Abstract
The current definition of the meter has been based on ‘the speed of light’ since 1983. Length standard measurements are performed using laser interferometers and are generally conducted in air. Specifically, a long length (referred to as “distance”) is measured in air. The accuracy of distance measurements in air is limited by the refractivity of air, which can be determined with an accuracy of only approximately 10-8. For smaller uncertainties, we must measure distances in vacuum. For example, both laser strain meters, which are used for tectonic measurements, and laser interferometer gravitational wave detectors employ long-baseline interferometers in vacuum. Since the sensitivity of gravitational wave detectors become worse by residual gases in vacuum, a pressure of less than 10-5 Pa is required for laser beam paths in the interferometer.
