History of the Search for Gravitational Waves
By Alan Chodos
The direct observation of gravitational waves, announced last year, has propelled LIGO into the headlines and has been widely recognized as the leading scientific achievement of the year. What is not always stressed, however, is how long and tortuous the road has been to get to the point where that discovery was possible.
The April Meeting session (held on January 30) "History of the search for gravitational waves" was organized by FHP to tell that story. Speakers were Virginia Trimble of UC Irvine, Barry Barish of Caltech, and Richard Isaacson of NSF (retired).
Trimble reminded the audience that the first experimental efforts to find gravitational waves were not interferometric, but rather used resonant metal bars. The person who led this effort was Joseph Weber, to whom Trimble was married from 1972 until Weber’s death in 2000.
Weber graduated from the US Naval Academy in 1940, and had a distinguished naval service career during World War II. After the war he joined the faculty at the University of Maryland, where he was a pioneer in the development of the maser and the laser, and then became interested in the possibility of detecting gravitational waves as predicted by Einstein’s general relativity.
Weber began his experiments in the mid 1960’s, and by the late ‘60s he was reporting positive results. However, in Trimble’s terminology, by the mid ‘70s Weber had been "voted off the island". The physics community had decided that Weber’s results were not reproducible, and probably just statistical noise. Whether or not he ever saw gravitational waves, however, Weber introduced many experimental and analytical techniques that carried over to other experiments like LIGO.
Despite the skepticism of the community, Weber continued to improve and refine his instruments, for example experimenting with cryogenic techniques and increasing the sensitivity of the piezoelectric strain detectors. Later in his career he also became interested in using similar techniques to detect neutrinos.
In Trimble’s summation, "if Joe had not been looking for gravitational waves back then, perhaps no one would be looking for them even now."
Trimble was followed by Barry Barish, who led the LIGO Laboratory and collaboration in the crucial period 1994-2005. In his talk, Barish gave a comprehensive history of the scientific aspects of gravitational waves, from their first discussion by Einstein in 1916 to their discovery a century later. As Trimble had also mentioned, it was not universally agreed among theorists that general relativity actually predicted the existence of waves. There were tricky issues regarding singularities and coordinate choices. Barish recounted one incident in the mid-1930s, when Einstein himself, in collaboration with Rosen, submitted a paper to Physical Review asserting that waves did not exist. He received a referee’s report pointing out some errors in the analysis, which so offended Einstein that he never again published in Physical Review. However, the referee, Howard Percy Robertson, communicated to Einstein through Leopold Infeld; Einstein realized the error, revised the paper, and published it in a different journal. (Interestingly, Infeld later became the foremost skeptic about the existence of gravitational waves).
Barish acknowledged Weber’s pioneering contributions, including sensitive noise analysis, use of coincidence, and using time slides for estimating background. He mentioned early work on interferometry, both in Russia and the US, which led to serious experimental work beginning in the 1990s. LIGO started construction in 1994, with a two-stage plan that would result in LIGO first becoming operational, and then undergo significant upgrades to "Advanced LIGO", which would finally reach the sensitivity at which actual detection became probable. LIGO has separate laboratories in Louisiana and Washington State; coincident signals at the two locations is a necessary condition for detection.
Barish discussed the factors, at various frequencies, that limited the detectors’ sensitivity. Among them are seismic noise at low frequencies, thermal noise at intermediate frequencies, and high-frequency "shot noise". Mitigation of these effects is achieved by better suspension and isolation of the mirrors, and increasing the power of the laser to improve statistics and limit shot noise.
Barish noted that in addition to LIGO, other detectors are either beginning operation or are under construction worldwide, for example GEO and Virgo in Europe and KAGRA in Japan. With all these operational, sensitivity and frequency range will be enhanced, and directional information on any observed sources will be greatly improved.
Following Barish was Richard Isaacson, who played a pivotal role in making sure LIGO received the necessary support for what has been the largest facility ever funded by NSF. He noted the seminal contributions of what he called the "three musketeers", Rai Weiss of MIT and Kip Thorne and Ron Drever of Caltech. However, he pointed out that many others were key contributors along the way.
Many people worked on prototype interferometers in the ‘70s, including Weiss at MIT, Drever and his group at Glasgow, and a group at the Max Planck Institute, Garching. In the ‘80s, the MIT and Caltech groups started to come together, Drever moved to Caltech, and a proposal for the LIGO laboratory was submitted to NSF, with Rochus "Robbie" Vogt as the Project Director. But Isaacson pointed to the appointment of Barry Barish as the Director in 1994 as a watershed moment. Barish brought "big science" experience from high-energy physics to bear on LIGO. He helped develop the two-stage LIGO/Advanced LIGO plan, established the LIGO Scientific Collaboration (LSC), with more than a thousand scientists worldwide, to operate in tandem with the laboratory, but with separate governance, and he introduced "modern project management and planning techniques to break down this very complex project into understandable modules."
As a result, Isaacson stated, "LIGO was successfully completed ... on time, on scope, and on budget."
In addition to the LIGO facility and the LSC, Isaacson mentioned the numerical relativity community, dating back to early work in the ‘60s and ‘70s, and culminating in the ability to predict the signals that LIGO should see from various astrophysical events. "We have finally extracted the details of the exact dynamical predictions from Einstein’s theory, and solved the fundamental gravitational 2-body problem with radiation."
Isaacson went on to describe the relationship of LIGO to NSF, which has contributed about $1.1B over 40 years. He mentioned prominent support from Physics Division Director Marcel Bardon, and from Erich Bloch, who was the NSF Director under whom LIGO began construction. In addition, he cited numerous program officers who were instrumental in supporting theoretical gravitational research, computing initiatives, and the LIGO laboratory itself.
As Isaacson concluded, "There are lots of men and women who deserve recognition for their essential intellectual contributions and decades of hard work building the world’s most sensitive instrument."