 1916 that gravitational waves would exist, but thought them too weak to ever be detected. By the 1960s, technological advances such as the laser and new insights into possible astrophysics sources made it conceivable that Einstein was wrong and that gravitational waves might actually be detectable.
The first person to build a gravitational-wave detector was Joseph Weber of the University of Maryland. Weber's detectors, built in the 1960s, used large aluminum cylinders, or bars, that would be driven to vibrate by passing gravitational waves. Other researchers elsewhere, including the late Ronald W. P. Drever at the University of Glasgow in Scotland—later a professor of physics at Caltech—soon followed Weber's lead.
When those experiments proved unsuccessful, the focus of the field began shifting to a different type of detector called a gravitational-wave interferometer, invented independently by Weiss at MIT and, in rudimentary form, by several others. In this instrument, gravitational waves stretch and squeeze space by an infinitesimal amount while widely separated mirrors hanging by wires "ride" the oscillations, moving apart and together ever so slightly. This mirror motion is measured with laser light using a technique called interferometry.
In the late 1960s, Weiss began laying conceptual foundations for these interferometers. In parallel, Thorne, along with his students and postdocs at Caltech, worked to improve the theory of gravitational waves, and estimated the details, strengths, and frequencies of the waves that would be produced by objects in our universe such as black holes, neutron stars, and supernovas.
In 1972, Thorne, with his student Bill Press (MS '71, PhD '73), published the first of many articles that would appear over the next three decades, summarizing what was known about the gravitational-wave sources and formulating a vision for gravitational-wave astronomy.
"LIGO would not exist without Kip's vision for the scientific potential of gravitational waves and his amazing gift for sharing that vision with other scientists," says Stan Whitcomb (BS '73), the chief scientist for the LIGO Laboratory at Caltech, who began working on the project in 1980.
Also in 1972, Weiss published a detailed analysis of his interferometers. He identified all of the major obstacles that could prevent the instruments from detecting gravitational waves, such as vibrations of the earth and of the mirrors, and he invented techniques to deal with each obstacle. At this stage, it became evident that large interferometers, several kilometers or more in size, might possibly prove successful—as, indeed, they ultimately did with LIGO and its 4-kilometer-long arms. Also evident was the fact that perfecting the interferometers would be exceedingly difficult: a passing gravitational wave would induce mirror motions 1,000 times smaller than a proton, and these infinitesimal changes would have to be measured. That's 100 million times smaller than an atom, and a trillion times smaller than the wavelength of the light being used in the measurement.
Triggered by Weiss's work, Drever's research group in Glasgow switched from bars to interferometers, as did a research group in Garching, Germany, led by Heinz Billing. By 1975, there were three prototype interferometers under development at MIT, Glasgow, and Garching.
A Fateful Hotel Room Discussion
At first Thorne was skeptical of Weiss's interferometer idea. "I even wrote, in a textbook, that it was not very promising," he says. But that changed when Thorne studied, in depth, Weiss's 1972 analysis. Thorne came to call it a "tour de force" and a "blueprint for the future."
In 1975, Weiss invited Thorne to speak at a NASA committee meeting in Washington, D.C., about cosmology and gravitation experiments in space. Hotel rooms that summer were fully booked, so the two shared a room, where they stayed up all night talking. Thorne came away so excited by the experimental prospects that he went home and proposed creating an experimental gravity group at Caltech to work on interferometers in parall