books

Physics and Philosophy Meet Practical Engineering

Reviewed by Robert M. Wald

Students often are introduced to the revolutionary nature of time in special relativity via Albert Einstein's famous thought experiment: Suppose that lightning strikes the front and back ends of a train in such a way that an observer on the station platform determines the strikes to occur simultaneously. Then an observer riding on the train will not regard the strikes as being simultaneous. Given the assumed constancy of the speed of light, the calculations required to show that result are quite simple. The remarkable insight of Einstein was to discard the Newtonian notion of absolute time and view simultaneity in purely operational terms. That insight was key to the discovery of special relativity.

It is commonly assumed that Einstein's inspiration about simultaneity arose from deep philosophical musings very far removed from his mundane, day−job activities as a patent clerk. In Einstein's Clocks, Poincaré's Maps, Peter Galison makes a convincing case that this description is not the complete story. Galison's main thesis is that, rather than being the product of one man's isolated attempt to resolve deep problems in physics by pure thought, the discovery of special relativity should be viewed as arising naturally from a convergence of ideas in physics, philosophy, and practical engineering that, in the author's words, had produced a "critical opalescence" by the turn of the 20th century.

In the late 19th century, clock synchronization was considered in purely operational terms for a number of eminently practical reasons. Various ideas were proposed and implemented—though most did not work well—to synchronize clocks within cities and thereby eliminate annoying discrepancies between different city clocks. Clock synchronization between cities became essential for creating reliable railroad schedules. More importantly, accurate clock synchronization over large distances was needed for longitude determination. By the late 19th century, telegraphic signals sent over transoceanic cables enabled clocks to be synchronized worldwide with sufficient accuracy that one had to correct for the delay due to the transmission of the telegraphic signal. Thus the same clock−synchronization prescription used in Einstein's thought experiments was already in use in real, entirely practical contexts.

The central Figure in Galison's account is not Einstein but Henri Poincaré. Well known to physicists for his major contributions to pure mathematics and to classical mechanics, Poincaré devoted a significant portion of his activities to practical engineering issues. He was deeply involved in the establishment of international standards of units and measurement, and even made a serious proposal for the decimalization of time units. Most significantly, he was a permanent member of the French Bureau of Longitude and served as scientific secretary to its mission to Quito, Ecuador. Galison persuasively argues that such experiences helped shape Poincaré's view—expressed in his 1898 essay "The Measure of Time"—that simultaneity is merely a convention. It is possible, although the historical record is not clear, that Poincaré's essay may have had an important influence on Einstein.

Galison also argues—less compellingly, in my opinion—that the practical aspects of time synchronization must have directly influenced Einstein's thought. Bern, Switzerland, inaugurated its synchronization of city clocks in 1890. The young Einstein of 1905 was fascinated by machines. Numerous patent applications for the coordination of clocks via electric signals passed through the Bern patent office during his tenure (though the historical record does not make clear which ones may have been evaluated by Einstein himself). Surely, argues the author, such life experiences must have helped inspire Einstein to take an operational view of simultaneity.

If the historical record presented by Galison shows that Einstein as isolated thinker is not all that was behind relativity, it seems to me also to show that critical opalescence cannot be the whole story. No one was in a better position to take advantage of the critical opalescence than Poincaré. By 1900, he understood how Hendrik Lorentz's "local time" could be interpreted as the physical time measured by a moving observer. He had by then adopted an operational view of simultaneity, very likely influenced by his work for the Bureau of Longitude. His mathematical abilities certainly exceeded Einstein's. Yet Poincaré not only failed to discover special relativity, he failed to embrace it after Einstein discovered it. Surely Einstein's deep conviction that the principle of relativity must be fundamentally embedded in the laws of physics—and his willingness to give up on the ether or anything else that did not accord with that view—was crucial to the discovery of special relativity and cannot be explained as merely a byproduct of a critical opalescence.

In any case, Einstein's Clocks, Poincaré's Maps is a fascinating, beautifully written account of the era preceding the discovery of special relativity. It is both scholarly and highly engaging, and it will serve as an important and valuable reference on the history of special relativity.

Robert M. Wald is the Charles H. Swift Distinguished Service Professor in the department of physics at the University of Chicago. His main research interests involve the theory of quantum phenomena in strong gravitational fields such as those near a black hole.