Big Engineering Helps Catch an Elusive Wave
Sounds from the sky
That sound you heard in the detection of gravitational waves announced last week was a new tone in the music of the spheres that people have been listening to since antiquity. Tuning into just the right frequency, as researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) managed to do, and capturing the distinctive glissando of gravitational waves passing through the earth’s atmosphere mark a triumph not only of science but also of engineering.
The innovative, persistent work of one engineering professor, Joseph Weber, laid the groundwork for both the theory and instruments at the heart of the LIGO discovery. And scores of sophisticated technical advances in instrumentation originated in the course of LIGO’s construction, thanks to the work of countless engineers across multiple, varied disciplines of the field.
Big science, hallelujah
As with all “big science” projects, the engineering dimensions of the work are inextricably entwined among the threads of theory, inquiry, and data that researchers follow in their efforts to extend the frontiers of human knowledge and capability.
Engineering the search for waves
On the engineering faculty at the University of Maryland, Joseph Weber followed fundamental engineering practices of asking important, relevant questions, testing his design solutions over and over, and learning from failure in his pioneering work on the question of how to detect gravitational waves.
He started work in the 1950’s on building tools to hear the vanishingly faint ripples of cosmic phenomena that many people doubted were even there. On this key, but unproven, part of the General Theory of Relativity, Einstein himself famously reversed himself on the existence of gravitational waves before reaffirming the necessity of their existence.
Learning to listen
Weber persisted in his work for over 40 years, and while his claims for evidence of gravitational waves never stuck, the force of his thinking did. He died 15 years ago, just before LIGO started initial measurements, but his questions about gravitational wave detection inspired students and peers to continue applying his tools and methods.
Their efforts found dramatic success in the confirmation that Einstein was ultimately right about gravitational waves. Weber’s work pointed the way to the idea of listening for waves as they wend their barely detectable way through the galaxy, opening human ears for the first time to the sounds of the universe working to keep our feet on the ground beneath us.
The Weber story
Dogged pursuit of his goals and imperviousness to failure animated Weber his whole life.
At age five, he lost the ability to speak after being struck by a bus. Having to relearn the mechanics of speech, he emerged from therapy with a totally different sound to his voice.
In World War II, Weber was assigned aircraft carrier duty in the Pacific theater. He survived the sinking of his ship, the USS Lexington, in the 1942 Battle of the Coral Sea. Posted to the Mediterranean in command of a submarine-chasing ship, he ended up part of the landing force in the invasion of Sicily a year later. His expertise in radio and radar later earned him responsibility for the Navy’s program in electronic countermeasures.
Accomplishments in this field landed him a job on the engineering faculty at the University of Maryland in 1948. Writing his dissertation at night, Weber taught and researched during the days on topics of study that gave shape to the emerging field of lasers.
From lasers to waves
Nobel Prizes eventually went to the people who made use of Weber’s findings to build the first laser-generating instruments. Weber himself, while personally close to these researchers, was largely lost in the hubbub. However, the next direction of his research marked the first moves in the 60-plus-year journey towards the September 2015 detection of gravitational waves that was announced last week.
Gravitational waves, Weber deduced, could be better heard than seen. He imagined, in essence, very sensitive, very large wind chimes that would resonate with the passing of gravitational waves through their midst.
Hard to hear
To try and detect these exceedingly faint signals, he built a “resonant bar antenna,” an aluminum cylinder suspended in a vacuum chamber. The cylinder was hooked up to highly sensitive instruments designed first to detect vibrations in the cylinders caused by waves hitting them and then to translate these vibrations into sound.
To confirm that the signal was coming from a gravitational wave, Weber built two such antennae and located them several hundred miles apart. If both bars were to detect the same wave, Weber thought, they would ring in the same way, just a few thousandths of a second apart in time. Repetition of the signal across distance would eliminate the possibility that a bar might be registering local, accidental phenomena.
Not necessarily the big news
In the late 1960’s, Weber published findings he believed to show evidence of gravitational waves making his bars chime. But others failed to corroborate his findings, and their results – plus Weber’s apparent mistakes with data – suggested Weber’s signals were originating with other, random wave-generating phenomena. He persisted in this work for decades thereafter but never found wide acceptance of either his theories or findings.
