Editing Absolute zero

{{short description|Lowest theoretical temperature}}
{{About|the minimum temperature limit|other uses|Absolute Zero (disambiguation)}}
{{Use dmy dates|date=May 2025}}
{{More citations needed|date=December 2022}}
[[File:CelsiusKelvin.svg|thumb|upright=0.5|Zero [[kelvin]] (−273.15&nbsp;°C) is defined as absolute zero.]]
'''Absolute zero''' is the lowest possible [[temperature]], a state at which a system's [[internal energy]], and in ideal cases [[entropy]], reach their minimum values. The absolute zero is defined as 0&nbsp;K on the [[Kelvin scale]], equivalent to −273.15&nbsp;°C on the [[Celsius|Celsius scale]],<ref name="sib2115">{{Cite web |title=SI Brochure: The International System of Units (SI) – 9th edition (updated in 2022) |url=https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507 |access-date=7 September 2022 |publisher=BIPM |page=133 |quote=[...], it remains common practice to express a thermodynamic temperature, symbol T, in terms of its difference from the reference temperature T<sub>0</sub> = 273.15 K, close to the ice point. This difference is called the Celsius temperature.}}</ref><ref name="arora">{{Cite book |last=Arora |first=C. P. |url=https://books.google.com/books?id=w8GhW3J8RHIC&pg=PA43 |title=Thermodynamics |publisher=Tata McGraw-Hill |year=2001 |isbn=978-0-07-462014-4 |at=Table 2.4 page 43}}</ref> and −459.67&nbsp;°F on the [[Fahrenheit scale]].<ref>{{Cite web |last=Zielinski |first=Sarah |date=1 January 2008 |title=Absolute Zero |url=http://www.smithsonianmag.com/science-nature/absolute-zero-200801.html |url-status=dead |archive-url=https://web.archive.org/web/20130401180715/http://www.smithsonianmag.com/science-nature/absolute-zero-200801.html |archive-date=1 April 2013 |access-date=26 January 2012 |publisher=Smithsonian Institution}}</ref> The Kelvin and Rankine temperature scales set their zero points at absolute zero by design. This limit can be estimated by extrapolating the [[ideal gas law]] to the temperature at which the volume or pressure of a classical gas becomes zero.

In the [[quantum mechanics]], matter at absolute zero occupies its [[ground state]], the configuration with the lowest possible [[internal energy]]. At this temperature, the particles exhibit minimal motion, retaining only what is mandated by the [[Uncertainty principle|Heisenberg uncertainty principle]] and, for a system of fermions, the [[Pauli exclusion principle]]. Even if absolute zero could be achieved, this residual quantum motion would persist, as it cannot be eliminated. 

Although absolute zero can be approached, it cannot be reached. Some [[Isentropic process|isentropic processes]], such as [[adiabatic demagnetization]], can lower the system's temperature without relying on a colder medium. Nevertheless, the [[third law of thermodynamics]] implies that no physical process can reach absolute zero in a finite number of steps. As a system nears this limit, further reductions in temperature become increasingly difficult, regardless of the cooling method used. In the 21st century, scientists have achieved temperatures below 100 picokelvin (pK). At such ultra-low temperatures, matter displays exotic quantum phenomena such as [[superconductivity]], [[superfluidity]], and [[Bose–Einstein condensation]]. 

== Ideal gas laws ==
[[File:Gas_thermometer_and_absolute_zero.svg|thumb|Pressure–temperature plots for three different gas samples, measured at constant volume, all extrapolate to zero at the same point, the absolute zero.]]
For an [[ideal gas]], the pressure at constant volume decreases linearly with temperature, and the volume at constant pressure also decreases linearly with temperature. When these relationships are expressed using the Celsius scale, both pressure and volume extrapolate to zero at approximately −273.15 °C. This implies the existence of a lower bound on temperature, beyond which the gas would have negative pressure or volume—an unphysical result.{{Cn|date=June 2025}}

