COMMENT OBITUARY Ronald Drever (1931–2017) Experimental physicist key to the detection of gravitational waves.

onald (Ron) Drever was theoretical physicist Kip Thorne. a hands-on physicist with There, he initiated plans for a 40-metre a child-like joy for experi- prototype gravitational-wave detector. Rmentation. He co-invented several Meanwhile, a German group started important techniques to directly up a 30-metre prototype, and the ARCHIVES CALTECH detect gravitational waves — ripples Massachusetts Institute of Technol- in space-time created by accelerat- ogy (MIT) in Cambridge also began a ing masses. He co-founded the Laser study of the science, concept and cost Interferometer Gravitational-wave of a device. Observatory (LIGO) project, which In 1983, the Caltech and MIT announced the first direct detection research groups joined forces to of those long-sought waves in 2016. build and operate a full-scale device: Drever, who died aged 85 in Scot- LIGO. The collaboration was initially land on 7 March, was an intuitive and directed by a committee of Drever, imaginative physicist who thought pri- Thorne and myself. But we had differ- marily in pictures. Those pictures — of ent visions and were unable to make concepts or devices — gave him an decisions. In 1987, the project moved elegant way to circumvent a lot of analytical new research programme at Glasgow for the forward under a single director, Rochus Vogt, reasoning, and provided a way to think about detection of gravitational waves. who helped us to write a definitive proposal problems that often resulted in an invention. First, Drever devised a more-sensitive and that attracted the funds to design and con- As a child in Scotland, guided by an engi- broader-band version of the ‘acoustic bar’ struct the initial detector. The project — the neer uncle, Drever assembled a television gravitational-wave detector that others were largest ever funded by the US National receiver from parts left over from the Second using at the time to try to replicate the 1969 Science Foundation — consisted of two World War. He did not do well in preparatory result. But in the early 1970s, several groups L-shaped detectors, each with arms 4 kilome- school, but came into his own at the Univer- were exploring a technique that involved laser tres long, in Washington state and Louisiana. sity of Glasgow, UK. There he studied nuclear interferometers. The idea was to measure the Drever was a strong contributor to the con- physics, including particle detectors and their distortions of space-time caused by gravita- ceptualization of LIGO. But he struggled to electronics. His 1958 PhD thesis was on radia- tional waves by timing how long laser light move from the freedom of table-top science tion counters for nuclear decay. took to travel between mirrors in an interfer- to the rigorous schedule and firm decision- As a lecturer at Glasgow, Drever tested ometer. Drever, as well as experimenters in making necessary to pin down a large-scale an idea newly proposed by theorists. This Germany, showed that scattered laser light project. In such projects, thorough engineer- was that objects on Earth might have one made noise that restricted the sensitivity of ing practice and careful analysis take priority inertial mass when travelling in the plane the interferometer. over intuition and pictorial reasoning. of the Milky Way, and another when travel- One solution was to stabilize the frequency In 1994, LIGO’s then-director Barry Barish ling perpendicular to it, owing to the uneven of the laser. On a sabbatical to Harvard Uni- supported a decision made at Caltech to give distribution of the Galaxy’s mass. He and a versity in Cambridge, Massachusetts, in 1979, Drever a separate laboratory, where he could scientist at Yale University in New Haven, Drever learned about a method devised dur- develop new ideas and techniques for future Connecticut, independently checked the ing the Second World War to stabilize the fre- gravitational-wave detectors. LIGO started motions of nuclei in lithium atoms to see quencies of microwaves for radar by reflecting collecting data in 2002, and detected gravi- whether they were affected by the orientation them with a resonant cavity. Drever and his tational waves from a pair of colliding black of the nuclei relative to the Galactic plane. colleagues adapted the technique for lasers holes in 2015. This led to many prizes and Drever did his experiment in his parents’ and optical cavities, creating what is now awards for Drever and his colleagues. back garden, away from the magnetic dis- called Pound–Drever–Hall cavity stabiliza- It is well known that Drever and I had turbances of the university, using equipment tion. That has become a central technique in different views about the direction for tech- borrowed from the teaching labs. Neither saw precision optical systems, including LIGO. nical development for LIGO. I disagreed with any effect, thereby establishing the uniform- Other ideas further improved laser inter- him about the use of optical cavities; it turned ity of space to an unprecedented precision. It ferometer detectors in the 1970s. Drever, with out he was right. I held out for a solid-state was a landmark result. colleagues in Germany and Glasgow, came up laser while he insisted on a green argon one; The work took Drever into cosmology and with systems for power and signal recycling Drever was wrong on that one. But we always astrophysics. When pulsars were discovered to increase the sensitivity of the interferom- respected each other’s views, and as LIGO’s in 1967, he searched for γ-rays that might eter and to adjust its response. Both systems construction progressed we became close accompany the radio-wave pulsations. After involved adding more partially reflecting mir- colleagues and friends. ■ researchers announced in 1969 that they had rors to the input and output components of detected gravitational waves coming from the the instrument. Rainer Weiss is emeritus professor of Galactic Centre (a result later found to be spu- By this time, Drever was working at the physics at the Massachusetts Institute of rious), Drever hunted for radio pulses com- California Institute of Technology (Caltech) Technology in Cambridge. ing from the same location. And he began a in Pasadena, having been invited by US e-mail: weiss@ligo.mit.edu

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