Japan to Begin Pioneering Hunt for Gravitational Waves the Underground KAGRA Detector Will Deploy Ambitious Technology to Improve Sensitivity

Home , LIGO

NEWS IN FOCUS PHYSICS Tantalizing signs of POLITICS Violence in NEW YEAR Gene-editing, MATERIALS The scramble superconductivity at near- Nicaragua engulfs open access and seals with to understand a twisted room temperature p.12 scientists p.11 sensors to shape 2019 p.13 form of graphene p.15 THE ASAHI SHIMBUN VIA GETTY

Japan’s Kamioka Gravitational Wave Detector is scheduled to start up in 2019, joining a global network of interferometers.

PHYSICS Japan to begin pioneering hunt for gravitational waves The underground KAGRA detector will deploy ambitious technology to improve sensitivity.

BY DAVIDE CASTELVECCHI When operations begin later this year, their job past few years, these machines have begun will be to bounce infrared laser beams back and to detect gravitational waves — long-sought nside a house-sized scaffolding wrapped forth along two 3-kilometre, high-vacuum ripples in the fabric of space-time, created by in thick plastic sheets, Takayuki Tomaru pipes, ready to sense the passage of gravitational cataclysmic cosmic events such as the merging is in full clean-room attire. The physicist, waves (see ‘Japan’s wave hunter’). of two black holes or the collision of two Iwho works at the High Energy Accelerator The ¥16.4-billion (US$148-million) obser- neutron stars. Research Organization (KEK) in Tsukuba, vatory — Japan’s Kamioka Gravitational With the addition of KAGRA, the growing Japan, is performing one of the most delicate Wave Detector (KAGRA) — will work on global network of detectors will enable astro- and crucial tasks in the construction of a grav- the same principle as the two detectors of the physicists to locate the position of these feeble itational-wave observatory: installing one of Laser Interferometer Gravitational-Wave cosmic signals in the sky with greatly increased the machine’s four mirrors, each a 23-kilogram Observatory (LIGO) in the United States precision. They will be able to dissect the cylinder of solid sapphire known as a test mass. and the Virgo solo machine in Italy. In the waves’ properties, such as how they are

