- Person
- Albert A. Michelson
- Lived
- 1852–1931
- Field
- Optics
- From
- Prussian-born American
The story
Stars are so far away that even through the largest telescopes they appear as points of light, blurred by the Earth's atmosphere. Their true sizes could only be estimated indirectly. Albert Michelson realised that his interferometer, which compares light arriving by two different paths, could resolve details far finer than any single telescope could show.
The underlying idea of stellar interferometry had been suggested by the French physicist Hippolyte Fizeau in 1868, but Michelson turned it into a working instrument. At the Mount Wilson Observatory in southern California he built a 20-foot (about 6-metre) interferometer mounted on the front of the 100-inch (2.5-metre) Hooker telescope. A periscope-like arrangement of mirrors gathered light from two points up to 20 feet apart and directed it into the telescope, producing interference fringes that could be watched through the eyepiece.
Working with the astronomer Francis Pease, and helped by John August Anderson, Michelson turned the instrument on Betelgeuse, the bright red giant in the constellation Orion. On 13 December 1920 Betelgeuse became the first star outside the Solar System to have its angular diameter measured. The team found a value of about 0.047 arcseconds, a tiny angle that nevertheless showed the star to be enormous, many times wider than the Sun.
The measurement of stellar diameters and of the separations of binary stars took up more and more of Michelson's later life. His result held up well: a study published in 2000 with the Infrared Spatial Interferometer gave a diameter entirely consistent with his finding eighty years earlier.
Astronomical interferometry was revived from the 1970s, and Michelson's principle of combining light from separated apertures now works over baselines of hundreds of metres at observatories such as the European Southern Observatory's Very Large Telescope Interferometer, the CHARA Array and the U.S. Navy's NPOI. These instruments measure the sizes and shapes of stars, track close binary systems and map the dusty shells that surround dying giants like Betelgeuse itself.
Why it matters
Michelson's technique turned stars from points of light into objects with measurable sizes, and his approach remains central to the highest-resolution astronomy done 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 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'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
- Betelgeuse Wikipedia
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