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“slave.” The slave clock sent an electromagnetic impulse every thirty seconds to the pendulum master clock, which then, in turn, regulated the slave clock. This device was accurate to within one second a year, precise enough to measure microseconds, or millionths of a second. In a microsecond a sound wave will have moved only one-third of a millimetre.
    Shortt’s timekeeper was usurped only eight years later, when Warren A. Marrison, an engineer at Bell Laboratories in the United States, designed the first quartz-crystal clock, using the vibrational frequency of an electrically charged quartz crystal. By the mid-1940s, quartzclocks had achieved a degree of accuracy within one second every thirty years. The world of the very small—the crystal lattice of silicon molecules that make up quartz—had become the new pendulum.
    The pace of precision timekeeping continued to accelerate. In 1948 Harold Lyons created the first atomic clock, using the natural resonant frequency of an atom. By the mid-1950s this atomic clock had evolved into the cesium-beam atomic clock, which is still in use today to broadcast Co-ordinated Universal Time and which has an accuracy of about one nanosecond a day. A nanosecond is a billionth of second, the time it takes light to travel thirty centimetres in a vacuum. Computers take between two to four nanoseconds to process a single calculation. The children’s odyssey in C. S. Lewis’s The Lion, the Witch and the Wardrobe , where an imagined lifetime of adventure is crammed into a couple of seconds, must have been scaled to nanoseconds, each second in that world equalling a nanosecond of ours.
    This kind of accuracy has allowed scientists to measure the duration of a second, long defined (somewhat tautologically) as the “sixtieth part of the sixtieth part of the twenty-fourth part of a day.” According to the cesium-beam atomic clock, a second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom at zero degrees kelvin. It’s not an elegant definition, but it’s absolute.
    Clocks have become so accurate now that a curious thing has happened: they have outstripped the master clock—the earth’s revolution—that clockmakers have always used to calibrate their most precise chronometers. With the advent of the atomic clock, the yearly slowing of the earth’s revolutions could be measured accurately—and it turned outto be three milliseconds a year. The earth was now an unreliable timepiece. But that wasn’t all: the earth was also becoming an irregular timepiece. When the Boxing Day tsunami of 2004 struck, it changed the angular momentum of the planet’s spin, speeding it up like a spinning figure skater pulling her arms closer to her body. As a consequence our days are now three millionths of a second shorter. But the divorce of abstract time from earthly time is far from complete, and there is an ongoing battle between astronomers and physicists about what our atomic timepieces should be based on. Astronomers argue for “natural time,” where leap seconds are added every few years to accommodate the slowing earth, while physicists argue for “technological time,” the final separation of time from its earthly orgins. The earth might yet be relegated to chronological history.
    And all the while, smaller and smaller durations of time, like the bays within bays of a shoreline, continue to be discovered. The femtosecond, one millionth of a billionth of a second, was measured soon after the nanosecond. From the perspective of the femtosecond we humans are unmoving statues that exist an eternity. There could be a whole civilization overlapped with our own for whom femtoseconds are like our seconds. The Femtonians could be living invisibly among us as if we were so many figurines. Should their science became advanced enough, a genius among them might announce that the