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Atomic clocks

January 13, 2004

A public radio commentary

A year has gone by and that means a cesium atom has oscillated some 300 trillion times. I'm talking, of course, about atomic clocks. They work like grandfather clocks, but here the pendulum that keeps the time is the oscillation of the atom. America stores its atomic clocks in a bunker in Boulder, Colorado. Inside, fifty atomic clocks click away, sending the time to Paris.

There a team of clock-watchers combine these results with about 200 other atomic clocks from around the world. Their readings are averaged together to calculate what's called Coordinated Universal Time. It's accurate to better than one second in three million years. To my human senses that's unimaginable, I mean a year goes by and I barely feel it. So, can one second in three million years be the least bit important? Well, yes.

Here's an example that happened to me, and no doubt has happened to you. I was travelling in a jet and as we approached the ground we saw nothing but clouds. In fact, the ceiling, as they call it, was 100 feet above the ground. So, the pilot had to blindly maneuver the plane to 100 feet above the runway. The pilot used the Global Positioning System, or GPS, to put the plane in exactly the right place. As the jet approached the runway, the pilot received a signal from several satellites. The exact time it took for that signal to arrive determined the jet's position. If an error of even a billionth of a second occured, the position of the jet would be off by one foot. That's significant for the landing I was involved with, but even more so, if the jet is an F-14 Tomcat landing on an aircraft carrier. There, a foot or two is the difference between life and death.

The accuracy of atomic clocks even affects our telephone calls! To pack lots of calls on a single line, the phone company chops conversations into tiny packets, then packs these small bits very efficiently and sends them down the phone line. At the other end, a device reassembles these bits into a coherent conversation. This is done in the time it takes to say a single word. Thus, requiring a very accurate system of clocks. If all this makes you feel a slave to the clock, here's some solace: The clocks are actually slaves to us. You see a year is defined by us, by people. We define it as the rotation of the Earth. But our definition isn't perfect: The Earth wobbles and wiggles around its axis, causing random fluctuations in the length of a year. This means that the atomic-time grid falls out of tune with our sense of seasons, with our definition of a year. And although this year is an exception, usually the atomic timekeepers must bring their cherished, ultra-precise clocks back in line by adding a few seconds.

Copyright 2004 William S. Hammack Enterprises