To resolve this, the concept of absolute temperature is introduced, with 0 kelvins defined as the point at which pressure or volume would vanish in an ideal gas. This temperature corresponds to −273.15 °C, and is referred to as absolute zero. The ideal gas law is therefore formulated in terms of absolute temperature to remain consistent with observed gas behavior and physical limits.{{Cn|date=June 2025}}

==Absolute temperature scales==
[[Absolute temperature]] is conventionally measured in [[kelvin|Kelvin scale]] (using [[Celsius]]-scaled increments)<ref name="sib2115" /> and, more rarely, in [[Rankine scale]] (using [[Fahrenheit]]-scaled increments). Absolute temperature measurement is uniquely determined by 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 of this standard comes from the [[Maxwell–Boltzmann distribution]]. It can also be found in [[Fermi–Dirac statistics]] (for particles of half-integer [[Spin (physics)|spin]]) and [[Bose–Einstein statistics]] (for particles of integer spin). All of these define the relative numbers of particles in a system as decreasing [[exponential function]]s of energy (at the particle level) over ''kT'', with ''k'' representing the [[Boltzmann constant]] and ''T'' representing the temperature observed at the [[macroscopic]] level.{{cn|date=September 2023}}

== Unattainability of absolute zero ==
[[Image:Can T=0 be reached.jpg|450px|thumb|Left side: Absolute zero could be reached in a finite number of steps if {{math|1=''S''(0, ''X''<sub>1</sub>) ≠ ''S''(0, ''X''<sub>2</sub>)}}. Right: An infinite number of steps is needed since {{math|1=''S''(0, ''X''<sub>1</sub>) = ''S''(0, ''X''<sub>2</sub>)}}. Here, {{math|1=''X''}} is some controllable parameter of the system, such as its volume or pressure.]]
The [[third law of thermodynamics]] concerns the behavior of [[entropy]] as temperature approaches absolute zero. It states that the entropy of a system approaches a constant minimum at 0&nbsp;K. For a perfect crystal, this minimum is taken to be zero, since the system would be in a state of perfect order with only one [[Microstate (statistical mechanics)|microstate]] available. In some systems, there may be more than one microstate at minimum energy and there is some residual entropy at 0&nbsp;K. 

Several other formulations of the third law exist. [[Nernst heat theorem]] holds that the change in entropy for any constant-temperature process tends to zero as the temperature approaches zero.<ref name="f802">{{cite book |last=Atkins |first=Peter William |title=Atkins' Physical Chemistry |last2=Paula |first2=Julio De |last3=Keeler |first3=James |date=2018 |publisher=Oxford University Press |isbn=978-0-19-876986-6 |edition=11th |publication-place=Oxford, United Kingdom ; New York, NY |page= |pages=93–96}}</ref> A key consequence is that absolute zero cannot be reached, since removing heat becomes increasingly inefficient and entropy changes vanish. This unattainability principle means no physical process can cool a system to absolute zero in a finite number of steps or finite time.<ref name="q183">{{cite book |last=Shell |first=M. Scott |title=Thermodynamics and Statistical Mechanics |date=2015-04-16 |publisher=Cambridge University Press |isbn=978-1-107-01453-4 |publication-place=Cambridge |page= |pages=312–315}}</ref>

== Thermal properties at low temperatures ==
Using the [[Debye model]], the [[specific heat capacity|specific heat]] and entropy of a pure crystal are proportional to ''T''<sup>&nbsp;3</sup>, while the [[enthalpy]] and [[chemical potential]] are proportional to ''T''<sup>&nbsp;4</sup> (Guggenheim, p.&nbsp;111). These quantities drop toward their ''T''&nbsp;=&nbsp;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&nbsp;K. Even the less detailed [[Einstein solid|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 phenomena were unanticipated.

