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The Hunt for Gravitational Waves Goes Underground

The new Japanese facility, to be ready by the year’s end, will be Asia’s first detector and the world’s first below ground

14 September 2015 began like most other mornings for 32-year-old Marco Drago. The post-doctoral student was sitting at his computer at the Albert Einstein Institute in Hanover, Germany, watching data stream live from the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Livingston, Louisiana. Suddenly, he noticed something unusual on his screen — a compressed wave — and gave a start.

It was the signal LIGO scientists had been waiting for more than 40 years — a chirp signifying the first direct observation of gravitational waves. Sometimes referred to as the “holy grail of astrophysics” because it marks a crucial piece in the puzzle that is Albert Einstein’s general theory of relativity, detecting gravitational waves has eluded physicists for close to a century.

But with more sophisticated and sensitive detectors — called interferometers — being built, physicists hope to repeat that September day many times over. To date, gravitational waves have been spotted three other times. In addition to LIGO, there are now two other detection facilities — Virgo in Pisa, Italy, and GEO600 in Hanover, Germany. A fourth, situated in Japan, is currently under construction and expected to come online in late 2019 or early 2020. KAGRA, as the new interferometer is called, will be the first one built underground.

Professor Takaaki Kajita, Nobel Prize in Physics (2015), Director of at the University of Tokyo, says, “One of the noise sources for gravitational wave detectors is that from seismic activities. At the surface, seismic noises are generally high, due to various reasons such as human activities. Deep underground, seismic noises are much lower and stable, which is why we decided to locate the KAGRA detector deep in underground.”

Seeking the holy grail of astrophysics

Einstein first predicted the existence of gravitational waves in 1916, postulating that violent cosmic events, such as two black holes merging, distorts the space and time around them. Waves that result from such massive objects moving about in space create ripples that emanate outwards through the cosmos, similar to how ripples move across the surface of a pond.

Although the source of gravitational waves are cataclysmic events — the September event stemmed from two black holes, one 29 times the mass of the Sun and the other 36 times, slamming into one another 1.3 billion years ago — the waves diminish over time and distance as they travel to Earth. Because gravitational waves are relatively weak, they can only be detected with extremely sensitive equipment.

Interferometers, which detect gravitational waves, are L-shaped optical devices with kilometers-long arms arranged at right angles to one other. Within each vacuumed arm, a laser beam bounces back and forth many times between the mirrors suspended by a glass thread. Under normal circumstances, the laser light from one arm meets the light from the other arm, cancelling each other out. But when a gravitational wave passes through, it warps space-time and the laser beams no longer match up, resulting in a small amount of light striking the detector.

The design to detect the waves has been hailed as ingenious. “I think this will be one of the major breakthroughs in physics for a long time,” said Szabolcs Marka from Columbia University professor, and one of the LIGO scientists. To celebrate the accomplishment, the Nobel Committee awarded the 2017 Physics prize to the three US scientists — Rainer Weiss, Kip Thorne and Barry Barish — for their “decisive contributions to the LIGO detector and the observation of gravitational waves.”

A new type of detector

Constructing interferometers, however, is not without its challenges. Physicists have to make them extremely sensitive to the infinitesimal changes a gravitational wave would cause to the distance between the two mirrors. At the same time, they have to prevent other noises from interfering with the detector, which might lead to false readings. Each arm of an interferometer is a vacuum swaddled in layers of steel and concrete, creating an environment isolated from the tiniest vibrations.

Japan’s new KAGRA detector, estimated to cost US$150 million, will be similar to its US and European predecessors, barring two key features — its location underground and the use of mirrors chilled to extremely low temperatures. It is nestled 200 metres below Mount Ikenoyama in the Gifu prefecture of central Japan, in what used to be an old zinc mine. The Kamioka mine is also home to Super-Kamiokande, one of the world’s largest neutrino detectors.

Unlike existing interferometers, KAGRA won’t be as susceptible to seismic vibrations — thanks to its position below the Earth’s surface. Moving the detector underground cuts the seismic noise factor by 100, says Kajita, who is spearheading the new facility.

But building a detector underground comes with certain disadvantages. An underground cave environment will never be as clean as detectors in above-ground facilities. So the most sensitive parts of Japan’s detector will have to be kept in special tents, with filtered air constantly cycled in. The old mine also presents an extremely wet and humid setting, with humidity predicted to range between 75 to 100 percent. To combat this, KAGRA’s planners have designed the tunnel floors to tilt at an angle of 0.2 degrees, presenting yet another technological challenge to its construction.

Making up for that is the promise KAGRA that will, hopefully, be able to detect more gravitational waves because of its improved sensitivity. Helping to reduce noise disturbances further are the special mirrors that will be installed in its arms. The mirrors are made of sapphire, a material chosen specially for its ability to withstand extremely low temperatures. Physicists plan to cool the mirrors to 20 Kelvin, or -253 degrees Celsius — because most molecules cease to vibrate at such incredibly low temperatures. The result is fewer disturbances, which will help researchers pick up signals from gravitational waves more easily.

Professor Kajita added, “Any single gravitational wave detector cannot determine the direction of the source of a gravitational wave. Only a joint analysis of the data from largely separated gravitational wave detectors can tell the source direction. When KAGRA becomes operational, it will work together with the LIGO and Virgo detectors, share data with them for joint analysis, which will allow researchers to pinpoint sources of the waves with much greater accuracy.”

Kajita has been guiding KAGRA’s development since the facility was first approved in 2010. He shared the 2015 Nobel Prize in Physics for discovering that the neutrinos have mass, after demonstrating that subatomic particles come in three “flavours,” and oscillate from one to another.

Kajita is one of a handful of eminent international scientists who took part in the Global Young Scientists Summit 2018, an annual event held in Singapore. The five-day event, organised by the National Research Foundation Singapore, aims to facilitate interactions of bright, young international researchers with leading researchers to discuss key areas of science and research, technology innovation and society, and solutions to global challenges.

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