{{otheruses2|Absolute Zero}}
'''Absolute zero''' describes a theoretical system that neither emits nor absorbs energy. It is the point at which particles have a minimum energy, determined by [[quantum mechanics|quantum mechanical]] effects, which is called the [[zero-point energy]].
By international agreement, absolute zero is defined as precisely 0 K on the [[Kelvin]] scale, which is a [[thermodynamic temperature|thermodynamic (absolute) temperature]] scale, and –273.15 °C on the [[Celsius]] scale.<ref>[http://www1.bipm.org/en/si/si_brochure/chapter2/2-1/2-1-1/kelvin.html International Agreement (Absolute Zero)], [[International Bureau of Weights and Measures|BIPM]]</ref> Absolute zero is also precisely equivalent to 0 °R on the [[Rankine scale]] (also a thermodynamic temperature scale), and –459.67 °F on the [[Fahrenheit]] scale.
It is not possible to cool any substance to 0 K,<ref>{{cite book |year=1996 |first=Jeremy Dunning |last=Davies |title=Concise Thermodynamics |publisher=Horwood Publishing |isbn=1898563152 |pages=43}}</ref> but scientists have made great advancements in achieving temperatures close to absolute zero, where matter exhibits odd [[Bose–Einstein condensate|quantum effects]] such as [[superconductivity]] and [[superfluid]]ity. In 2003, researchers at [[Massachusetts Institute of Technology|MIT]] achieved a record low of 500 [[Kelvin#SI prefixed forms of kelvin|pK]] (0.50 nK).<ref>[http://web.mit.edu/newsoffice/2003/cooling.html MIT team achieves coldest temperature ever.]</ref>
== History ==
One of the first to discuss the possibility of an "absolute cold" on such a scale was [[Robert Boyle]] who in his 1665 ''New Experiments and Observations touching Cold'', stated the dispute which is the ''primum frigidum'' is very well known among naturalists, some contending for the earth, others for water, others for the air, and some of the moderns for [[niter|nitre]], but all seeming to agree that:
{{cquote|There is some body or other that is of its own nature ''supremely cold'' and by participation of which all other bodies obtain that quality.}}
===Limit to the 'degree of cold' ===
The question whether there is a limit to the degree of cold possible, and, if so, where the zero must be placed, was first attacked by the French physicist [[Guillaume Amontons]] in 1702, in connection with his improvements in the air thermometer. In his instrument temperatures were indicated by the height at which a column of mercury was sustained by a certain mass of air, the volume or "spring" which of course varied with the heat to which it was exposed. Amontons therefore argued that the zero of his thermometer would be that temperature at which the spring of the air in it was reduced to nothing. On the scale he used the boiling-point of water was marked at +73 and the melting-point of ice at 51, so that the zero of his scale was equivalent to about –240 on the Celsius scale.
This remarkably close approximation to the modern value of –273.15 °C for the zero of the air-thermometer was further improved on by [[Johann Heinrich Lambert]] (''Pyrometrie'', 1779), who gave the value –270 °C and observed that this temperature might be regarded as absolute cold.
Values of this order for the absolute zero were not, however, universally accepted about this period. [[Pierre-Simon Laplace|Laplace]] and [[Antoine Lavoisier|Lavoisier]], for instance, in their treatise on heat (1780), arrived at values ranging from 1500 to 3000 below the freezing-point of water, and thought that in any case it must be at least 600 below, while [[John Dalton]] in his ''Chemical Philosophy'' gave ten calculations of this value, and finally adopted –3000 °C as the natural zero of temperature.
=== Lord Kelvin's work ===
After [[James Prescott Joule|J. P. Joule]] had determined the mechanical equivalent of heat, [[William Thomson, 1st Baron Kelvin|Lord Kelvin]] approached the question from an entirely different point of view, and in 1848 devised a scale of absolute temperature which was independent of the properties of any particular substance and was based solely on the fundamental [[laws of thermodynamics]]. It followed from the principles on which this scale was constructed that its zero was placed at –273.15 °C, at almost precisely the same point as the zero of the air-thermometer.<ref>[http://www.1911encyclopedia.org/Cold Cold] – Britannica 1911</ref>
== Achieving record temperatures near absolute zero ==
It can be shown from the laws of [[thermodynamics]] that absolute zero can never be achieved artificially, though it is possible to reach temperatures close to it through the use of [[cryocoolers]]. This is the same principle that ensures no [[machine]] can be 100% efficient.
At very low temperatures in the vicinity of absolute zero, matter exhibits many unusual properties including [[superconductor|superconductivity]], [[superfluid]]ity, and [[Bose-Einstein condensate|Bose-Einstein condensation]]. In order to study such [[phenomenon|phenomena]], [[scientist]]s have worked to obtain ever lower temperatures.
