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Between the ticks

December 1st, 2009 by Gene

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We’re going to do this one on time. Come along with us as we figure out how. Atomic clocks, today on Engineering Works!

Time never stops. We’ve been keeping track of it for a long time, and we’ve done it a lot of different ways – sundials, dripping water, candles with marks on them, springs and gears and pendulums, quartz crystals and electricity.

All of these timekeepers have one thing in common. They keep track of the interval between one tick and the next. And they all have a problem — the same problem. The intervals they measure aren’t always the same. They’re probably not that different, but they vary – a little or a lot. If you need to measure time exactly – to navigate a space probe or use a global positioning system – they’re not good enough.

This is where special clocks called atomic clocks come in. Instead of pendulums and gears or even quartz crystals, atomic clocks use the vibration between the nucleus and electrons of atoms – usually cesium atoms – to set the interval we use to measure time passing. Even this interval varies a little. But not much. The atomic clock at the Naval Observatory near Washington, D.C., is accurate to within about one second in 20 million years.

If you think this is accurate, clocks based on hydrogen atoms do even better over the short term. But over longer periods of time, cesium is better.

Time’s up. We’ve got to go now.

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