KAGRA: Japan’s Underground Einstein Wave Detector

On June 1, U.S. and European physicists published the latest results in their quest for gravitational waves — tiny ripples in spacetime, generated by energetic events like the collision and merger of distant black holes. Meanwhile, a new gravitational wave observatory is under construction in Japan. Sky & Telescope Contributing Editor Govert Schilling visited the KAGRA detector in 2016.

In a huge cave deep within Mount Ikeno in Western Japan, construction workers are building the world’s next big laser interferometer. An initial version of KAGRA (Kamioka Gravitational-wave Detector) was completed and tested in March and April 2016. Now, new equipment is installed to create the ‘baseline’ version of the instrument. Additional mirrors, towering suspension systems, new lasers, cryogenic cooling units — it’s a huge operation, hopefully finished by late 2018.

The bus stop at the Mozumi post office.
Govert Schilling

From Tokyo’s Ueno train station, it’s a relaxing two-hour drive with the luxurious Shinkansen ‘bullet train’ to Toyama on Japan’s west coast. From there, an early-morning bus takes me into the mountains. It’s a stunning 75-minute ride along steep overgrown mountain slopes, covered with wisps of fog. I’ve been told to leave the bus at the Mozumi post office bus stop. Mozumi is a very small mining village just across the border of Gifu prefecture. Mining activities in this region date back to the 8th century. The Kamioka zinc and lead mine — named after another town some 10 kilometers down the road — suspended its operations in 2001, but some miner families still live here.

Mozumi village
Govert Schilling

The KAGRA headquarters are located in a new building overlooking the village. Respecting Japanese tradition, I change my shoes for the slippers provided at the building’s entrance. Unfortunately, they’re all much too small for my large Dutch feet. Yoichi Aso of the Japanese Gravitational Wave Project Office shows me around the small control room, where a 10-minute morning briefing takes place. He then provides me with a hard hat and a fluorescent safety vest and we drive some five kilometers to the entrance of the mine, about a thousand meters below the summit, in the side of Mount Ikeno.

The Kamioka mine has turned into a versatile physics facility over the past few decades. 1991 saw the start of the excavation of the huge cave that is now home to Super-Kamiokande — one of the world’s largest neutrino detector experiments. Other underground physics experiments at the Kamioka Observatory are the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) and the Xenon Detector for Weakly Interacting Massive Particles (XMASS), which is searching for dark matter.

The central area of KAGRA. 
Govert Schilling

Through yet another horizontal tunnel, Aso takes me to the central area of KAGRA, where upgrade activities are in full swing. It’s an impressive sight: a huge cavern, filled with gleaming vacuum tanks, portal cranes, scaffolding, forklifts, beam pipes with huge bolted flanges, and racks of electronics. Thanks to the weird contrast between the rough-hewn rock and the high-tech equipment it all looks like the secret underground laboratory of some mad scientist in a sci-fi movie — it’s surreal.

The disadvantages of building a gravitational-wave laboratory underground are also very apparent. The rocky walls have been treated with an anti-dust coating, but the cave can never be as clean as the central buildings of the two LIGO detectors in the USA (Laser Interferometer Gravitational-Wave Observatory) or the European Virgo detector. The most sensitive parts of the equipment are all placed underneath ‘clean booths’: large plastic overpressure tents into which filtered air is blown.

A much bigger problem is water. As any speleologist knows, caves are notoriously wet. The relative humidity in the KAGRA cavern is anywhere between 75 and 100 percent. The water emerges through the walls of the caves and the two 3-kilometer tunnels. It even wells up through the tunnel floors, thanks to groundwater pressure. It’s an incredible amount: some 500 tons of water per hour.

The view down one of the KAGRA tunnels.
Govert Schilling

Aso takes me to the near end of one of the damp and dimly lit tunnels. The stainless steel beam pipes will probably not suffer too much, but water has accumulated into small puddles on the tunnel floor, and I can clearly make out a constant dripping sound. Parts of the ceiling have been covered with huge sheets of plastic. To facilitate draining, the tunnel floors are slanted by some two degrees. Because of the tilt, the KAGRA mirror surfaces must be slightly tilted, too — another technological challenge.

The walls and ceiling of the central cavern are plastic lined to prevent water damage.
Govert Schilling

Most of the walls of the central cavern are also plastic lined. But here, too, water is the main enemy. The problem was particularly severe in the spring of 2015, when the cave was flooded in some places by up to 10 centimeters of water. Water was dripping from the ceiling of the cave onto the clean booths. The installation of the vacuum system had to be suspended for several months.

Back in Tokyo, at the Mitaka campus of the National Astronomical Observatory of Japan (NAOJ), Italian-born project director Raffaele Flaminio is well aware that the problem is not yet under control. But, he says, the same has happened in the Gran Sasso underground particle physics laboratory in the Italian Alps. ‘Just after construction, there was water everywhere. Now, it’s solved. We will find a solution, too.’

When KAGRA is operational, hopefully in late 2018 or early 2019, there will be four large laser interferometers working together. Observing data will be shared between the American, European, and Japanese groups for joint analysis. Running four detectors in concert further reduces the false alarm rate. Moreover, if the Einstein waves from a neutron star or black hole merger are detected by four independent instruments, pinpointing the event on the sky can be done with a relatively high precision. Follow-up observations by automated counterpart searches with ground-based optical telescopes are going to be much more efficient.

Just a few years further in the future, hopefully in 2022, there will be a fifth large interferometer, in India. Known as LIGO India, it could be described as an Asian ‘outpost’ of the LIGO project, using U.S.-built equipment. Eventually, it will be an almost exact copy of the current Advanced LIGO detectors, with 4-kilometer arms.

Albert Einstein once said: "Look deep into Nature, and then you will understand everything better." The same is true for the new sense that gravitational-wave astronomy is providing us with. We’ve learned to surf the waves of spacetime. The journey is far from over — it’s only just beginning.


This story is an edited excerpt of the final chapter of Govert Schilling’s new book ‘Ripples in Spacetime. Einstein, Gravitational Waves, and the Future of Astronomy’, to be published in late July by Harvard University Press.

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About Govert Schilling

Sky & Telescope Contributing Editor Govert Schilling lives in the Netherlands, but loves to explore his home planet. In July, Harvard University Press will publish his new book, ‘Ripples in Spacetime. Einstein, Gravitational Waves, and the Future of Astronomy’.
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