- Person
- Albert A. Michelson
- Lived
- 1852–1931
- Field
- Optics
- From
- Prussian-born American
The story
In the nineteenth century, physicists believed that light, like sound or ripples on water, needed a medium to travel through. They called it the luminiferous aether and supposed it filled all of space. As the Earth orbits the Sun, it should be moving through this aether, creating an "aether wind" that would make light travel at slightly different speeds in different directions. The expected difference was tiny, since the Earth's orbital speed is about one ten-thousandth of the speed of light.
Albert Michelson had already tried to detect the effect in Germany in 1881 with his newly invented interferometer, but the instrument was not sensitive enough to be conclusive. In 1885, as professor of physics at the Case School of Applied Science in Cleveland, Ohio, he began working with the chemist Edward Morley of neighbouring Western Reserve University. In 1886 they first confirmed an earlier experiment by Hippolyte Fizeau on light travelling through moving water.
For the aether test they built a far better instrument. Mirrors bounced the light back and forth so that each beam travelled about 11 metres, and the optics were mounted on a sandstone block about a foot thick, floating on a trough of mercury so it could be turned smoothly with a single push. The apparatus sat in the basement of a stone dormitory to cut down vibration. In concentrated observations in July 1887 they watched the fringes as the block rotated through the points of the compass. They expected a shift of about 0.4 of a fringe; what they saw was at most a small fraction of that, within experimental error. The results were published in the American Journal of Science that November.
The Michelson–Morley experiment became perhaps the most famous null result in the history of science. Physicists including George FitzGerald and Hendrik Lorentz devised explanations, among them Lorentz's contraction equations. In 1905 Albert Einstein's special theory of relativity did away with the aether altogether, taking the constancy of the speed of light as a basic principle. Einstein later wrote that without the embarrassment the experiment caused, no one would have welcomed relativity as a partial redemption.
Michelson–Morley type experiments have been repeated many times with ever greater sensitivity; optical resonator tests in 2009 confirmed the absence of any aether wind to about one part in 10^17. Together with related experiments, they remain one of the fundamental tests of special relativity.
Why it matters
By carefully finding nothing, Michelson and Morley helped clear the way for relativity, the theory behind technologies from particle accelerators to satellite navigation.
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 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
- Michelson–Morley experiment Wikipedia
- The Michelson-Morley Experiment History of University Circle in Cleveland (Cleveland State University)
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