Measuring Up

As fast as the speed of light

The report from Paris sounded like a nonevent, yet scientists considered it major news. In a rare display of international comity, the 46-member General Conference on Weights and Measures unanimously redefined the meter, the world's basic unit of length.* Instead of being viewed simply as an arbitrary length, the meter will henceforth be defined in terms of another base measure, time--specifically, the distance light travels through space in 1/299,792,458 of a second.

One of physics' sacred constants, the speed of light is exactly 299,792,458 meters per second. If in the future it is measured with greater accuracy--or, more unlikely, is found to have shifted--the length of the meter will change as well.

In the 1790s, revolutionary France tried to bring order to the existing hodgepodge of weights and measures by adopting the metric system. Its scientists confidently set the meter as 1/10,000,000 of the distance between the North Pole and the equator. Making that measurement, however, turned out to be impossibly difficult not only because the earth is far from a perfect, unchanging sphere but because of France's internal turmoil. The government's surveyors were arrested as royalist spies, narrowly escaping the guillotine.

By 1889, scientists had defined the meter as the distance between two marks on a platinum-iridium bar kept under controlled conditions near Paris. Still, even this measurement, accurate to one part in a million, was eventually adjudged unsatisfactory. In 1960 the meter was redefined as 1,650,763.73 wave lengths of the reddish-orange light emitted by krypton 86, a rare atmospheric gas.

Although the krypton meter was accurate to four parts per billion, it led to errors of up to 5 ft. in such experiments as measuring the distance from the earth to the moon--an irritant to scientists exploring the nuances of relativity or movements of continents. Now, by redefining the meter in terms of time, the scientists are using the most accurately known base measure. With its incredibly precise atomic clocks, the Bureau of Standards can measure the second to better than one part in 10 trillion. The new standard, to be sure, makes no significant difference for workaday tasks. Still, there is something comforting in the fact that the scientific quest for accuracy is alive and well.

* Even the nonmetric U.S. has legally established the yard as exactly .91440183 meter. This file is automatically generated by a robot program, so viewer discretion is required.