A question of time

When the history of the twenty-first century comes to be written, one of the most puzzling questions to be asked will be why, in the information age, millions of people still pay to dial a number on their phones to find out the time.

Nearly 80 years after its formation, Britain’s speaking clock, the world’s original telephone time service, remains an essential part of British life. This is despite the nearly ubiquity of timing displays—at least not on mobile phones that people drop to call 123 from a certain line.

For some people, sometimes accuracy matters. There are peaks in the use of spoken clock, for example, on New Year’s Eve, or when clocks are set one hour forward and backward, respectively, the start and end of British Summer Time.

There is another way, at least in the UK. BBC radio regularly broadcasts the same time signal used to set the speaking clock – affectionately known as pips. In fact, it has become as much a feature of few shows as the material planned around it. Time is more than a British institution; It is woven into the cultural fabric of everyday life.

Pips are drawn from an atomic clock held at the National Physical Laboratory (NPL) in Teddington, near London. One of the most accurate in the world, the NPL clock is tuned for regular bursts of light emitted by cesium atoms when they are excited by microwaves. The clock loses about one second every 138 million years—accuracy enough for an hour-long late commuter who forgot to set his clock at night, but not accurate enough for some.

In a paper published on Nature’s website this week, Time Lords in the United States describe the latest advances in chronometry, and one that is as superior to atomic pips as those pips were to the mechanical devices they replaced.

Researchers have created a clock based not on cesium but on strontium. More importantly, it uses much, much higher optical frequencies. This gives devices, called optical clocks, more accuracy than those that rely on microwaves. The new optical clock, for example, won’t lose a second even if it runs for 5 billion years.

It is also extremely stable – another key measure of timekeeping. (Accuracy defines how closely the clock’s output matches the desired time signal, while stability is a measure of how stable the output is. A clock that loses exactly one second each day is inaccurate but stable , for example.)

The unveiling of a super-precise strontium optical clock comes just months after a related group revealed a device based on ytterbium. Other laboratories around the world have their own designs. Inevitably, the increased accuracy and reliability of optical clocks is fueling debate about whether they can be used to determine the end time, and redefine the other. (There are no official plans to do so, but plans are underway to redefine other SI units.) These are prime times for metrology: a world view on page 455 Attempts to measure another fundamental constant Describes: Big G.

Nature has a special stake in the race to develop new atomic clocks. Back in January 2003, we published a news feature that surveyed the scene and tried to predict what would happen (D. Adam Nature 421, 207–208; 2003). Within a decade, the piece suggested, optical clocks could rise to prominence and raise new debate about the definition of the other.

The ten-year event horizon is a major cornerstone of scientific journalism, and most promised breakthroughs fail to materialize on deadlines. In contrast, the latest developments in atomic timekeeping have come on time. well almost.

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