lowest temperature theoretically possible, characterized by complete absence of heat, at approximately –273.16°C (–459.69° F), or zero degree on the Kelvin scale (0 K). See HEAT,; TEMPERATURE,.

The concept of absolute zero temperature was first deduced from experiments with gases; when a fixed volume of gas is cooled, its pressure decreases with its temperature. Although this experiment cannot be conducted below the liquefaction point of the gas, a plot of the experimental values of pressure versus temperature can be extrapolated to zero pressure. The temperature at which the pressure would be zero is the absolute zero temperature. This experimental concept of a gas-thermometer temperature scale and of absolute zero was subsequently shown to be consistent with the theoretical definitions of absolute zero.

Absolute zero cannot be reached experimentally, although it can be closely approached. Special procedures are needed to reach very low, or cryogenic, temperatures (see CRYOGENICS,). Liquid HELIUM (q.v.), which has a normal boiling point of 4.2 K (–268.9° C/–452.0° F), can be produced by cryostats, extremely well-insulated vessels, based on a design by the American mechanical engineer Samuel Collins (1898–1984). If the helium is then evaporated at reduced pressures, temperatures as low as 0.7 K can be obtained. Lower temperatures require the adiabatic (no heat transfer) demagnetization of paramagnetic substances (substances of low magnetizability), such as chrome alum, while they are being surrounded with a liquid helium bath (see THERMODYNAMICS,). The method, which was first developed in 1937 by the Canadian-American chemist William Giauque, utilizes a magnetic field that initially aligns the ionic magnets of the material. If the magnetic field is removed, the magnets again assume their random orientation, reducing the thermal energy of the material and thus its temperature. Temperatures as low as 0.002 K have been reached with the demagnetization of paramagnetic salts, and the demagnetization of atomic nuclei has yielded temperatures as low as 0.00001 K.

Temperature measurements at values close to absolute zero also present special problems. Gas thermometers can only be used up to the liquefaction point of helium. At lower temperatures, electric and magnetic measurements must be used to determine the effective temperature.

The concept of absolute zero temperature is also important in theoretical considerations. According to the third law of thermodynamics, the entropy, or state of disorder, of a pure crystal is zero at absolute zero temperature; this is of considerable importance in analyzing chemical reactions and in quantum physics.         F.La., FRED LANDIS, M.S., D.Sc.

See also CHEMICAL REACTION,; QUANTUM THEORY,; SUPERCONDUCTIVITY,; SUPERFLUIDITY,.