From Classroom Haiku to Nobel Prize: The Story of LIGO's Birth
How Rainer Weiss's 'Gedanken' experiment in a general relativity class led to the groundbreaking detection of gravitational waves.
In "Black Hole Blues and Other Songs from Outer Space" by Janna Levin, the engineering story of LIGO is depicted:
They’re constructing a recording device, not a telescope. The instrument—scientific and musical—will, if it succeeds, record Lilliputian modulations in the shape of space. Only the most aggressive motion of great astrophysical masses can ring spacetime enough to register at the detectors. Colliding black holes slosh waves in spacetime, as can colliding neutron stars, pulsars, exploding stars, and as yet unimagined astrophysical spacetime cataracts.
In a less than perfect analogy, astrophysical calamities are the finger pickers, spacetime is the set of strings, and the experimental apparatus is like the body of the guitar. Or, moving up a few dimensions, astrophysical calamities are the mallets, spacetime is the skin of a three-dimensional drum, and the apparatus records the modulations in the shape of the drum to play the silent score back to us as sound.
A key figure in the development of this "musical instrument" recording the universe's symphony was Rainer Weiss. A prominent physicist, Weiss was one of the chief architects behind the Laser Interferometer Gravitational-Wave Observatory (LIGO). Born in 1932 in Berlin, he and his family fled to the United States during World War II. From an early age, he had a fiery passion for science. Weiss pursued Physics at MIT, later joining its faculty, where his groundbreaking work in gravitational physics and astrophysics began. He was essential in developing the concept and design for LIGO, marking a huge leap forward in the effort to detect gravitational waves. His phenomenal contributions were recognized in 2017 when he shared the Nobel Prize in Physics with Kip S. Thorne and Barry C. Barish for their decisive work on the LIGO detector and the observation of gravitational waves.
The idea came to him during a course he taught on the obscure subject of general relativity, Einstein’s theory of curved spacetime, as a junior professor. Rai says, “[MIT] figured that, hell, I had been to Princeton, so I must know something about relativity, right? . . . Well, what I knew about relativity you could stick in this finger. I mean general relativity. I’m not talking about special relativity.
“And I couldn’t admit I didn’t know general relativity. I mean, here I had started this whole research program to study gravity and I tell them that I don’t know anything about general relativity. I didn’t . . . so okay, I had a major problem on my hands. And I had to be sort of a day ahead of the students. Now all of us have been caught out that way, but I had just been caught out. I couldn’t say no.
“So I teach this relativity course. Now, the reason why that figures in the LIGO story is because that’s where LIGO got invented, in that course. This was about 1968 or 1969, and I was, as I say, one day ahead of the students. I had a terrible time with the mathematics. And I tried to do everything by making a Gedankenexperiment out of it. You see, I was trying to learn it myself. I mean, in the process of learning it, the mathematics was beyond what I really understood. But I kept trying to understand.
“I gave as a problem, as a Gedanken problem, the idea, ‘Well, let’s measure gravitational waves by sending light beams between things,’ because that was something you could solve. The idea was that here was an object. You’d put another object here and make a right triangle of objects, floating freely in a vacuum. And we’d send light beams between them and then be able to figure out, ‘What does the gravitational wave do to the time it takes light to go between those things?’ It was a very stylized problem, like a haiku, you know? You’d never think that it was of any value.”
The idea: Suspend mirrors so they’re free to rock parallel to the earth and watch them toss on the passing gravitational wave. Keep track of the distance between them, and their motions will record the changing shape of spacetime. Since light’s speed is a constant, the time it takes for light to race the track measures the length of the course. If the light travel time is a little longer, the distance between the mirrors has stretched. If the light travel time is a little shorter, the distance between the mirrors has squeezed.
Precision clocks are not good enough to distinguish minuscule variations in travel time. Rai’s idea was to use the floating mirrors to build a far more precise instrument, an interferometer (the roots of the word are “interfere” and “measure”). Instead of bouncing light along one arm, an interferometer sends light down two arms arranged in an L. Laser light is split into two beams, so that one beam travels along one arm of the L and the other travels along the orthogonal arm of the L. Each beam bounces off a mirror at the far end and returns down the respective arm to interfere back at the original apex. The recombined light is then split into two outputs. If the light travels the same distance in each direction, then the light in one output will recombine perfectly so that the output is bright. Light in the other output will combine in perfect cancellation so that the output is dark. If the arms are not the same length the light will come together but imperfectly, out of sync in a sense. The light will interfere with itself.
This beautiful idea underscores the birth of a transformative concept that would ultimately take shape as the Laser Interferometer Gravitational-Wave Observatory (LIGO). This story serves as a powerful testament to the might of inventive thinking, and emphasises how a singular idea, nurtured by a relentless pursuit of knowledge and comprehension, can dramatically reshape our understanding of the cosmos.
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