2/1/12 - the ultimate clock

In today's excerpt - the technology of our daily lives has become so advanced that the need for accuracy in clocks presses hard the most accurate of today's clocks - which are accurate to five parts per 100,000,000,000,000,000 (or five in 1016). Improving this accuracy is absolutely necessary for such things as improved GPS navigation, improved satellite communication, and improved detection of faults in the massive communication networks we now depend on:

"Today those who would build a more accurate clock must advance into the frontiers of physics and engineering in several directions at once. They are cobbling lasers that spit out pulses a quadrillionth of a second long together with chambers that chill atoms to a few millionths of a degree above absolute zero. They are snaring individual ions in tar pits of light and magnetism and manipulating the spin of electrons in their orbits.  

 "And thanks to major technical advances, the art of ultraprecise timekeeping is progressing with a speed not seen for 30 years or more. These days a good cesium beam clock, of the kind Symmetricom sells for $50,000, will tick off seconds true to about a microsecond a month, its frequency accurate to five parts in 1013. The primary time standard for the U.S., a cesium fountain clock installed in 1999 by the National Institute of Standards and Technology (NIST) at its Boulder, Colo., laboratory, is good to five parts in 1016 (usually written simply as 10-16). That is 1000 times the accuracy of NIST's best clock in 1975. Successful prototypes of new clock designs - devices that extract time from calcium atoms or mercury ions instead of cesium - have recently attained accuracy in the 10-18 range, a 100-fold improvement in a decade.  

 "Accuracy may not be quite the right word. The second was defined in 1967 by international fiat to be 'the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 13enable utilities and communications firms to trace faults in their networks, and enhance geologists' ability to pinpoint earthquakes and nuclear bomb tests. Astronomers could use them to connect telescopes in ways that dramatically sharpen their images. And inexpensive, microchip-size atomic clocks are likely to have myriad uses not yet imagined." 3 atom.' Leave aside for the moment what that means: the point is that to measure a second, you have to look at cesium. The best clocks now don't - so, strictly speaking, they don't measure seconds. That is one predicament the clock makers face.  

 "Further down the road lies a more fundamental limitation: as Albert Einstein theorized and experiment has confirmed, time is not absolute. The rate of any clock slows down when gravity gets stronger or when the clock moves quickly relative to its observer - even a single photon emitted as an electron reorients its magnetic poles or jumps from one orbit to another. By putting ultraprecise clocks on the space station, scientists hope to put relativity theory through its toughest tests yet. But now that clocks have achieved a precision of 10-18 - proportions that correspond to a deviation of less than half a second over the age of the universe - the effects of relativity have started to test the scientists. No technology exists that can synchronize clocks around the world with such exactness.

"So why bother to improve atomic clocks? The duration of the second can already be measured to 14 decimal places, a precision 1,000 times that of any other fundamental unit. ... More stable and portable clock designs could ... be a big boon to navigation, enhancing the accuracy and reliability of the Global Positioning System and of Galileo, a competing system under development in Europe. Better clocks would help NASA track its satellites. 


W. Wayt Gibbs


"Ultimate Clocks"


Scientific American, Special Edition: A Matter of Time


Spring 2012


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