One model that estimates the properties of an [[electron]] gas at absolute zero in metals is the [[Fermi gas]]. The electrons, being [[fermion]]s, must be in different quantum states, which leads the electrons to get very high typical [[velocities]], even at absolute zero. The maximum energy that electrons can have at absolute zero is called the [[Fermi energy]]. The Fermi temperature is defined as this maximum energy divided by the Boltzmann constant, and is on the order of 80,000 K for typical electron densities found in metals. For temperatures significantly below the Fermi temperature, the electrons behave in almost the same way as at absolute zero. This explains the failure of the classical [[equipartition theorem]] for metals that eluded classical physicists in the late 19th century.

=== Gibbs free energy ===
Since the relation between changes in [[Gibbs free energy]] (''G''), the enthalpy (''H'') and the entropy is
:<math> \Delta G = \Delta H - T \Delta S \,</math>

thus, as ''T'' decreases, Δ''G'' and Δ''H'' approach each other (so long as Δ''S'' is bounded). Experimentally, it is found that all spontaneous processes (including [[chemical reaction]]s) result in a decrease in ''G'' as they proceed toward [[thermodynamic equilibrium|equilibrium]]. If Δ''S'' and/or ''T'' are small, the condition Δ''G''&nbsp;<&nbsp;0 may imply that Δ''H''&nbsp;<&nbsp;0, which would indicate an [[exothermic]] reaction. However, this is not required; [[endothermic]] reactions can proceed spontaneously if the ''T''Δ''S'' term is large enough.

Moreover, the slopes of the [[derivative]]s of Δ''G'' and Δ''H'' converge and are equal to zero at ''T''&nbsp;=&nbsp;0. This ensures that Δ''G'' and Δ''H'' are nearly the same over a considerable range of temperatures and justifies the approximate [[empiricism|empirical]] Principle of Thomsen and Berthelot, which states that ''the equilibrium state to which a system proceeds is the one that evolves the greatest amount of heat'', i.e., an actual process is the ''most exothermic one'' (Callen, pp.&nbsp;186–187).

==History==
[[File:Robert Boyle 0001.jpg|thumb|upright=1.05|[[Robert Boyle]] pioneered the idea of an absolute zero.]]
One of the first to discuss the possibility of an absolute minimal temperature was [[Robert Boyle]]. His 1665 ''New Experiments and Observations touching Cold'', articulated the dispute known as the ''primum frigidum''.<ref>{{Cite book |last=Stanford |first=John Frederick |author-link=John Frederick Stanford |url=https://books.google.com/books?id=8vRaAAAAMAAJ&pg=PA651 |title=The Stanford Dictionary of Anglicised Words and Phrases |year=1892}}</ref> The concept was well known among naturalists of the time. Some contended an absolute minimum temperature occurred within earth (as one of the four [[classical element]]s), others within water, others air, and some more recently within [[nitre]]. But all of them seemed to agree that, "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."<ref>{{Cite book |last=Boyle |first=Robert |title=New Experiments and Observations touching Cold |year=1665}}</ref>

