the origin from which thermodynamic temperature is counted; it lies 273.16 K below the temperature of the triple point of water (0.01°C) (273.15°C below the zero of the Celsius scale, see Temperature scales). The existence of a thermodynamic temperature scale and of absolute zero of temperature follows from the second law of thermodynamics. As the temperature approaches absolute zero, the thermal characteristics of a substance tend to zero: entropy, heat capacity, the coefficient of thermal expansion and others. In the ideas of classical physics, at absolute zero the energy of the thermal (random) motion of the molecules and atoms of a substance is zero. According to quantum mechanics, however, at absolute zero of temperature the atoms and molecules sitting at the sites of a crystal lattice are not completely at rest: they perform “zero-point” vibrations and possess what is called zero-point energy. If the mass of the atoms and the energy of interaction between them are very small, zero-point vibrations can prevent a crystal lattice from forming. This is the case with 3He and 4He, which remain liquid at atmospheric pressure down to the lowest temperatures yet reached.
Obtaining temperatures that come extremely close to absolute zero of temperature is a difficult experimental problem (see Low temperatures), but temperatures only millionths of a degree away from absolute zero have already been reached.