- Person
- Albert A. Michelson
- Lived
- 1852–1931
- Field
- Optics
- From
- Prussian-born American
The story
Albert Michelson returned to the speed of light again and again throughout his life. His first precise measurement, made at the U.S. Naval Academy in 1879, had made his name, and he refined it in 1883. In 1906, however, an electrical method used by the National Bureau of Standards produced a lower value, and other measurements followed that trend. The question needed a definitive answer, and Michelson set out to provide one.
From 1920 he planned a measurement from the Mount Wilson Observatory in southern California. Light would travel from Mount Wilson to a mirror on Lookout Mountain, a prominent bump on the south ridge of Mount San Antonio, known as Old Baldy, some 22 miles (about 35 kilometres) away, and back again.
The distance had to be known exactly. In 1922 the United States Coast and Geodetic Survey began two years of painstaking measurement of the baseline using newly available invar tapes, made of an alloy that barely expands with heat; the survey's estimated error was less than one part in a million. With the baseline established in 1924, Michelson carried out measurements over the next two years and published a value of 299,796 kilometres per second, with an uncertainty of just 4.
The work was not easy. Smoke from forest fires created haze that blurred the mirror image, and the Santa Barbara earthquake of June 1925 may have shifted the carefully surveyed baseline. Even so, the result came within about 4 kilometres per second of the exact value adopted today, 299,792.458 kilometres per second.
Michelson was still not finished. In 1929, with Francis Pease and Fred Pearson, he began measuring light inside a tube about 1.6 kilometres long and three feet wide at the Irvine Ranch near Santa Ana, pumped nearly empty of air; repeated reflections stretched the light path to about 5 miles. It was the first measurement of the speed of light in an almost perfect vacuum. Michelson died in 1931 with only 36 of the 233 planned series completed, and his colleagues published the final result in 1935.
His relentless pursuit of precision helped make the speed of light one of the best-known constants in physics, the value on which the modern metre is now based.
Why it matters
Michelson's mountain-top measurement came within a few kilometres per second of today's exact value, a triumph of experimental patience that underpins modern standards of length.
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 measured the metre in wavelengths of lightIn 1892–93 in Paris, Albert Michelson measured the international prototype metre in wavelengths of cadmium's red light, showing a unit of length could be defined by nature.
- 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.
- 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
- 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.