Absolute zero

Absolute zero is the temperature corresponding to 0 K. At this temperature the molecules and atoms have as close to no kinetic energy as quantumn mechanics will allow. Since the kinetic energy is related to the motion of the atoms, 0 K corresponds to the point at which all atoms are motionless. Obviously, it is not possible for the atoms to move slower than to be motionless, thus it is not possible to achieve a temperature below 0 K.

(This definition is not quite correct. Absolute zero is not zero kinetic energy, rather it means that all things in the system are at their quantum "ground state" energy, the lowest energy they are "allowed" to have by the laws of physics. In many cases this is not zero.

Also, temperatures below 0 K can be achieved for certain thermodynamic systems, according to some (but probably not all) of the definitions of temperature used by physicists. See also population inversion and Laser which should cover this but probably don't yet.)

At very low temperatures in the vicinity of absolute zero, matter exhibits many unusual properties including superconductivity, superfluidity, and Bose-Einstein condensation. In order to study such phenomena, scientists have worked to obtain ever lower temperatures. To date the lowest temperature ever acheived was 20 nK (20 billionths of a degree above absolute zero). This record-setting low temperature was achieved in 1995 by scientists working for NIST in Boulder, Colorado. The work is described in the July 14, 1995 edition of the journal Science.

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