In 2017, three American researchers were awarded the Nobel Prize in Physics for their “significant contribution to gravitational wave observation using the laser interferometer LIGO.” While Dr. Einstein’s theory of general relativity has been experimentally demonstrated, direct observation of gravitational waves is so difficult that it is called Einstein’s Last Homework, and it took 100 years from prophecy to observation realization.
In order to observe gravitational waves directly, countries around the world are working together and Japan’s KAGRA is planning to start observation in 2019 as a low-temperature laser interferometer with the world’s largest 3km L-shaped observation facility.

To observe gravitational waves, observe the differential changes in the optical path of both arms. The more accurate the distance the laser beam runs, the more accurate it is.

Since the effect of gravitational waves appears as a small change in the distance between the mirror and the mirror installed 3 km away in each arm, the Gravitational Wave Telescope measures this change using laser interferometer technology. However, the measurement accuracy required for that is one tenth of a trillionth of a meter. In order to measure the ridge with such high accuracy, for example, if there is a tint of air in the part where the laser beam passes, the refractive index fluctuation becomes noise, so the space through which the laser beam passes should be ultra-high vacuum condition.
Especially in case of KAGRA, high technology is required to keep the vacuum duct with 80 cm diameter and 3 km length single arm, in an ultra-high vacuum condition.