In 1887, Albert Michelson built an experiment that he hoped would lead to the detection of luminiferous ether. At the time, physicists believed that the ether permeated the universe and served as the medium through which light waves moved, like the way waves traveled across the ocean.*
The experiment turned out to be a failure. The mystical ether didn’t exist. But the instrument that Michelson invented to conduct this research would detect, more than a century later, a very real, very significant astronomical phenomenon: gravitational waves, the ripples in the fabric of space and time, coming from a violent collision between two black holes.
Scientists at the Laser Interferometer Gravitational-Wave Observatory, or LIGO, used Michelson’s invention to make the first-ever direct observation of gravitational waves in September 2015. Albert Einstein predicted the existence of gravitational waves in 1916 as part of his general theory of relativity, but no one had detected them directly. Since their first find, the LIGO scientists have detected the waves three more times. On Tuesday, they were awarded the Nobel prize in physics for their efforts.
The instrument at the heart of Michelson’s research is called an interferometer, which manipulates light inside closed tubes to make tiny measurements of natural phenomena. At LIGO’s twin observatories in Washington state and Louisiana, scientists use laser light.
Each observatory has two steel tubes measuring four kilometers long and placed in an L shape. At the ends of each arm are mirrors. LIGO scientists aim laser light into the L-shaped tube and let it travel back and forth between the mirrors. When gravitational waves reach Earth, they ripple through the arms of the tube, stretching and shrinking the steel. The distortions change the distance that the light travels as it bounces around. The change is the size of about one-thousandth the width of a proton.
Michelson’s L-shaped interferometer was much smaller than LIGO’s; each arm of in the instrument measured 11 meters long. While they’re more sophisticated, LIGO’s interferometers are essentially giant versions of Michelson’s interferometer from the 1880s.
It’s not the size of LIGO’s interferometer that makes the difference. A massive interferometer is still just an interferometer, which isn’t capable of detecting something as tiny as waves in space-time. LIGO scientists needed to make some tweaks to Michelson’s design so that the instrument could detect the tiniest of changes. They added, among other features, mirrors that would increase the number of times the laser beam was reflected as it traveled through the tubes. The extra technology extended LIGO’s interferometers arms to 1,120 kilometers, making the instrument 144,000 times bigger than the one Michelson used—and sensitive enough to detect gravitational waves.
Michelson and the LIGO scientists have something else in common: They’re now both Nobel laureates. Michelson won the prize in physics in 1907 for his study of the speed and properties of light, becoming the first American to win a Nobel in science. He may not have found luminiferous ether, but he pioneered the technique that found something much more elusive.
* This article originally stated that luminiferous ether involved sound waves. We regret the error.
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