We’re conducting a series of blog postings in the lead up to the announcement of the 2021 Nobel Prize for Physics on October 5th, looking at past recipients and what they did after earning the prize. Michael Banks speaks with Takaaki Kajita, a pioneer in neutrino research, on the shift into gravitational-wave research.

Japan has dominated the world in neutrino science over the past half-century. Masatoshi Koshiba, a Japanese scientist, oversaw the building of a massive neutrino detector 1000 meters deep in a lead and zinc mine in Hida, Gifu Prefecture, in the 1980s. It was called Kamiokande, and it was a massive water tank surrounded with photomultiplier tubes to detect light flashes caused by neutrinos colliding with atomic nuclei in water molecules.

In the course of becoming one of the inventors of neutrino astronomy, Koshiba notably utilized the experiment to detect neutrinos from a distant supernova explosion. For his contribution on the detection of cosmic neutrinos, he shared the Nobel Prize in Physics in 2002.

Takaaki Kajita, a physics student at the time of Koshiba’s Nobel-winning work, was fascinated by the study of these phantom particles and opted to pursue a PhD at the University of Tokyo under Koshiba’s supervision. The work piqued Kajita’s interest sufficiently that he joined the Institute for Cosmic Ray Research (ICRR) at the University of Tokyo in 1988, where he worked on neutrino physics for the following two decades.

The successor to Kamiokande, the Super-Kamiokande experiment, commenced operating in 1996, and Kajita played a significant role in using the facility to prove that the ratio of electron to muon neutrinos originating from opposing sides of the Earth was different two years later. This meant that when these neutrinos – which are generated when cosmic rays contact with nuclei in the high atmosphere – traveled through the Earth, their flavor changed. For the first time, it was demonstrated that neutrinos must have mass, although at a low energy of around 0.1 eV.

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“Science has become critical in determining the course of civilization – and possibly the future of the Earth.”

Kajita followed in Koshiba’s footsteps, sharing half of the 2015 Nobel Prize in Physics with neutrino scientist Arthur McDonald “for the discovery of neutrino oscillations, which demonstrates that neutrinos have mass.” Experiments led by McDonald at the Sudbury Neutrino Observatory (SNO) established how many electron neutrinos generated in the Sun transform into muon neutrinos or tau neutrinos as they travel to Earth. The fact that roughly two-thirds of solar electron neutrinos change flavor by the time they reach the Earth was confirmed by SNO data.

Changing fields

Despite contributing to Japan’s great neutrino physics history, Kajita took the daring choice to alter study disciplines in 2008, before being awarded the Nobel Prize. “I wanted to develop something fresh that would be meaningful and fascinating after many years of neutrino research,” Kajita told Physics World. “Fortunately, our institute had been preparing a gravitational-wave experiment to follow Super-Kamiokande as the next major project.” The KAGRA gravitational-wave observatory, which would be situated underground near Super-Kamiokande, was that facility.

In 2008, Kajita was named director of ICRR, and he was instrumental in having KAGRA funded and built. KAGRA is a massive interferometer with two 3 km-long arms that divide a laser beam. The beams are numerous times reflected between mirrors hung at the ends of each arm before being merged at a detector. A gravitational wave is a ripple in space–time that can modify the distances between mirrors as it travels through an interferometer. A difference in how the laser light interferes at the detector detects this.

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KAGRA, which is 200 meters below, began construction in 2010 and will be operating in 2020. KAGRA is anticipated to join the international search for gravitational waves next year, owing to the effect of COVID-19. The project’s primary investigator is presently Kajita.

In addition to changing study topics, Kajita expanded his knowledge of science by joining the Science Council of Japan (SCJ) in 2017. The SCJ’s job is to offer recommendations to the government and the general public on various topics. He was nominated for president of the SCJ by the other SCJ members in 2020. “I have no idea why so many of them voted for me,” Kajita acknowledges. “However, I suppose the Nobel Prize had an impact on the vote!”

Kajita says he joined the SCJ to help the public understand the significance of fundamental science, which he believes is as as essential as performing the work itself. He argues, “Science has become so crucial for determining the course of civilization – or perhaps the future of the Earth.” “Physics is unquestionably an important branch of science. As a result, I hope that physicists devote more effort to tasks such as science policy.”

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