Last week in Paris, I walked along the north-south line connecting the Observatoire de Paris to the Palais du Luxembourg. A line of longitude: in fact, the line of longitude, if the French had had their way a little over a century ago. A politico-scientific battle was being fought in the late nineteenth century over the location of the Prime Meridian. Parisians, thinking only of considerations of nature and philosophy, argued that the line of zero longitude should go through l’Observatoire; the rest of the world, crass materialists that they were, noted that over seventy percent of the world’s shipping was already using Greenwich (nine minutes and twenty-one seconds to the west of Paris) as its standard of longitude. The French lost out to the British, prefiguring a similarly heated tussle over who would host the Olympic Games over a hundred years later.
These issues figure prominently in the book I was reading during my trip, Peter Galison’s Einstein’s Clocks, Poincare’s Maps: Empires of Time. It is a paradigmatic example of a engaging work of intellectual history, as it has a definite theme that is at once simple, interesting, and true. Einstein and Poincare, the obscure German theoretical physicist and the celebrated French mathematician and philosopher, were pivotal figures in the development of the special theory of relativity, whose centenary we are celebrating this year. Relativity has a reputation as an esoteric theory, and Einstein and Poincare are often thought of as abstract thinkers divorced from mundane matters of technology and experimentation. Galison argues convincingly that these thinkers’ practical concerns with the measurement of time — Einstein judging clock designs at his patent office in Bern, Poincare as President of the Bureau of Longitude — were in fact crucial to their recognition of the need for a new understanding of the fundamental nature of time itself.
In a Newtonian universe, time is universal — the amount of time elapsed between two events is precisely and uniquely defined, even if the events are widely separated in space. It may be difficult to actually measure the time between events, and this task was a constant preoccupation of nineteenth-century astronomers, surveyors, politicians, and businessmen. It’s easy enough to use the sun to determine your local time, but the advent of railroads made it necessary (as several unfortunate accidents proved) to sensibly coordinate time among far-flung locales, a program that eventually led to our current system of time zones. In the course of standardizing time across broad expanses of geography, it became clear that synchronization was an operational concept — you had to bounce some signals back and forth between locations, and taking into account the travel time of the signals themselves was of primary importance. Poincare’s work on longitude was intimately connected to precisely this problem, as was Einstein’s experience with novel clock designs. (At one point subterranean Paris featured tubes that would carry pulses of compressed air from a central station to clocks throughout the city, which would use the pulses as reference standards to guarantee as precise a degree of synchronization as possible. Einstein would have seen numerous proposals for electrical versions of such schemes.)
By itself, the need to synchronize time via exchanged signals does not lead you to relativity; it is equally characteristic of Newtonian absolute time. But when combined with the principle of relativity and the invariance of the speed of light, this insight led Einstein to understand that the notion of simultaneity of distant events is not universal, but depends on one’s frame of reference. (In general relativity, in which spacetime is curved, we need to go even further — the notion of simultaneity is not simply frame-dependent, it is completely ill-defined.) Time goes from being an absolute characteristic of the universe to something individual and personal, a measure of the distance traversed by a particular object through spacetime. Poincare (following Hendrik Lorentz) had worked his way to similar conclusions, but it was Einstein who showed how to completely abandon the absolute Newtonian time that other physicists felt still lurked unobserved in the background.
Did someone say that scientists are individual idiosyncratic human beings? Gleaming mathematical edifices like the special theory of relativity can give the impression of having dropped from the sky; it’s nice to be reminded of the messy contingent ways that real people happen to stumble upon them.