Science: New Time, New Length

Since the 19th century, science and industry have honored a holy of holies: a bar of platinum-iridium alloy, triple-locked in a subbasement at Sèvres on the outskirts of Paris. Near the ends of the bar were engraved two microscopically thin lines, and the distance between them was exactly one meter—by international agreement, the world standard of measurement. Around the globe, other countries had copies of the bar at Sèvres, and their traditional units of length—feet (3.28 to a meter), versts, li, or whatever —were defined by reference to it. But last week the sacred bar was in effect tossed on the scrap heap. A General Conference on Weights and Measures, meeting at Paris, made a wave length of light the new official standard of length. The meter is now denned as 1,650,763.73 wave lengths of the orange-red light given off by electrically excited krypton 86, a rare gas extracted from the atmosphere. The U.S. inch is 41.929.399 wave lengths.

The Old Last Word. Precise standards of measurement are a present-day perplexity. On a laboratory wall in the U.S. Bureau of Standards hangs a blow-up of a 16th century woodcut showing 16 men lined up heel to toe to define the rute, an old German measure of length related to the English rod. That was fine for the 1500s. But since then, each advance in technology has required better measurements. The standard meter bar, adopted by 28 nations, including the U.S.. in 1875, was considered the last word. Used with elaborate comparing devices, it could measure with an accuracy of one part in 10 million, and that seemed as good as would ever be needed.

But metrologists soon began to worry about the meter bar. It might be lost or destroyed, or its metal might change with time in subtle and unpredictable ways, altering the distance between the lines by a fatal few parts per million. Worst of all, as the years passed and technological requirements became even more devastatingly precise, the bar’s accuracy was no longer good enough. A master standard must be at least ten times more accurate than the practical measuring systems that are derived from it, and modern industry began to call for tolerances expressed in millionths of an inch. An error of this infinitesimal order in a gyro guidance system might send a space probe to the wrong planet, millions of miles off the track.

The New Answer. The waves of krypton 86 have none of these failings. They cannot be lost, destroyed, damaged or stolen (there is krypton in all air), and scientists believe that their length, which is determined by the properties of the krypton 86 atom, will never change at all. Anyone with the proper equipment (present cost about $100,000) can reproduce, even a million years from now, the standard unit of length adopted in 1960. By use of an interferometer—an optical device that counts wave lengths and fractions of them—the new light standard gives measurements accurate to one part in 100 million. This, think the metrologists, should suffice for quite a while.

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Besides adopting a new standard of length, the conference adopted a new standard of time. Until now the second has been defined as a specific fraction (1/86,400th) of the day, which is one rotation of the earth. This is not good enough for modern science, especially astronomy, because the period of the earth’s rotation varies slightly in ways that are not properly understood. So, decreed the conference, from now on the second will be a fraction of the year, the earth’s revolution around the sun. In case this very stable interval should change in the future, the second was defined as 1/31,556,925,9747th of the year 1900.