 collective support.”
Listening for a wobble
On Sept. 14, 2015, at approximately 5:51 a.m. EDT, a gravitational wave — a ripple from a distant part of the universe — passed through the Earth, generating an almost imperceptible, fleeting wobble in the world that would have gone completely unnoticed save for two massive, identical instruments, designed to listen for such cosmic distortions.
The Laser Interferometer Gravitational-wave Observatory, or LIGO, consists of two L-shaped interferometers, each 4 kilometers in length, separated by 1,865 miles. On Sept. 14, 2015, scientists picked up a very faint wobble in the instruments and soon confirmed that the interferometers had been infinitesimally stretched — by just one-ten-thousandth the diameter of a proton — and that this miniscule distortion arose from a passing gravitational wave.
The LIGO Scientific Collaboration, with the Caltech-MIT LIGO Laboratory and more than 1,000 scientists at universities and observatories around the world, confirmed the signal as the first direct detection of a gravitational wave by an instrument on Earth. The scientists further decoded the signal to determine that the gravitational wave was the product of a violent collision between two massive black holes 1.3 billion years ago.
The momentous result confirmed the theory of general relativity proposed by Albert Einstein, who almost exactly 100 years earlier had predicted the existence of gravitational waves but assumed that they would be virtually impossible to detect from Earth. Since this first discovery, LIGO has detected three other gravitational wave signals, also generated by pairs of spiraling, colliding black holes; the most announced of a detection came just last week.
“We are incredibly proud of Rai and his colleagues for their vision and courage that led to this great achievement,” says Michael Sipser, the Donner Professor of Mathematics and dean of the School of Science at MIT. “It is a wonderful day for them, for MIT, for risk-taking and boldness, and for all of science.”
A gravitational blueprint
The detection was an especially long-awaited payoff for Weiss, who came up with the initial design for LIGO some 50 years ago. He has since been instrumental in shaping and championing the idea as it developed from a desktop prototype to LIGO’s final, observatory-scale form.
In 1967, Weiss, then an assistant professor of physics at MIT, was asked by his department to teach an introductory course in general relativity — a subject he knew little about. A few years earlier, the American physicist Joseph Weber had claimed to have made the first detection of gravitational waves, using resonant bars — long, aluminum cylinders that should ring at a certain frequency in response to a gravitational wave. When his students asked him to explain how these Weber bars worked, Weiss found that he couldn't.
No one in the scientific community had been able to replicate Weber’s results. Weiss had a very different idea for how to do it, and assigned the problem to his students, instructing them to design the simplest experiment they could to detect a gravitational wave. Weiss himself came up with a design: Build an L-shaped interferometer and shine a light down the length of each arm, at the end of which hangs a free-floating mirror. The lasers should bounce off the mirrors and head back along each arm, arriving where they started at the exact same time. If a gravitational wave passes through, it should “stretch” or displace the mirrors ever so slightly, and thus change the lasers’ arrival times.
Weiss refined the idea over a summer in MIT’s historic Building 20, a wooden structure built during World War II to develop radar technology. The building, meant to be temporary and known to many as the “Plywood Palace,” lived on to germinate and support innovative, high-risk projects. During that time, Weiss came to the conclusion that his design could indeed detect gravitational waves, if built to large enough dimensions. His design would serve as the essential blueprint for LIGO.
An observatory takes shape
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