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Alumni Interviews

Exploring the Future
The Power of Big Science

Senior Scientist, LIGO Laboratory, California Institute of Technology (Caltech)

Koji Arai

December 20, 2021

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Dr. Koji Arai, Senior Scientist at Caltech, is one of the main architects of the “historic moment.” The technology he pursued at the Graduate School of Science certainly contributed to the proof of Einstein's prediction.

――The world got excited at the first report on detection of gravitational waves in February 2016.

Gravitational waves are a phenomenon predicted in 1916 as a logical consequence of Einstein's theory of general relativity. Wherever there is mass, there is distortion of spacetime; and, when the mass moves, the distortion is propagated as a wave. That wave is a gravitational wave. However, as the distortion is extremely minute, it was believed that observation would be difficult. Although observation experiments started in the late 1960s, gravitational waves had never been detected for a period of half a century.

As I myself, since I was a student, have been setting my sights on observation of gravitational waves for more than 20 years, I am filled with deep emotion. As a matter of fact, as the instrument for observation was also based on the technology I invented when I was a student, it is the greatest pleasure to think that “the instrument to which I made a contribution detected the gravitational wave.”

LIGO Hanford Observatory
ivingston Observatory (bottom). They are 3,002 km away from each other to prevent erroneous observation due to noise. ©LIGO Lab

――What sort of technology is it in concrete terms?

Gravitational waves are detected using a laser beam and mirrors. The laser beam is split into two orthogonal directions (i.e. arms) and mirrors are placed a few kilometers away at the end of each arm. You need to adjust the instrument in such a way that the reflected light waves coming back from the mirrors match exactly at the orthogonal meeting point. However, when there is a distortion in spacetime due to gravitational waves, the lengths of the arms changes slightly, resulting in mismatch of the waves. The underlying principle of the laser interferometer gravitational wave observation device, the mainstream detection device today, is based on the detection of this mismatch.

The interferometer mirrors hang down from pendula and are always swinging several μm or so. To maintain the instrument in a condition capable of detecting gravitational waves, one needs to control this swinging. When I was at the Graduate School of Science, I engaged in research on that technology, which was summarized in my doctoral dissertation. The value of this technology was recognized and used for the LIGO.

Dr. Arai, Senior Scientist, inspecting the device he himself built (left) and his colleague, an engineer (right). ©LIGO Lab / Corey Gray

―― What motivated you to go overseas?

I have worked for the LIGO Laboratory at the California Institute of Technology (Caltech) since 2009. Being an international joint experiment mainly supported by Caltech and the Massachusetts Institute of Technology (MIT), LIGO is a huge project with more than 1,000 researchers putting their names on as co-researchers.

Before that, I engaged in “TAMA300,” an observational experiment of the National Astronomical Observatory of Japan, since 1995, the year I started my study at the graduate school. At one stage, after the successful operation of the device in 1999, our device boasted to be the most sensitive device in the world. After having worked for the National Astronomical Observatory of Japan for 10 years, I decided to take up a new challenge to engage in research activities using the most advanced devices at LIGO. That is why I went to the US. When I went there, I came to realize that the research at the Graduate School of Science and the things we cultivated through the development of TAMA300 have been highly respected by LIGO and other projects. I think such accumulation of technology continues to benefit the KAGRA interferometer program currently pursued in Japan, as well.

――What is your message to students?

Through teamwork of many, one can achieve a huge goal, which could never be done by an individual. That is the whole point of big science. In addition, you can acquire the skills and capabilities that are widely required in society through collaborative activities with many people, for example making an experiment plan in a logical manner, putting it into practice through dialogue & cooperation with other people, and evaluating the result which forms the basis for your next activities. If you develop these skills and capabilities, you should be able to use them not just for academic research, but in other areas as well. There are a variety of scientific research projects, which are carried out through teamwork. I advise you, by all means, to feel the enjoyment of achieving something through team efforts in the Faculty of Science.

Interview and text: Masatsugu Kayahara
​Photography: Junichi Kaizuka

Originally published in The School of Science Brochure 2017

Koji Arai
Senior Scientist, LIGO Laboratory, California Institute of Technology (Caltech)
Graduated from Department of Physics, Faculty of Science, the University of Tokyo in 1995; Completed Master’s Course, Department of Physics, Graduate School of Science, the University of Tokyo in 1997; Studied between 1997 and 1999 in Doctoral Course, Department of Physics, Graduate School of Science, the University of Tokyo, and obtained Ph.D. in 2002 (Doctoral Dissertation). After serving as Research Associate and Assistant Professor at the National Astronomical Observatory of Japan between 1999 and 2009, joined LIGO Laboratory, Caltech in 2009 and has the current title since 2016.
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