Roots take hold
But Weber’s methods and theories did take root in the minds of some researchers. Following his work with resonant bar antennae, or “Weber bars” as they had come to be known, some people working on gravitational wave detection, such as MIT’s Rainer Weiss, theorized that pairs of laser beams would work better than aluminum cylinders as gravitational wave detection devices.
They would have to be tuned to register only phenomena bearing the signature supposed to belong to gravitational waves. If such a wave were to cross the path of a beam, it would disrupt the oscillation cycle of that particular beam and throw it out of sync with the partner beam, something sufficiently sensitive instruments would then be able to detect.
Just as Weber had done with his cylinders, researchers called for beams to be built far apart to validate the possible detection of any waves passing through their paths. Also following Weber’s example, they hoped to record the evidence of gravitational waves as an audible signal, the chime of a wave rippling through the universe to be captured in the tick-tock of laser beams going ever so slightly and briefly out of phase, one a fraction of a second after the other.
But can you get there from here?
However, building the facility that could enable such research to be carried out faced a significant obstacle: the technologies required to do the work did not exist.
Rich Isaacson, a physicist in his own right, was the NSF program officer working on funding the project (see p. 14 at the link) in the early 1990’s. In a New Yorker article, he noted,
“[LIGO] never should have been built. It was a couple of maniacs running around, with no signal ever having been discovered, talking about pushing vacuum technology and laser technology and materials technology and seismic isolation and feedback systems orders of magnitude beyond the current state of the art, using materials that hadn’t been invented yet.”
Nevertheless, Isaacson and a clutch of gravitational wave detection researchers, including many of Weber’s students and peers, persuaded the National Science Foundation to put over $270 million towards the construction of what came to be LIGO.
Engineering comes to the fore
Weber’s work helped provide a model for LIGO, but for it to become a reality, engineers had to imagine, design, and build a raft of complex, cutting-edge technologies. And they all had to work together at degrees of precision and calibration never before made operational. As the facility’s website says:
“LIGO is a masterpiece of complex and sophisticated engineering. Super-stabilized lasers, unprecedented vacuum systems, the purest optics, extraordinary vibration isolation, and servo controls all work symbiotically for one singular purpose: To sense the ephemeral passage of a gravitational wave.”
Everything comes together
Instruments at the two LIGO facilities captured results proving that a gravitational wave had passed through their sensors.
And this, it turns out, is exactly what happened on September 14, 2015, the very first day of a new round of LIGO operations. After extensive double- and triple-checking, LIGO scientists confirmed that the fleeting chirp they had heard did in fact indicate a gravitational wave passing through their laser beams. It had originated over a billion years ago in the implosion of two black holes across the galaxy collapsing into one, newly formed black hole approximately the size of the state of Maine.
Viva Weber
Coming 15 years after Weber’s death, the discovery nevertheless occasioned unequivocal recognition that his work made him the “father of gravitational wave detection.” And while he gained most of his fame eventually as a physicist, Weber’s engineering roots clearly anchor LIGO’s design and functioning.
He addressed himself to a big, relevant problem – how to detect gravitational waves – imagined various answers, and designed and improved his chosen solutions over many years. He persisted through repeated cycles of failure, and while he himself never achieved final success, the lessons gleaned from his career-long efforts provided the template for the instrument that allows us to hear part of the soundtrack to the universe that we have never heard before.
Eric Iversen is VP for Learning and Communications at Start Engineering. He has written and spoken widely on engineering education in the K-12 arena. You can write to him about this topic, especially when he gets stuff wrong, at eiversen@start-engineering.com.
You can also follow along on Twitter @StartEngNow.
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Photo credits:
Merging Black Holes, courtesy of NASA Ames Research Center; Joseph Weber head shot, courtesy of University of Maryland A. James Clark School of Engineering; LIGO Hanford, courtesy of LIGO; gravitational wave results, courtesy of American Physical Society and used by permission;