Einstein's Clocks, Poincaré's Maps by Peter Galison

Nowadays we take time too much for granted. In 1880, a public clock in Geneva needed three faces to show the time in Geneva, Paris and Bern. The challenge for scientists and engineers was to find a reliable way of standardising time across cities and countries by connecting clocks to a central "mother clock". In Paris an ingenious city-wide system of clocks was regulated by compressed air piped beneath pavements. Heath Robinson would have been proud of it, but by the end of the century electricity offered an easier way of unifying time. According to the historian of science Peter Galison, synchronising clocks remained an urgent issue in the 20th century: "Time coordination in the Central Europe of 1902-05 was not merely an arcane thought experiment; rather, it critically concerned the clock industry, the military, and the railroads," as well as being a symbol "of the interconnected, sped-up world of modernity."

Nowadays we take time too much for granted. In 1880, a public clock in Geneva needed three faces to show the time in Geneva, Paris and Bern. The challenge for scientists and engineers was to find a reliable way of standardising time across cities and countries by connecting clocks to a central "mother clock". In Paris an ingenious city-wide system of clocks was regulated by compressed air piped beneath pavements. Heath Robinson would have been proud of it, but by the end of the century electricity offered an easier way of unifying time. According to the historian of science Peter Galison, synchronising clocks remained an urgent issue in the 20th century: "Time coordination in the Central Europe of 1902-05 was not merely an arcane thought experiment; rather, it critically concerned the clock industry, the military, and the railroads," as well as being a symbol "of the interconnected, sped-up world of modernity."

It was this practical problem that provided Albert Einstein with a crucial clue to the complexities of relativity. In May 1905, after a day of intense discussion with his favourite "sounding-board" Michele Besso, Einstein finally found the answer he had spent nine years searching for. The key lay in "an analysis of the concept of time". Pointing out two far-flung clock towers, Einstein explained: "Time cannot be absolutely defined, and there is an inseparable relation between time and signal velocity."

Galison's book explores notions of simultaneity in order to explain the origins of relativity. As well as Einstein, he brings a less familiar figure into the relativity equation: Henri Poincaré, the extraordinary French mathematician, philosopher and physicist "who produced, quite independently of Einstein, a detailed mathematical physics incorporating the relativity principle".

From the 1890s Poincaré worked at the Bureau of Longitude. There the new global network of telegraph cables was used to map the globe by sending time signals to the four corners of the world. But time is a harsh master and Poincaré has been relegated to a footnote in the history of relativity. By bringing him out of Einstein's shadow, Galison argues that Einstein's theory was part of a widespread material and intellectual concern with the cultures of time.

One of the far-reaching implications of relativity is that our sense of simultaneity is an illusion. There is no such thing as a universal "now". Einstein rejected Newton's notion of a "universally audible tick-tock". In his work as a patent officer in 1905, Einstein examined proposals for cutting-edge electrotechnology, including synchronised clocks.

According to Galison, to talk about simultaneity at the beginning of the 20th century was to ask how to coordinate clocks. For both Einstein and Poincaré the answer was to use an electric signal, and deduct the time taken for the signal to travel between the clocks. It was an idea with "breathtaking consequences for concepts of time and space, for the new relativity theory, for modern physics" and for "our very model of secure scientific knowledge". The problem of coordinating clocks helped Einstein see that Newton's absolute time was wrong: his theological concept had no place in a technological and secular age.

Galison is right to argue that the history of relativity is about more than one man and that it's an idea rooted in practical concerns, in time-keeping and map-making, as well as the higher abstractions of philosophy and physics. He makes few concessions to the non-specialist reader, but this impressive study is absorbing, original and full of insights into the revolutionaries of time.

P D Smith's illustrated biography of Einstein is published by Haus

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