*In [[September 2003]], [[Massachusetts Institute of Technology|MIT]] announced a record cold temperature of 450 pK, or 4.5 × 10<sup>-10</sup>  K in a Bose-Einstein condensate of sodium atoms. This was performed by [[Wolfgang Ketterle]] and colleagues at MIT.<ref>Leanhardt, A. ''et al.'' (2003) ''Science'' '''301''' 1513. [http://physicsweb.org/article/news/7/9/8 Physicsweb news report]</ref>
*As of [[February 2003]], the [[Boomerang Nebula]], with a temperature of –272.15 degrees Celsius; 1 K, is the coldest place known outside a laboratory. The [[nebula]] is 5000 light-years from [[Earth]] and is in the constellation [[Centaurus]].<ref>{{cite news |author=Stephen Cauchi |title=Coolest bow tie in the universe |publisher=smh.com.au |date=February 21, 2003 |url=http://web.archive.org/web/20060901031441/http://www.smh.com.au/articles/2003/02/20/1045638427695.html |accessdate=2007-08-01 }} (Web archive)</ref>
*As of [[November 2000]], nuclear spin temperatures below 100 pK were reported for an experiment at the [[Helsinki University of Technology]]'s Low Temperature Lab. However, this was the temperature of one particular degree of freedom — a quantum property called nuclear spin — not the overall average thermodynamic temperature for all possible degrees of freedom.<ref>The experimental methods and results are presented in detail in T.A. Knuuttila’s Ph.D. thesis: [http://www.hut.fi/Yksikot/Kirjasto/Diss/2000/isbn9512252147/ ''Nuclear Magnetism and Superconductivity in Rhodium'']. Also the university’s press release on its achievement is [http://ltl.hut.fi/Low-Temp-Record.html here]</ref>
*In 1994, researchers at [[National Institute of Standards and Technology|NIST]] achieved a then-record cold temperature of 700 [[Kelvin#SI prefixed forms of kelvin|nK]] (billionths of a kelvin).
==Thermodynamics near absolute zero==
At temperatures near 0 K, nearly all molecular motion ceases and <math>\Delta</math>''S'' = 0 for any [[adiabatic process]]. Pure substances can (ideally) form perfect [[crystal]]s as ''T''<math>\to</math>0. [[Max Planck|Planck's]] strong form of the [[third law of thermodynamics]] states that the [[entropy]] of a perfect crystal vanishes at absolute zero. However, if the lowest energy state is [[degenerate energy level|degenerate]] (more than one [[microstate (statistical mechanics)|microstate]]), this cannot be true. The original [[Walther Nernst|Nernst]] ''heat theorem'' makes the weaker and less controversial claim that the entropy ''change'' for any isothermal process approaches zero as ''T''<math>\to</math>0
:<math> \lim_{T \to 0} \Delta S = 0 </math>
which implies that the entropy of a perfect crystal simply approaches a constant value.
''The [[Third Law of Thermodynamics|Nernst postulate]] identifies the [[isotherm]] T = 0 as coincident with the [[adiabat]] S = 0, although other isotherms and adiabats are distinct. As no two adiabats intersect, no other adiabat can [[Line-line intersection|intersect]] the T = 0 isotherm. Consequently no adiabatic process initiated at nonzero temperature can lead to zero temperature.'' (≈ Callen, pp. 189-190)
An even stronger assertion is that ''It is impossible by any procedure to reduce the temperature of a system to zero in a finite number of operations.'' (≈ Guggenheim, p. 157)
A perfect crystal is one in which the internal [[lattice (group)|lattice]] structure extends uninterrupted in all directions. The perfect order can be represented by translational [[symmetry]] along three (not usually [[orthogonality|orthogonal]]) [[Cartesian coordinate system|axes]]. Every lattice element of the structure is in its proper place, whether it is a single atom or a molecular grouping. For [[chemical substance|substances]] which have two (or more) stable crystalline forms, such as [[diamond]] and [[graphite]] for [[carbon]], there is a kind of "chemical degeneracy". The question remains whether both can have zero entropy at ''T'' = 0 even though each is perfectly ordered.
Perfect crystals never occur in practice; imperfections, and even entire amorphous materials, simply get "frozen in" at low temperatures, so transitions to more stable states do not occur.
