absolute zero

Zero kelvin (−273.15 °C) is defined as absolute zero. Absolute zero is the theoretically coldest temperature, a state at which a system's internal energy, and in ideal cases entropy, reach their minimum values. The Kelvin scale is defined so that absolute zero is 0 K, equivalent to −273.15 °C on the Celsius scale, and −459.67 °F on the Fahrenheit scale. The Kelvin and Rankine temperature scales set their zero points at absolute zero by definition. This limit can be estimated by extrapolating the ideal gas law to the temperature at which the volume or pressure of a classical gas becomes zero.

Although absolute zero can be approached, it cannot be reached. Some isentropic processes, such as adiabatic expansion, can lower the system's temperature without relying on a colder medium. Nevertheless, the third law of thermodynamics implies that no physical process can reach absolute zero in a finite number of steps. As a system nears this limit, further reductions in temperature become increasingly difficult, regardless of the cooling method used. In 2018, scientists at the University of Bremen achieved temperatures as low as 38 picokelvin (pK), 38 billionths of a degree above absolute zero. At these low temperatures, matter displays exotic quantum mechanical phenomena such as superconductivity, superfluidity, and Bose–Einstein condensation. The particles still exhibit zero-point energy motion, as mandated by the Heisenberg uncertainty principle and, for a system of fermions, the Pauli exclusion principle.