Absolute Zero

Absolute Zero, lowest temperature theoretically possible, characterized by complete absence of heat (thermal energy). Absolute zero is -273.15°C (-459.67°F), or zero degrees on the Kelvin scale (0 K). 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). Using combinations of lasers, magnetic fields, and radio waves, scientists have cooled magnetically confined atoms to within a few hundred billionths of a degree above absolute zero.

As substances are cooled to temperatures close to absolute zero, the effects of quantum mechanics become more important than the effects of thermal energy. Unusual states of matter can occur such as superfluidity, in which all resistance to flow disappears; superconductivity, in which all resistance to electric current disappears; and Bose-Einstein condensates, in which atoms in a gas behave like a single giant particle.

The concept of absolute zero temperature was first deduced in the late 17th and early 18th centuries from experiments with gases: When a fixed volume of gas is cooled, its pressure decreases with its temperature. The temperature at which the pressure would be zero is the absolute zero temperature.

The idea of absolute zero was clarified in the 19th century when the laws of thermodynamics were formulated. The second law, which defined entropy and prohibited perpetual motion machines, also suggested an absolute temperature scale with an absolute zero point of temperature. The third law of thermodynamics stated that absolute zero cannot be attained by any procedure in a finite number of steps. A temperature of absolute zero can be approached, but it can never be reached. In 1848 the British physicist William Thomson Kelvin developed an absolute temperature scale that started at absolute zero. Known as the Kelvin scale, it is widely used in many fields of science.

In the 20th century, quantum mechanics revealed new properties of atoms and particles. Even if an atom theoretically reached absolute zero and had all its thermal energy removed, it would still show quantum effects. As atoms are cooled closer and closer to absolute zero, quantum effects become much more obvious and important.

Studying quantum effects in atoms and particles is one of the main areas of research into temperatures near absolute zero. Other areas of research include controlling chemical reactions of molecules at ultracold temperature using electrical and magnetic fields.