Absolute temperature

Absolute temperature is temperature measured from absolute zero, the lowest thermodynamic temperature. Its SI unit is the kelvin, whose zero corresponds to absolute zero.

Kelvin: The SI base unit of thermodynamic temperature, defined by fixing the Boltzmann constant. Absolute temperature is commonly reported in kelvins, with zero at absolute zero.
Ideal gas law: An equation relating pressure, volume, amount, and absolute temperature for an ideal gas. Its temperature variable must be measured from absolute zero.
Celsius: A temperature scale that sets water's freezing point at zero degrees and boiling point at 100 degrees under standard conditions. Its zero is offset from absolute zero, unlike the kelvin scale.
Boltzmann distribution: A probability distribution giving the equilibrium populations of energy states at a specified temperature. Its exponential weighting uses absolute temperature to set the scale of thermal fluctuations.
Third law of thermodynamics: The thermodynamic principle that a system's entropy approaches a limiting value as temperature approaches absolute zero. It explains why absolute zero cannot be reached by a finite sequence of processes.
Absolute zero: The limiting temperature at which a system has its minimum possible thermal energy. It supplies the zero point from which absolute temperature is measured.
Carnot cycle: An ideal reversible heat-engine cycle operating between two thermal reservoirs. Its maximum efficiency depends on the reservoirs' absolute temperatures.
Fahrenheit: A temperature scale that assigns 32 degrees to water's freezing point and 212 degrees to its boiling point under standard conditions. Its zero is conventional rather than absolute.
Equipartition theorem: A classical result assigning an average energy of one-half kT to each quadratic degree of freedom at thermal equilibrium. The theorem directly links kelvin temperature to average microscopic energy.
Laser cooling: Methods that use light to reduce the motion and temperature of atoms or ions. It approaches absolute zero while illustrating the limits of practical cooling.

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