- Person
- Albert A. Michelson
- Lived
- 1852–1931
- Field
- Optics
- From
- Prussian-born American
The story
The speed of light was one of the great puzzles of nineteenth-century physics. The French physicists Hippolyte Fizeau and Léon Foucault had measured it in the mid-1800s, but their results left plenty of room for improvement. As early as 1869, while serving as an officer in the U.S. Navy, Albert Michelson began thinking about how to repeat Foucault's rotating-mirror method with better optics and a much longer baseline.
As a physics instructor at the U.S. Naval Academy in Annapolis, he first experimented with light speed as part of a classroom demonstration in 1877. In 1878 he made preliminary measurements with largely improvised equipment, and his work came to the attention of Simon Newcomb, director of the Nautical Almanac Office, who was planning a study of his own.
Michelson's formal experiments took place in June and July 1879, from a frame building he constructed along the Academy's north sea wall. Light from a source passed through a slit, struck a rapidly spinning mirror, travelled to a distant fixed mirror and returned. In the meantime the spinning mirror had turned slightly, so the returning beam landed a little to one side of the slit. Measuring that displacement, the distances involved and the mirror's rate of rotation revealed how fast the light had travelled.
He took great care to reduce errors. He checked his 100-foot steel measuring tape against a copy of the official standard yard and found it was actually 100.006 feet long, and he repeated his measurements many times, keeping each one independent, so that averaging would reduce random error. Michelson published a value of 299,910 kilometres per second, plus or minus 50, far more accurate than earlier results. The work made the 26-year-old officer famous.
He then joined Newcomb in Washington to assist with his measurements, beginning a long professional collaboration and friendship. Michelson kept refining his methods for the next half-century, culminating in his measurements at Mount Wilson in the 1920s and in an evacuated tube near Santa Ana, completed after his death. The value accepted today is exactly 299,792.458 kilometres per second, and it now defines the metre.
Why it matters
Michelson's measurement set a new standard of accuracy for one of the fundamental constants of nature, essential to physics, astronomy and navigation, and his careful handling of errors became a model for experimental science.
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.
- 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.
Sources
- Albert A. Michelson Wikipedia
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