NEWS IN FOCUS

oriented in space, better than ever before, sensitive prototype detector in the world, says not letting the coolers’ vibrations creep in the ultimately revealing more about the elusive physicist Raffaele Flaminio, who has for sev- opposite direction. cosmic objects that produce them. eral years been a leading KAGRA researcher The prospects for a large, Japanese-led But KAGRA will not just be more of the at the National Astronomical Observatory of gravitational-wave detector suddenly improved same — it could prove to be an important test Japan. But TAMA wasn’t expected to make a towards the end of that decade, when Kajita bed for future detectors. “KAGRA is carry- discovery. Because gravitational waves stretch stepped in to champion such a project. He had ing out tests of two concepts that may prove the dimensions of space, the waves’ effects led a rebuilt Super-Kamiokande that made important for the future of gravitational-wave are most noticeable over long distances, major breakthroughs in neutrino science, and astronomy,” says Rainer Weiss, a physicist at putting smaller detectors at a disadvantage. he lent his credibility to KAGRA as someone the Massachusetts Institute of Technology in Moreover, human-generated vibrations made who knew how to manage a big-science project. Cambridge who co-founded LIGO. TAMA — located in Tokyo — a non-starter, It was a similar role to the one that Barry Barish, One innovation is that it is the first major Kajita explains. a physicist at the California Institute of Tech- interferometer to be built underground. In the 1990s, researchers in Europe nology (Caltech) in Pasadena, had for LIGO, Its two arms stretch inside tunnels under and the United States secured funding says University of Tokyo physicist Shinji Mount Ikenoyama, near Japan’s north coast. to build LIGO, Miyoki. “We believe this is an advantage, because seis- two 4-kilometre In 2010, Japan’s parliament approved mic noises are typically two orders of mag- interferometers in “It’s a daunting funding for the project, which also has funding nitude smaller underground,” says Takaaki Washington state and but courageous partners including South Korea and Taiwan. Kajita, a physicist at the University of Tokyo Louisiana, and the thing that KAGRA’s name — chosen from 600 sugges- who is KAGRA’s principal investigator. 3-kilometre Virgo, KAGRA has tions made by the public — is also a reference And whereas LIGO’s and Virgo’s mirrors but Japanese physi- done.” to kagura, a dance for the gods that is part of operate at room temperature, KAGRA’s will be cists faced an uphill Japan’s ancient Shinto tradition, says Miyoki, kept very cold, at 20 kelvin, to cut down noise struggle for funding. A further setback came who has worked on TAMA and CLIO, and from thermal vibrations. in 2001, when a serious and costly accident now has a leading role at KAGRA. If KAGRA works as planned, it could at Super-Kamiokande, a massive neutrino provide crucial know-how for the field. The observatory also under Mount Ikenoyama, A PROBLEM OF LOCATION use of cryogenics, in particular, might be made the Japanese government wary of Construction began in 2012, and KAGRA’s essential if future detectors are to offer vastly funding big-science projects. 6 kilometres of tunnels were dug in less improved sensitivity, says physicist David Still, Japanese researchers kept working on than 2 years. Shoemaker at the Massachusetts Institute of interferometer development, pursuing the But the location also poses a problem: the Technology, who is LIGO’s spokesperson. cold-mirror route. In 2006, the Cryogenic mountain’s rock is porous and soaked in water. Laser Interferometer Observatory (CLIO) KAGRA physicist Keiko Kokeyama recalls A WORK IN PROGRESS began operating in a tunnel at Kamioka. The visiting the site in 2014, when she was work- Japan was an early starter in the race to 100-metre prototype was the first to have cryo- ing on LIGO at Caltech. “Inside the tunnel, it detect gravitational waves, the existence of genically cooled mirrors, and took two dec- was raining very hard,” she says, and the floor which Albert Einstein predicted more than ades to perfect, Kajita says. That’s in large part was covered in mud. Keeping the tunnels dry a century ago. Researchers at the Univer- because cryogenic cooling poses a conundrum required an extra layer of lining, says Kokeyama, sity of Tokyo built prototype interferom- for gravitational-wave science. “Coolers are who oversees the interferometer’s laser source, eters in the early 1990s, following work by mechanical things,” Kajita says, so they create as well as having other roles. physicists in the United States, the United vibrations of their own. Researchers had to Each spring, when the snow above ground Kingdom and Germany. When TAMA, work out how to keep the coolers in physical melts, the tunnels’ drainage system takes a machine with 300-metre arms, began contact with the mirrors’ suspensions — so away up to 1,000 tonnes of water per hour. operating in 1998, it was the largest, most that they could keep the mirrors cold — while This probably means that KAGRA will have to schedule yearly shutdowns in the wettest months, Kajita says. “With such conditions, I JAPAN’S WAVE HUNTER think it’s unrealistic to operate.” The Kamioka Gravitational Wave Detector (KAGRA) is the world’s fourth major gravitational-wave detector — The team hopes that the machine will be and Asia’s rst. Due to open in late 2019, it is the rst one to be built underground, and to have cryogenically cooled mirrors, operating at around 20 kelvin. Both innovations should help KAGRA to separate the cosmic ready before the end of 2019, in time to join ripples from background noise. a year-long observation run that LIGO and Virgo are due to start in March. When KAGRA starts up, the world’s gravitational-wave community will be watching. LIGO is planning an upgrade called LIGO Voyager that will also When a gravitational wave passes Test mass through Earth, one arm stretches have cold mirrors — although not as cold as and the other shrinks, and then KAGRA’s. And the US community is design- the opposite happens. ing a 40-kilometre cryogenic machine called Cosmic Explorer. Meanwhile, researchers in Europe hope to build an observatory called the Einstein Telescope, which will be a Laser 10-kilometre-sided triangle, and will be both source The pipes and cryogenic and underground. “I believe people silos contain a high vacuum. will learn from KAGRA,” says Kajita. “It’s a daunting but courageous thing that The laser source sends an infrared beam, which is split into Detector KAGRA has done,” Shoemaker says, “to two parts (one per arm); these read-out pursue several of the things we think we need bounce between the mirrors. for future detectors. It will help us enormously to have that experience.” ■