===Limit to the "degree of cold"===
The question of whether there is a limit to the degree of coldness possible, and, if so, where the zero must be placed, was first addressed by the French physicist [[Guillaume Amontons]] in 1703, in connection with his improvements in the [[gas thermometer|air thermometer]]. His instrument indicated temperatures by the height at which a certain mass of air sustained a column of mercury—the pressure, or "spring" of the air varying with temperature. Amontons therefore argued that the zero of his thermometer would be that temperature at which the spring of the air was reduced to nothing.<ref>{{Cite journal |last=Amontons |date=18 April 1703 |title=Le thermomètre rèduit à une mesure fixe & certaine, & le moyen d'y rapporter les observations faites avec les anciens Thermométres |trans-title=The thermometer reduced to a fixed & certain measurement, & the means of relating to it observations made with old thermometers |url=https://www.biodiversitylibrary.org/item/87349#page/216/mode/1up |journal=Histoire de l'Académie Royale des Sciences, avec les Mémoires de Mathématique et de Physique pour la même Année |language=French |pages=50–56}}  Amontons described the relation between his new thermometer (which was based on the expansion and contraction of alcohol (''esprit de vin'')) and the old thermometer (which was based on air).  From p. 52:  ''" […] d'où il paroît que l'extrême froid de ce Thermométre seroit celui qui réduiroit l'air à ne soutenir aucune charge par son ressort, […] "''  ([…] whence it appears that the extreme cold of this [air] thermometer would be that which would reduce the air to supporting no load by its spring, […])  In other words, the lowest temperature which can be measured by a thermometer which is based on the expansion and contraction of air is that temperature at which the air's pressure ("spring") has decreased to zero.</ref> He used a scale that marked the boiling point of water at +73 and the melting point of ice at +{{frac|51|1|2}}, so that the zero was equivalent to about −240 on the Celsius scale.<ref name="AS2016">{{Cite EB1911|wstitle=Cold}}</ref> Amontons held that the absolute zero cannot be reached, so never attempted to compute it explicitly.<ref>{{Cite journal |last=Talbot |first=G. R. |last2=Pacey |first2=A. C. |date=1972 |title=Antecedents of thermodynamics in the work of Guillaume Amontons |journal=Centaurus |volume=16 |issue=1 |pages=20–40 |bibcode=1972Cent...16...20T |doi=10.1111/j.1600-0498.1972.tb00163.x}}</ref> The value of −240&nbsp;°C, or "431 divisions [in Fahrenheit's thermometer] below the cold of freezing water"<ref>{{Cite book |last=Martine |first=George |title=Essays Medical and Philosophical |date=1740 |publisher=A. Millar |location=London, England, UK |page=291 |chapter=Essay VI: The various degrees of heat in bodies |chapter-url=https://books.google.com/books?id=tSm2Ws6bg0oC&pg=PA291}}</ref> was published by [[George Martine (physician)|George Martine]] in 1740.

This close approximation to the modern value of −273.15&nbsp;°C<ref name="sib2115"/> for the zero of the air thermometer was further improved upon in 1779 by [[Johann Heinrich Lambert]], who observed that {{convert|-270|C|F K}} might be regarded as absolute cold.<ref>{{Cite book |last=Lambert |first=Johann Heinrich |title=Pyrometrie |year=1779 |location=Berlin, Germany |oclc=165756016}}</ref>

Values of this order for the absolute zero were not, however, universally accepted about this period. [[Pierre-Simon Laplace]] and [[Antoine Lavoisier]], in their 1780 treatise on heat, arrived at values ranging from 1,500 to 3,000 below the freezing point of water, and thought that in any case it must be at least 600 below. [[John Dalton]] in his ''Chemical Philosophy'' gave ten calculations of this value, and finally adopted −3,000&nbsp;°C as the natural zero of temperature.

===Charles's law===
From 1787 to 1802, it was determined by [[Jacques Charles]] (unpublished), [[John Dalton]],<ref>J. Dalton (1802), [https://books.google.com/books?id=3qdJAAAAYAAJ&pg=PA595 "Essay II. On the force of steam or vapour from water and various other liquids, both in vacuum and in air" and Essay IV.  "On the expansion of elastic fluids by heat" ], ''Memoirs of the Literary and Philosophical Society of Manchester'', vol. 8, pt. 2, pp. 550–574, 595–602.</ref> and [[Joseph Louis Gay-Lussac]]<ref>{{Citation |last=Gay-Lussac, J. L. |title=Recherches sur la dilatation des gaz et des vapeurs |work=Annales de Chimie |volume=XLIII |page=137 |year=1802 |author-link=Joseph Louis Gay-Lussac}}. [http://web.lemoyne.edu/~giunta/gaygas.html English translation (extract).]</ref> that, at constant pressure, ideal gases expanded or contracted their volume linearly ([[Charles's law]]) by about 1/273 parts per degree Celsius of temperature's change up or down, between 0° and 100°&nbsp;C. This suggested that the volume of a gas cooled at about −273&nbsp;°C would reach zero.

