‘copen- not obvious consequences of quantum hagen interpretation’ — the first general mechanics: rigid slits, for instance, and a attempt to understand the world of photographic plate that darkens irreversibly atoms according to quantum mechanics. where an electron hits it. But London did much more than create the These properties are a real intellectual first theory of the chemical bond, and has challenge to understand from first princi- not had the recognition he deserves. ples; the first thing one realizes is that time,

He was among the few pioneers who for the measurer and the photographic COLLECTION TODAY ARCHIVES/PHYSICS VISUAL E. SEGRÉ deliberately chose, once atoms and mol- plate, has a sign — earlier or later. This sign ecules were understood, not to focus his is not contained in the quantum theory and research on further subdividing the atom has to be the result of the organizing princi- into its ultimate constituents, but on explor- ples of quantum particles assembled into ing how quantum theory could work, and very large macroscopic objects. This and be observed, on the macroscopic scale. the fact that the apparatus has a definite For a few years, London worked at trying position in space require that a quantum to found chemistry on quantum theory, description of it can only be given in terms but in the end was overwhelmed by Linus of a superposition of an unimaginably large Pauling’s more heuristic approach; he never Lone thinker: Fritz London took an opposite number of different quantum states. published his book on the subject. He then tack from both Albert Einstein and Niels Bohr. The electron interacting with it attaches became intrigued by the twin phenomena (entangles) one part of its wave function to of superfluidity and superconductivity, In reading about these debates I have the one batch of these states, the other part to a which, he was convinced, were macroscopic sensation of being a small boy who spots different batch. And these batches differ in manifestations of quantum mechanics. not one, but two undressed emperors. Niels so many ways that they can never be made In 1935, London was the first to propose Bohr’s ‘complementarity principle’ — that to cohere again; they represent two entirely that superfluidity was Bose–Einstein con- there are two incompatible but equally cor- separate macroscopic histories of the appa- densation, and then in the late 1930s, with rect ways of looking at things — was merely ratus. The message is that what is needed is his brother Heinz, he developed the first a way of using his prestige to promulgate an understanding of the macroscopic world heuristic theory of superconductivity. His a dubious philosophical view that would in terms of quantum mechanics. This is the pair of books on these subjects appeared keep physicists working with the wonder- direction that London chose. around 1950 and admirably framed the ful apparatus of quantum theory. Albert And that brings me to superfluid solids. questions that were soon to be answered — Einstein comes off a little better because he Moses Chan and his student Eun-Seong in the one case by Oliver Penrose, Lars at least saw that what Bohr had to say was Kim have recently shown that helium (and Onsager and Richard Feynman, and in the philosophically nonsense. But Einstein’s probably hydrogen), if solidified below a other by John Bardeen, Leon Cooper and greatest mistake was that he assumed that tenth of a degree Kelvin, flow through their Robert Schrieffer. But London fell ill in 1950 Bohr was right — that there is no alterna- own crystal lattice like a superfluid. (This and died in 1954, so he did not live to see the tive to complementarity and therefore that has yet to be confirmed, but I believe it.) triumphs of his intuitions. quantum mechanics must be wrong. This This means that a rigid object — the most He had paid, however, for his unpopular was a far greater mistake, as we now know, primitive of our physical intuitions — is not choice of subject matter — quantum theory than the cosmological constant. a system in a simple, single quantum- on the macroscopic scale — by having to At this point London took an opposite mechanical ground state, but only arises as settle for a job in the pre-war South. This tack from either Bohr or Einstein. He found a consequence of thermal fluctuations. meant being out of mainstream physics, it difficult to believe Bohr’s idea that there Thus, Albert Einstein’s clocks and rigid and may have resulted in him being was a real ‘complementarity’ even though measuring rods, which play such a key excluded from the Manhattan bomb project he had been an early contributor to that line role in the theory of relativity, must be on which all his early associates worked. of thinking. Instead he took the then radical not primitive but derived in a very com- In 1939, in an obscure paper called ‘The step of assuming that quantum mechanics plex way from the underlying quantum observation problem in quantum mechan- was not wrong, but right at all scales, includ- laws of microscopic physics. At which ics’,London and Edmond Bauer took on the ing the macroscopic. This explains why point I could immodestly take the oppor- notorious Bohr–Einstein debates. This is London was intrigued by the realization tunity to announce that after all, “more is the earliest paper I know of that expresses that in the ‘super’ forms of matter, he was different!” ■ the most common-sense approach to the seeing quantum theory showing itself on Philip W. Anderson is in the Department of uncertainty principle and the philosophy of the (relatively) everyday scale. Physics, Princeton University, Princeton,

quantum measurement. Taking London’s point of view, one New Jersey 08544, USA. ESSAY