Einstein's Clocks and Poincare's Maps

Published by Published by Sceptre,  2003, hardcover, illustrated, index, 390 pages, condition: new.

Clocks and trains, telegraphs and colonial the challenges of the late nineteenth century were an indispensable real-world background to the enormous theoretical breakthrough of relativity. And two giants at the foundations of modern science were converging, step by step, on the Albert Einstein, a young, obscure German physicist experimenting with measuring time using telegraph networks and with the coordination of clocks at train stations; and the renowned mathematician Henri Poincare, president of the French Bureau of Longitude, mapping time coordinates across continents. Each found that to understand the newly global world, he had to determine whether there existed a pure time in which simultaneity was absolute or whether time was relative. The historian of science Peter Galison has culled new information from rarely seen photographs, forgotten patents, and unexplored archives to tell the fascinating story of two scientists whose concrete, professional preoccupations engaged them in a silent race toward a theory that would conquer the empire of time.

This book is wrapped around one basic insight, heavily illustrated with anecdotes, stories, and biographical sketches. The insight is this: the theory of Special Relativity owes a great deal to the practical technical challenges of turn-of-the-century Europe. This claim is then elaborated through a close examination of the professional biographies of Poincare and Einstein, who both formulated versions of the same theory at virtually the same time.One of the striking aspects of relativity (both according to Einstein and Poincare) is that it defines simultaneity as "the thing you get when you have clocks synchronized with propagation-delayed light pulses." This may seem like an odd thing to build into the theory -- it certainly struck me as odd when I first encountered it. Galison's contribution in this book is to put that definition into the context of the technology of the 1900s. A point I hadn't appreciated until I read the book is that wide-area clock synchronization were a relatively new development in 1905 -- time zones as we know them were only established a few decades earlier, and as late as 1912, France used Paris solar time, offset by 9:21 minutes from Greenwich.

There are two corollaries to this novelty, both of which are elaborated by Galison. The standard technique for measuring longitude differences in the late 19th century was to synchronize clocks by telegraph, and then measure local solar time in each location. For long-distance measurements, the propagation time of the electrical pulses (on the order of tens of milliseconds) was well within the available precision of contemporary instruments. "Synchronize clocks by sending light pulses and correcting for transit time" wasn't some goofy thought-experiment -- it was the actual way clocks were synchronized in 1905. Furthermore, this is something Henri Poincare would have known very well, since he was president of the Bureau of Longitude and responsible for overseeing a huge program of long-distance clock synchronization.

Now the second corollary: in 1905, railroads and municipal governments had a huge number of clocks and wanted them all kept synchronized automatically. This meant that there was a steady stream of electrical gadgets being patented for clock synchronization -- especially in Switzerland, where clockmaking was a major national industry. And many of these patents would have landed on the desk of patent clerk A. Einstein, specially associated with patents on electrotechnical apparatuses. Like Poincare, Einstein would have been extremely aware of how clock synchronization worked in practice. When Einstein wrote about how to synchronize a dispersed set of synchronized clocks via timing pulses, he wasn't doing a thought experiment at all, he was describing the state of the art in actual timekeeping technology.Why does any of this matter? Partly, it's interesting simply as a historical excursion through an interesting and under-explored corner of technology. Partly, it's nice to know what Poincare and Einstein had in mind when they were formulating relativity. And finally, it strikes me as useful for better understanding the theory of relativity.

The relativistic definition of simultaneity seems strange if presented as a thought-experiment without context. It makes much more sense in a context of "this is how engineers synchronize clocks, which is a real and important problem for a whole bunch of practical reasons." Those textbook sketches of reference frames, with clocks and meter-sticks correspond to actual artifacts: 19th-century geographers were literally creating networks of synchronized clocks, coordinated by timing pulses. And the theory of special relativity has a lot more cohesion and intellectual force when you see it presented against this socio-technical context.