Using the [[Peter Debye|Debye]] model, the [[specific heat capacity|specific heat]] and entropy of a pure crystal are proportional to ''T''<sup> 3</sup>, while the [[enthalpy]] and [[chemical potential]] are proportional to ''T''<sup> 4</sup>. (Guggenheim, p. 111) These quantities drop toward their ''T'' = 0 limiting values and approach with ''zero'' slopes. For the specific heats at least, the limiting value itself is definitely zero, as borne out by experiments to below 10 K. Even the less detailed [[Albert Einstein|Einstein]] model shows this curious drop in specific heats. In fact, all specific heats vanish at absolute zero, not just those of crystals. Likewise for the coefficient of [[thermal expansion]]. [[Maxwell relations|Maxwell's relations]] show that various other quantities also vanish. These [[phenomenon|phenomena]] were unanticipated.
Since the relation between changes in the [[Gibbs free energy|Gibbs energy]], the enthalpy and the entropy is
:<math> \Delta G = \Delta H - T \Delta S \,</math>
it follows that as ''T'' decreases, Δ''G'' and Δ''H'' approach each other (so long as Δ''S'' is bounded). [[Experiment]]ally, it is found that most [[chemical reaction]]s are [[exothermic reaction|exothermic]] and release heat ''in the direction'' they are found to be going, toward [[thermodynamic equilibrium|equilbrium]]. That is, even at [[room temperature]] ''T'' is low enough so that the fact that (Δ''G'')<sub>''T,P''</sub> < 0 (usually) implies that Δ''H'' < 0. (In the opposite direction, each such reaction would of course absorb heat.)
More than that, the ''slopes'' of the temperature derivatives of Δ''G'' and Δ''H'' converge and ''are equal to zero'' at ''T'' = 0, which ensures that Δ''G'' and Δ''H'' are nearly the same over a considerable range of temperatures, justifying the approximate [[empiricism|empirical]] [[Principle of Thomsen and Berthelot]], which says that ''the equilibrium state to which a system proceeds is the one which evolves the greatest amount of heat'', i.e., an actual process is the ''most exothermic one''. (Callen, pp. 186-187)
==Absolute temperature scales==
As mentioned, absolute or [[thermodynamic temperature]] is conventionally measured in [[kelvin]]s ([[Celsius]]-size degrees), and increasingly rarely in the [[Rankine scale]] ([[Fahrenheit]]-size degrees). Absolute temperature is uniquely determined up to a multiplicative constant which specifies the size of the "degree", so the ''ratios'' of two absolute temperatures, ''T''<sub>2</sub>/''T''<sub>1</sub>, are the same in all scales. The most transparent definition comes from the classical [[Maxwell-Boltzmann distribution]] over energies, or from the quantum analogs: [[Fermi-Dirac statistics]] (particles of half-integer [[spin (physics)|spin]]) and [[Bose-Einstein statistics]] (particles of integer spin), all of which give the relative numbers of particles as (decreasing) [[exponential function]]s of energy over ''kT''. On a [[macroscopic]] level, a definition can be given in terms of the efficiencies of "reversible" [[heat engine]]s operating between hotter and colder thermal reservoirs.
==Negative temperatures==
{{main|Negative temperature}}
Certain semi-isolated systems (for example a system of non-interacting spins in a magnetic field) can achieve negative temperatures; however, they are not actually colder than absolute zero. They can be however thought of as "hotter than T = ∞", as energy will flow from a negative temperature system to any other system with positive temperature upon contact.
== See also ==
<div style="-moz-column-count:2; column-count:2;">
* [[Celsius]]
* [[Cosmic microwave background radiation]] (this [[spacetime]] currently has a background temperature of roughly 2.7 K)
* [[Delisle scale]]
* [[Fahrenheit]]
* [[Heat]]
* [[International Temperature Scale of 1990|ITS-90]]
* [[Kelvin]]
* [[Orders of magnitude (temperature)]]
* [[Rankine scale]]
* [[Thermodynamic temperature|Thermodynamic (absolute) temperature]]
* [[Triple point]]
</div>
==References==
* {{cite book | author=Herbert B. Callen | title=Thermodynamics, Chapter 10 | publisher=John Wiley & Sons, Inc. | year=1960}} Library of Congress Catalog Card No. 60-5597. The clearest presentation of the logical foundations of the subject.
* {{cite book | author=E.A. Guggenheim | title=Thermodynamics: An Advanced Treatment for Chemists and Physicists, 5th ed. | publisher=North Holland; John Wiley & Sons, Inc. | year=1967}} Library of Congress Catalog Card No. 60-20003. A remarkably astute and comprehensive treatise.
* {{cite book | author=G. S. Rushbrooke | title=Introduction to Statistical Mechanics | publisher=Oxford Univ. Press | year=1949}} The classic, compact introduction to the subject.
==Notes==
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