If time waits for no man, then neither does human ingenuity in measuring its passing. Throughout history, more stable and accurate clocks have led to advances in communications and navigation. Now a clock 1000 times more accurate than any of its predecessors has set another benchmark, and could even be used to create a more precise definition of how long a second is. The new clock is a variant on the atomic clocks that appeared in the 1950s. Atomic clocks usually work by measuring the frequency at which atoms resonate. For instance, the outer electrons of a caesium-133 atom resonate between two energy states exactly 9,192,631,770 times each second, emitting microwaves of exactly that frequency as they do so. This property has been used since 1967 to define what we mean by 1 second - it is officially the time it takes for a caesium atom to resonate 9,192,631,770 times.
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