![]() ![]() "In the future, we might want to navigate outside the solar system for long periods of time," says Lute Maleki, a senior scientist at NASA's Jet Propulsion Laboratory. Having more accurate clocks in hand could allow NASA engineers to pilot spacecraft at even greater distances. NASA engineers depend on accurate timekeepers to direct spacecraft around the cosmos. More accurate clocks could help pinpoint location even better. Location is determined by clocking the time it takes for a signal to reach a satellite and return again. Global Positioning Systems depend on highly accurate cesium clocks in satellites to measure distance. So why do we need such accurate timekeepers? It turns out they're important not so much for punctuality, but for science. It's unlikely anyone would worry about being a femtosecond late to work. The result, after more than 15 years of work and "several" million dollars in spending, is a clock that measures time by intervals 100,000 times shorter than those recorded by the best current clocks. When a mercury ion is cooled, it vibrates at just the right rate to "tune" the light from a laser oscillating a million billion times a second, and to keep that cadence steady. The NIST team settled upon a single mercury ion as the "heart" of their optical clock. As Bergquist explains, if a laser is tuned to the same frequency of an atom "it's like using a tuning fork - the laser and atom will oscillate in time." Next, they had to find an atom - like the cesium atoms in the atomic clock - that would keep the laser's oscillations at a steady pace. For every tick of the counter, the scientists know 100,000 laser ticks have gone by. Rather than directly counting a million billion oscillations a second, a highly accurate device (called a femtosecond pulsed laser) counts every 100,000th pulse of the laser. To surmount that problem, Bergquist, his colleague Scott Diddoms, also at NIST, and others used a method first developed by German physicists to accurately count each cycle of the laser's rapid oscillations. Since 1967, the cesium atomic clock has defined "one second" as the time it takes a cesium atom to vibrate 9,192,631,770 times. Rather than swinging once per second, like the grandfather clock, Cesium 133 atoms oscillate at a rate of 9,192,631,770 times a second. The most accurate clocks today measure time by locking the frequency of microwave beams to the frequency of vibrating atoms. Digital clocks use either the oscillations on the power line (60 cycles a second in the United States) or the oscillations of a quartz crystal and also count using digital counters. In quartz watches, time is measured by the oscillations of a quartz crystal and usually recorded using digital counters. In a grandfather clock, for example, a pendulum swings once every second and those swings are recorded by metal gears inside the clock. The first is something that creates a regular, periodic event, and the second is a device that will count, accumulate and display those events. All clocks operate using two main ingredients. ![]()
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