PROVING EINSTEIN RIGHT the Committed Team Behind LIGO's Detection of Gravitational Waves Is Contending with Ongoing Technical Challenges of a Mighty Big Laboratory

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Inspecting the large optic infrastructure at LIGO Livingston observatory. The maintenance is constant and complex.

PROVING EINSTEIN RIGHT The committed team behind LIGO's detection of gravitational waves is contending with ongoing technical challenges of a mighty big laboratory.

BY KATHERINE BOURZAC The LIGO detection was a physics land- The global fanfare surrounding these sci- mark. It provided direct, observational evi- entific breakthroughs has been the culmina- n 14 September 2015, David Shoe- dence for a prediction made by Einstein more tion of years of hard work. “The physics is maker, a physicist at MIT, woke up to than 100 years ago in his theory of general really the easy part,” says Shoemaker, now the BRYCE VICKMARK BRYCE some incredible news. Detectors in relativity. It was also the first time astrophysi- spokesperson for the LIGO Scientific Col- Othe states of Washington and Louisiana had cists witnessed black holes merging. laboration. More complex was the delicate picked up ripples in space-time — gravita- Since then, LIGO has announced several precision-engineering in designing, upgrad- tional waves — emanating from the collision more gravity wave detections, also emanating and now maintaining the facilities needed of two black holes. ing from black-hole mergers, with their latest to detect black hole collisions more than a Shoemaker and about 1,000 of his col- announcement of a fifth detection, this time billion light years away. Add to that the more leagues at the Laser Interferometer Gravita- of colliding neutron stars. And on 3 October earthly challenges of getting a group of about tional-Wave Observatory (LIGO) Scientific 2017, three members of the LIGO team — 1,000 opinionated scientists in a dozen coun- Collaboration checked and debated the data Barry Barish, Rainer Weiss and Kip Thorne tries to work on a common project, interpret for months, publishing the results on 11 Feb- — were awarded the Nobel Prize in Physics for complex data, and agree on wording for a ruary 2016. their work in detecting the first waves. research paper.

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The next big task for project leaders is to PHYSICAL STRENGTH Building LIGO was unlike any other large- figure out how to transition from a blue-sky The majority of inter-state collaborations scale physics undertaking, says Barish, a experiment to an operational, data-producing between Massachusetts and California are in Caltech physicist who was the project’s execu- the life sciences, but narrowing in on astrophysical observatory. Engineers have collaborative relationships between MIT and tive director from 1994 (when construction begun the next round of upgrades, which Caltech reveals a shift in the subject balance. began) to 2005. Designers of telescopes and brings new technical quandaries. The two institutions have co-authored the most particle accelerators can draw on a legacy of papers in the physical sciences. experimental knowhow. Gravitational wave CROSS-COUNTRY COLLABORATION physicists were starting from scratch. “It was a Detecting gravitational waves from even the Life sciences huge extrapolation,” says Barish. “Much of the most catastrophic celestial events, such as Physical sciences technology had not been proven.” black-hole or neutron-star collisions, requires Chemistry To detect gravitational waves, LIGO must exquisitely sensitive instruments. Earth and environmental sciences measure minuscule changes in a light beam’s In the 1960s, physicists, including MIT’s distance travelled — far smaller than the diam- Weiss, imagined using a laser interferometer eter of an atom. This meant keeping the long Massachusetts and California have established to detect these wrinkles in space-time. A laser the most bilateral institutional partnerships in steel tubes as empty of matter as possible to interferometer splits a beam of light and sends the life sciences in the Nature Index in 2016. avoid wayward gas molecules from interfer- it down two perpendicular tubes. The light ing with the light path. To ensure the cleanest reflects off a mirror at the end of either tube, 450 signal possible, the team planned to create “the then recombines at a detector. When the light- ultimate vacuum system,” says Barish, in paired waves combine, they create an interference pat- 400 pipes, four kilometres long. Such a large vac- tern, which adds up the peaks and troughs of uum had never been attempted. Engineers had the waves. Weiss reasoned that a passing gravi- 350 to fit the tubes with doors to allow engineers to tational wave would stretch and compress the access the equipment inside. space through which the beams of light travel, 300 The 340-millimetre-diameter mirrors at the extending and shortening their path. This ends of the tubes are “exquisite objects,” says warping of space-time would appear as distor- 250 LIGO Lab chief engineer, Dennis Coyne, who is tions in the interference pattern. based at Caltech. To clean these mirrors, engi-

Groups at Caltech were also exploring the 200 neers have to climb into the vacuum chamber idea. In 1980, the National Science Founda- between runs, spread on a layer of polymer, and tion (NSF) funded prototype interferometers 150 peel it off, bringing any dust with it. at Caltech and at MIT. institutional partnerships Bilateral Success was hard earned. The team ran the

