Cryogenics is the branch of physics dealing with the behavior of matter at very low temperatures, and with the production of those temperatures; the word comes from the Greek kruos meaning frost). Early cryogenics relied heavily on the Joule-Thomson effect by which temperature falls when a gas is permitted to expand without an external energy source. Using this, James Dewar liquefied hydrogen in 1895 (though not in quantity until 1898), and H. K. Onnes liquefied helium in 1908 at 4.2 kelvin (K) (see absolute zero).


Several cooling processes are used today. Down to about 4K the substance is placed in contact with liquefied gases which are permitted to evaporate, so removing heat energy by the so-called Joule-Thomson effect). The lowest that can be reached in this way is around 0.3K.


Further temperature decrease may be obtained by paramagnetic cooling (adiabatic demagnetization). Here a paramagnetic material (see paramagnetism) is placed in contact with the substance and with liquid helium, and subjected to a strong magnetic field, the heat so generated being removed by the helium. Then, away from the helium, the magnetic field is reduced to zero. By this means temperatures of the order of 10-2–10-3 K have been achieved (though, because of heat leak, such temperatures are always unstable).


A more complex process, nuclear adiabatic demagnetization, has been used to attain temperatures as low as 2 × 10-7 K.


Near absolute zero, substances can display strange properties. Liquid helium II has no viscosity and can flow up the sides of its container. Some elements display superconductivity: an electric current started in them will continue indefinitely.


Low temperatures can be used to preserve foods for periods of years. Much publicity has surrounded the idea of freezing people with terminal illnesses, so that at some date in the future, when medical science has advanced sufficiently, they may be revived and cured (a subject known as cryonics). Another idea mooted is that of freezing travelers on interstellar spacecraft (see suspended animation; sleeper ship).