The Metre Defined by Light

For most of the nineteenth century the metre was a bar of metal in a vault outside Paris. Every other metre in the world was a copy of it, and every copy had to be carried back periodically and compared. If the bar had been destroyed the unit would have gone with it. Michelson saw the way out almost as soon as he had built his interferometer. In 1892 and 1893 he measured the prototype metre against the red line of cadmium and found it to be 1,553,163.5 wavelengths — the first time a fundamental unit had been expressed in terms of a property of an atom rather than an object. It took another sixty-seven years for the definition to follow the measurement, but in 1960 the metre officially became 1,650,763.73 wavelengths of a krypton-86 transition. Nothing to store, nothing to lose, and any competent laboratory could now build the standard from scratch. In 1983 it changed again and more radically. The metre is now the distance light travels in one 299,792,458th of a second, which makes the speed of light EXACT by definition and hands the realisation of length to the second — the most accurately realised unit there is. Measuring c more precisely is no longer possible in principle; a better measurement now refines the metre instead. Every step of that is the instrument from blueprint 4. Lock a laser to an atomic transition, divide the exact c by its frequency to get an exact wavelength, and count fringes. The batch began by comparing a lens to a wave and ends with the metre itself defined that way.