Nine years later, the two universities submit- 100 first phase of observations through 2010 — and ted a joint proposal for LIGO. In their design, saw no astronomical signals. No characteris- interferometers with four-kilometre-long arms tic gravitational wave squiggles from colliding 50 would be sited in two distant locations — one in black holes or neutron stars, no hum from the Hanford, Washington, and the other in Living- Big Bang, just noise. LIGO picked up cars driv- ston, Louisiana. The detectors were constructed 0 ing by, engineers’ footsteps, small earthquakes, at a great enough distance (roughly 3,000 kilo- and even the vibrations of the guitar-string-like metres) to eliminate any shared sources of dis- Collaborations between MIT on the east coast suspension wires holding up the mirrors. tortion or ‘noise’ in the signal, such as seismic and Caltech on the west coast are dominated The team upgraded the detectors over five activity jostling the system. by the physical sciences. Collaboration score years, replacing the vibrating steel wires with The equipment and observatories would be (CS) sums the fractional count of collaborative quieter silica fibres, as well as other adjustments operated by LIGO Laboratory, a collaboration papers from the two partnering institutions. to compensate for systematic errors. LIGO between Caltech and MIT. These two institu- detected gravitational waves in September tions are among the top five US contributors 22 2015 thanks to what Barish calls “the world’s to the authorship of physical sciences articles Caltech best shock absorbers”. The team suspended in the journals included in the index, collabo- 20 the mirrors below a series of three weights that rating on 209 papers in 2016. LIGO’s activities would soak up vibrations, like shock absorb- 18 Caltech and MIT and data analysis would be coordinated by the collaborations ers. Another detection was made in December LIGO Scientific Collaboration, a much larger contributed to 10 2015, and again in January and August 2017. 16 community of members from more than 100 times the share of Getting over all these hurdles has brought the institutions and 18 countries. authorship in the LIGO engineers and scientists together. “We’re 14 physical sciences compared to any very close-knit,” says Shoemaker. The group

12 other subject in the makes decisions as a team — in some cases, a Nature Index 2016. very large team. “Imagine writing a paper with “IMAGINE WRITING CS 1,000 authors who all have an opinion about 10 what should be in it,” says executive director of

A PAPER WITH 1,000 8 LIGO Lab, David Reitze. Conference calls rou- MIT tinely happen at 7 am California time to include AUTHORS WHO ALL HAVE 6 the European groups; dedicated members in Australia must dial in at midnight. AN OPINION ABOUT 4 NEW FRONTIERS WHAT SHOULD BE 2 Now that LIGO has shown it is possible to detect gravitational waves, the team hopes IN IT.” 0 to do it much more routinely — to become a

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MIT's David Shoemaker says the physics was the easy part of getting LIGO up and running. The engineering and collaborative logistics were much harder to overcome.

gravitational wave observatory, akin to a large Despite these objections, some upgrades telescope. This transition from ambitious phys- “I AM EXCITED can’t be delayed. The facilities need new roofs. ics experiment to everyday observatory comes The infrastructure, including the steel pipes with its own problems, says Shoemaker. housing the laser beams, is 20 years old, and

TO BE WOKEN IN THE VICKMARK BRYCE LIGO was designed with upgrades planned, the quality of the vacuum is degrading. and engineers want to keep making them. The MIDDLE OF THE NIGHT The Louisiana site has problems.Tiny holes payoff for physicists will be con-siderable. are appearing along the tubes. “It’s a 10,000 The deeper into the Universe they can see, the TO SEE WIGGLES THAT cubic-metre vacuum system, and finding greater the likelihood of observing astronomi- and repairing these leaks is a challenge,” says cal phenomena such as a spiralling pair of neu- DON'T LOOK LIKE Reitze. Their hypothesis is that the steel might tron stars. The detectors will soon go offline be corroding due to contact with mouse urine again until autumn 2018 for more upgrades. MODELS OF and with the chlorine from black widow spider LIGO’s reach is currently limited by the poison. Wasps build their nests on the tubes, power of the laser and the quality of the vac- ANYTHING.” and put the spiders inside to feed their young. uum. So far, the lasers have not been turned In probing the secrets of the Universe, says to their full power, in part due to problems Reitze, “we’re up against nature.” with scattered light reflecting off the tube and is working on how it could be implemented at LIGO participants dream of less mundane re-entering the beam. Engineers plan to add the LIGO observatories. concerns: what will they do when they detect a protective layer to the tube walls that will Some stakeholders object to the breaks. a signal that they know is a gravitational wave, absorb scattered light. They are also working Funders want the observatory they paid for but that doesn’t look like anything that theo- on long-term strategies to improve sensitivity running, and so do astrophysicists who want rists have predicted would emanate from a even further, including adopting a cutting-edge to keep searching for signals. Black holes are known astronomical phenomenon? “I am quantum optics technique called light squeez- considered to be the best laboratory for study- excited to be woken up in the middle of the ing that could make the signal clearer. So far ing general relativity, and physicists have only night to see wiggles that don’t look like our this work is in theoretical stages, but the group just gained access to them through LIGO. models of anything,” says Shoemaker. ■

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