Temperature Scales Below 1 Kelvin

The quest to measure temperature accurately began in ancient Greece with the invention of the thermoscope—an open, oil‐inglass device that was the forebear of the familiar, sealed, liquid‐in‐glass thermometer. Since that time, many new types of thermometer have been invented to serve contemporary science and commerce. The two extremes of temperature will, however, always remain out of reach of our devices. The highest temperature attained by a physical phenomenon, that of the universe at its birth, is certainly beyond our measurement capabilities, although not our ability to estimate—on the order of ${10}^{23}$ kelvin. The third law of thermodynamics forbids experiments from ever reaching the lower limit, absolute zero, although they may approach it arbitrarily closely. Efforts are under way to develop a temperature scale t hat would reach as low as 0.001 kelvin and extend the present international scale, which is compromising many experiments conducted below 0.65 K.