A Helping Hand
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thesis A helping hand One hundred years ago last month, a the rudiments of quantum field theory research paper appeared in the journal “Grossmann, you have learns to write down Lagrangian functions Zeitschrift für Mathematik und Physik, in manifestly covariant form, thereby co-authored by a Swiss mathematician to help me, or else I’ll ensuring the relativistic invariance of the from ETH Zürich. Written with a physicist go crazy!” resulting theory. colleague, the paper presented the results of It may be, of course, that someone in the a two-year collaborative effort, which had role of Grossmann would have existed, even flowered from a friendship formed when if Grossmann himself had not. Gravity is the two men were students some years intimately linked with spacetime curvature before. Their paper of January 1914 stands derivative and more. This mathematical and differential geometry. Even if Einstein as a historical monument to the power of machinery and language, Sauer argues, was had failed to see precisely how, someone timely collaboration. originally brought into the theory largely by else presumably would have eventually The physicist lacked the mathematical Grossmann. He not only pointed Einstein succeeded, learning the right mathematical knowledge to give his deepest insights to crucial mathematical papers — such as language on their own, or meeting someone, concrete form. The mathematician helped to a 1901 article on the absolute differential like Grossmann, who could help. This kind of show him how, bringing recent mathematical calculus by Gregorio Ricci-Curbastro and mathematics — a step up in abstraction from theories to bear. The result was a paper Tullio Levi-Civita — but took real steps in the mathematics of classical mechanics — entitled ‘Outline of a general theory of forging the emerging theory into these terms. was increasingly in the air. relativity and a theory of gravitation’. Indeed, it seems it was Grossmann At the time of Grossman and Einstein, Most physicists will have guessed by who kicked off the use in physics of the Emmy Noether was making major advances now that the physicist in question was now ubiquitous term ‘tensor’, favouring in abstract algebra and group theory, and Albert Einstein. The mathematician was the idea that equations written in terms of bringing these into the foundations of field one Marcel Grossmann. History records tensors would be manifestly of the same theory. Following general relativity came that Einstein discovered general relativity form in all coordinate systems. Sauer notes the Hilbert-space formulation of quantum in complete form in 1915, yet what many that Ricci-Curbastro and Levi-Civita had theory, Paul Dirac’s relativistic quantum physicists might not know is just how never used ‘tensor’ in this context, instead theory, field theories with broken symmetries significant a role Grossmann played in calling these objects “systèmes covariants ou and the rise of mathematical physics as the giving that theory its final form. Indeed, contrevariants”. Grossmann, Sauer’s study guiding light of physics. Grossmann’s imprint the Einstein–Grossmann paper of 1914 of the notebooks suggests, “actually helped is all over today’s theories of cosmology and came within a hair’s breadth of describing clarify the very mathematical status of the string theory. the correct equations of general relativity. objects that were entering the centre stage of After his work with Einstein, “We had come at that time already very their theoretical efforts.” Marcel Grossmann did further research in close to the solution,” Einstein later wrote. Einstein’s notebooks make pointed synthetic and non-Euclidean geometry, but “At the time we lacked only a few relations reference to Grossmann’s contributions to the also became very active outside of research. of a formal nature, without which the link development of the mathematical formalism. In the years following the First World War, between the formulas and already known On one page, where an expression appears he helped found and run a twice-weekly laws cannot be attained.” for the Riemann–Christoffel tensor — an magazine promoting open debate on Swiss Even today, however, no one has written object with four indices capturing the national affairs and spent considerable energy a biography of Marcel Grossmann. He local curvature of a manifold — Einstein in efforts to reform the Swiss education never claimed to be a co-inventor of the has written “Grossmann tensor of fourth system. Beneficial education, in his view, theory, merely a helper. He and Einstein manifold”. It was Grossmann who showed would not only instil specific knowledge but remained friends for two decades until how to reduce this tensor to an object with should “create a frame of mind that enables Grossmann’s death in 1936. The historical only two indices that would be suitable for a the young person at the end of his studies to evidence — as historian Tim Sauer points gravitational field equation, pointing the way swim about without swimming coach and out in a new examination of Einstein’s to the correct mathematical objects needed without swimming rings, even when the notebooks and other documents of the period to develop fully covariant field equations. current would flow in unexpected directions.” (http://arXiv.org/abs/1312.4068) — clearly I’m sure none of this is news to physicists He remained all the time a staunch believer shows that Grossmann played a hugely working with general relativity, who have in broad education, perhaps instructed from influential role in pushing Einstein towards long known of Grossmann’s contributions. A his own experience with Einstein of the vast his final theory. prominent series of international conferences potential for a little knowledge from other Open almost any modern textbook on even bears his name. Yet it might also be fields to make a big difference. general relativity and you’ll encounter several said that Grossmann not only helped forge When Einstein first approached his friend, chapters of mathematical preliminaries. general relativity, but was instrumental in seeking help in 1912, he did so with the For example, a popular undergraduate changing the mathematical language of following words: “Grossmann, you have to textbook by Bernard Schutz starts out physics. His tensor notation left a mark help me, or else I’ll go crazy!” It is fortunate covering topics such as covariant and not only on the form of general relativity, for physics that Grossman helped. ❐ contravariant tensors, measures of curvature but indeed on all fundamental theories on differentiable manifolds, the covariant developed since. Every physicist studying MARK BUCHANAN 84 NATURE PHYSICS | VOL 10 | FEBRUARY 2014 | www.nature.com/naturephysics © 2014 Macmillan Publishers Limited. All rights reserved.