Absolute zero is the zero point of thermodynamic temperature, corresponding to exactly 0 kelvin (K), −273.15 °C, or −459.67 °F. It represents the cold limit of ordinary equilibrium systems, rather than merely an exceptionally low temperature on an arbitrary scale. Although classical mechanics associates this limit with the disappearance of thermal motion, quantum mechanics allows matter to retain zero-point motion. Absolute zero therefore does not mean that every particle becomes motionless or that all energy disappears. (nist.gov)
Temperature scales and historical development
The concept emerged from investigations of gases and thermodynamics. At fixed volume, the pressure of a sufficiently dilute gas falls approximately linearly as its temperature decreases. Extrapolating that relationship suggests a temperature at which pressure would vanish. Such extrapolation identifies a limiting temperature, not an experiment in which a real gas remains gaseous all the way to zero: real substances depart from ideal-gas behavior as they are cooled. William Thomson, later Lord Kelvin, proposed an absolute temperature scale in 1848, helping establish temperature independently of the properties of a particular thermometer substance. (nist.gov)
In the International System of Units, the kelvin has the same interval size as the degree Celsius. Thus a temperature difference of 1 K equals a difference of 1 °C, while their numerical origins differ:
[ t/^\circ\mathrm{C}=T/\mathrm{K}-273.15. ]
The kelvin’s magnitude is defined by fixing the Boltzmann constant at exactly (1.380649\times10^{-23}\ \mathrm{J,K^{-1}}). This connects temperature with microscopic energy without making a particular material’s freezing point the fundamental definition. The unit symbol is K, without a degree sign. (nist.gov)
Microscopic meaning
In statistical mechanics, temperature governs the relative occupation of available energy states. For a system following the Boltzmann distribution, the probability of occupying a state of energy (E_i) is proportional to
[ \exp!\left(-\frac{E_i}{k_{\mathrm B}T}\right). ]
At positive temperatures, lower-energy states are favored. As temperature approaches zero, occupation concentrates in the lowest-energy state or states, called the ground state. Thermal excitation above this lowest-energy configuration consequently disappears in the ideal equilibrium limit. (mpg.de)
This does not imply zero internal energy. Quantum systems can possess zero-point energy even in their ground states. For example, a quantum harmonic oscillator has a ground-state energy
[ E_0=\frac{1}{2}\hbar\omega, ]
where (\omega) is its angular frequency and (\hbar) is the reduced Planck constant. This energy lies above the minimum of the corresponding classical potential. The uncertainty principle prevents the oscillator from simultaneously having perfectly definite position and momentum; its ground state therefore retains quantum fluctuations rather than classical stillness. These fluctuations are not thermal agitation. (ocw.mit.edu)
Entropy and the third law
Absolute zero is central to the third law of thermodynamics. A standard formulation states that the entropy of a pure, perfect crystal approaches zero as its temperature approaches zero, provided its equilibrium ground state is unique. Microscopically, a unique ground state gives a single accessible configuration in the zero-temperature limit. This establishes a reference for absolute entropy rather than merely for entropy differences. (chem.purdue.edu)
The qualification matters: zero temperature does not automatically guarantee zero entropy for every conceivable system. Ground-state degeneracy or frozen configurational disorder can produce residual entropy. Thus the familiar statement about a perfect crystal should not be generalized into a claim that all possible forms of matter become identical, perfectly ordered, or devoid of quantum behavior at zero temperature. (arxiv.org)
A related formulation is the unattainability principle: exact absolute zero cannot be reached by a finite cooling procedure with finite physical resources. A 2017 theoretical treatment quantified this limitation through restrictions on cooling time and resources. Approaching zero and reaching it exactly are therefore physically distinct tasks. The entropy formulation and unattainability formulation are closely connected, but their precise equivalence depends on the assumptions governing the permitted processes. (nature.com)
Experimental approaches
Cryogenics provides several routes toward extremely low temperatures. A dilution refrigerator uses helium-3 and helium-4 mixtures to reach the millikelvin range; NIST describes a system operating at approximately 30 mK. Such temperatures remain above zero, despite being only a small fraction of a kelvin from it. (nist.gov)
For dilute atomic gases, laser cooling reduces atomic motion through controlled interactions with light. Further evaporative cooling removes more energetic atoms, allowing the remaining trapped sample to become colder. Combining these techniques enabled the production of a Bose–Einstein condensate, in which many atoms share a common quantum state. Bose–Einstein condensation in dilute alkali gases was first achieved in 1995 and recognized by the 2001 Nobel Prize in Physics. A condensate is not evidence that absolute zero has been reached: condensation occurs at a finite temperature. (mediaplayer.nobelprize.org)
Negative absolute temperatures
Certain specially prepared systems with an upper bound on their accessible energy can be described as having negative absolute temperatures. Their energy-state populations are inverted: higher-energy states are preferentially occupied rather than lower-energy ones. In this statistical description, they are hotter than any positive-temperature system, not colder than absolute zero. They cannot be obtained simply by continuing ordinary cooling through 0 K. (mpg.de)
The thermodynamic interpretation of negative temperatures has prompted debate over entropy definitions. Nevertheless, the population-inverted states do not represent a temperature below absolute zero in the ordinary sense of greater coldness. Their preparation and interpretation concern bounded energy spectra, rather than removal of thermal excitation beyond the ground-state limit. (nature.com)