Random reminder

Press, 2 June 1984, Page 28

Random reminder

METRE IN THE MOONLIGHT

Everyone once knew what a “yard” was. It was about so much -- 0.9144 metre, to be exact — and had been defined in real units since 1963. But as real units go. what IS a metre? In 1790 the French revolutionaries knew what a metre ought to be. A metre ought to be an exact fraction of a polar meridian. Through Paris. The British, miffed, ignored it. Unaware of this, through not reading “The Times,” the French surveyors set off in 1791. In 1798 they struggled back. Result: a platinum bar, 10 million of which would reach from the Equator to the North Pole. We now know by satellite that the 1799 bar was 0.22 in too short, but this makes no difference to anything. Except for the annoying feeling that if they had played their cards better we would not now be stuck with the figure 9.8 in gravity sums and the figure 2.997 in speed-of-light sums. It could have been 10 or 3. But not both. Now read on. The 1799 bar was replaced in 1889 by thirty (British-made) iridium bars, each still defined as “a metre,” each actually 1 metre plus-or-minus 0.000001 m. Six-figure accuracy was replaced, like the physical bars, in 1960 by so many wavelengths of a particular krypton light. Nine-figure accuracy.

Unfortunately also in 1960 was invented the laser, which apart from zapping Star Wars villains can measure lengths to eleven decimal places. This is not a lot of use if your metre itself goes fuzzy around the edges at nine decimal places. A metre is not now — since October — a quantity measurable to plus-or-minus any number of decimal places. It is an absolute, like the speed of light. A metre is defined as, are you taking notes?, 0ne299,792,458th of the distance travelled by light in a vacuum in one second. What is a second? we hear intelligent people cry. Here we are on much firmer ground, made secure by the atomic caesium clock, its vibrations kept constant by the speed of light. Thirteen figure precision is normal. Twenty-figure precison, when it arrives, will not aifect the metre. A metre is a fraction of a second, plus-or-minus zero. Sportspeople, like astronomers, can now relax. The centuries of “near enough” were not good enough. The new fixed metre takes away those niggling worries. Its precision, for sports, is equivalent to l/100th of a second in a marathon lasting 3000 years. Or to put it another way, if a shotput were to be hurled at the moon and back, you might be 0.1 mm out