- Person
- Albert A. Michelson
- Lived
- 1852–1931
- Field
- Optics
- From
- Prussian-born American
The story
For most of history, units of length rested on physical objects. From 1889 the metre was defined by an international prototype bar of platinum-iridium kept near Paris. Bars can be damaged or lost, they can change subtly over time, and their length depends on temperature, so every comparison carried some uncertainty. The American scientist Charles Sanders Peirce had already suggested tying the metre to the wavelength of a line in the spectrum of light. Albert Michelson took up the idea and made it work.
He had the ideal tool. His interferometer splits a beam of light, sends the two halves along separate paths and recombines them, so that differences in distance appear as a pattern of bright and dark fringes that can be counted one wavelength at a time. After 1887, he and Edward Morley turned to using light waves as a standard of length.
At the request of the International Committee for Weights and Measures, Michelson travelled to Paris. In 1892 and 1893, working with the French metrologist Jean-René Benoît, he measured the international prototype metre in terms of the wavelength of the red light emitted by cadmium. It came to about 1.55 million wavelengths, and the measurement was accurate to a small fraction of a single wave. The work showed that a unit of length could be tied to a constant of nature that a well-equipped laboratory anywhere could reproduce, rather than to one bar in a vault.
The achievement was among those cited when Michelson received the 1907 Nobel Prize in Physics for his optical precision instruments and the measurements made with them. Benoît, Charles Fabry and Alfred Perot repeated the cadmium measurement in 1906, and the cadmium red line was used to define the ångström, a small unit for spectroscopy, in 1907.
Michelson's idea was later written into the definition of the metre itself. In 1960 the metre became 1,650,763.73 wavelengths of an orange line of krypton-86, chosen because natural cadmium's line turned out to be a cluster of closely spaced lines. The new standard was linked back to the cadmium measurements. After the invention of the laser, precision improved again, and since 1983 the metre has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second.
Today any laboratory with a stabilised laser can realise the metre without consulting an artefact. Laser interferometers working on Michelson's principle measure distances, angles, vibrations and forces, and guide machines that position parts to within trillionths of a metre.
Why it matters
Michelson showed that length could be measured against light itself, freeing the metre from fragile metal bars and pointing the way to the light-based standard used by science and industry today.
About Albert A. Michelson
Physicist who measured the speed of light with unmatched precision, performed the Michelson–Morley experiment and in 1907 became the first American to win a Nobel Prize in science.
More about Albert A. Michelson
- Michelson built Chicago's renowned physics departmentIn 1892 Albert Michelson became the first head of physics at the newly founded University of Chicago, leading it until 1929 and making it one of America's leading centres of physics.
- Michelson's interferometer now detects gravitational wavesThe interferometer Albert Michelson invented in 1881 became one of physics' most important instruments; the LIGO detectors that first caught gravitational waves in 2015 are Michelson interferometers.
- Michelson measured the size of a star for the first timeIn 1920 Albert Michelson and Francis Pease measured the diameter of the red giant Betelgeuse, the first time the size of any star other than the Sun had been measured.
- Michelson's famous null result paved the way for relativityIn 1887 Albert Michelson and Edward Morley found no trace of the supposed "aether", a result that became one of the classic foundations of Einstein's theory of relativity.
- Michelson timed light across 22 miles of CaliforniaUsing a 22-mile light path between Mount Wilson and Mount San Antonio, Albert Michelson measured the speed of light in 1926 as 299,796 kilometres per second, plus or minus 4.
- A young naval officer measured light's speed in 1879In June and July 1879, Albert Michelson, a 26-year-old instructor at the U.S. Naval Academy, measured the speed of light as 299,910 kilometres per second, far more accurately than anyone before.
Sources
- Albert A. Michelson Wikipedia
- History of the metre Wikipedia
- Albert A. Michelson: Biographical NobelPrize.org
Another act of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.