===Lord Kelvin's work===
After [[James Prescott 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 that was independent of the properties of any particular substance and was based on [[Nicolas Léonard Sadi Carnot|Carnot]]'s theory of the Motive Power of Heat and data published by [[Henri Victor Regnault]].<ref>{{Cite journal |last=Thomson |first=William |author-link=Lord Kelvin |date=1848 |title=On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat, and calculated from Regnault's observations. |url=https://www.biodiversitylibrary.org/item/87114#page/72/mode/2up |journal=Proceedings of the Cambridge Philosophical Society |volume=1 |pages=66–71}}</ref> It followed from the principles on which this scale was constructed that its zero was placed at −273&nbsp;°C, at almost precisely the same point as the zero of the air thermometer,<ref name="AS2016" /> where the air volume would reach "nothing". This value was not immediately accepted; values ranging from {{convert|-271.1|C}} to {{convert|-274.5|C}}, derived from laboratory measurements and observations of [[Atmospheric refraction#Astronomical refraction|astronomical refraction]], remained in use in the early 20th century.<ref>{{Citation |last=Newcomb |first=Simon |title=A Compendium of Spherical Astronomy |date=1906 |page=175 |place=New York |publisher=The Macmillan Company |oclc=64423127 |author-link=Simon Newcomb}}.</ref>

===The race to absolute zero===
{{see also|Timeline of low-temperature technology}}
[[File:Leiden - Kamerlingh Onnes Building - Commemorative plaque.jpg|thumb|upright=1.2|Commemorative plaque in Leiden]]
With a better theoretical understanding of absolute zero, scientists were eager to reach this temperature in the lab.<ref name="MyUser_YouTube_November_23_2016c">{{Cite web |title=ABSOLUTE ZERO – PBS NOVA DOCUMENTARY (full length) |url=https://www.youtube.com/watch?v=mTFRgosx4aQ&t=894s |url-status=dead |archive-url=https://web.archive.org/web/20170406015107/https://www.youtube.com/watch?v=mTFRgosx4aQ |archive-date=6 April 2017 |access-date=23 November 2016 |newspaper=YouTube}}</ref> By 1845, [[Michael Faraday]] had managed to liquefy most gases then known to exist, and reached a new record for lowest temperatures by reaching {{convert|-130|C|F K}}. Faraday believed that certain gases, such as oxygen, nitrogen, and [[hydrogen]], were permanent gases and could not be liquefied.<ref>[http://www.scienceclarified.com/Co-Di/Cryogenics.html Cryogenics]. Scienceclarified.com. Retrieved on 22 July 2012.</ref> Decades later, in 1873 Dutch theoretical scientist [[Johannes Diderik van der Waals]] demonstrated that these gases could be liquefied, but only under conditions of very high pressure and very low temperatures. In 1877, [[Louis Paul Cailletet]] in France and [[Raoul Pictet]] in Switzerland succeeded in producing the first droplets of [[liquid air]] at {{convert|-195|C|F K}}. This was followed in 1883 by the production of liquid oxygen {{convert|-218|C|F K}} by the Polish professors [[Zygmunt Wróblewski]] and [[Karol Olszewski]].

Scottish chemist and physicist [[James Dewar]] and Dutch physicist [[Heike Kamerlingh Onnes]] took on the challenge to liquefy the remaining gases, hydrogen and [[helium]]. In 1898, after 20 years of effort, Dewar was the first to liquefy hydrogen, reaching a new low-temperature record of {{convert|-252|C|F K}}. However, Kamerlingh Onnes, his rival, was the first to liquefy helium, in 1908, using several precooling stages and the [[Hampson–Linde cycle]]. He lowered the temperature to the boiling point of helium {{convert|-269|C|F K}}. By reducing the pressure of the liquid helium, he achieved an even lower temperature, near 1.5 K. These were the [[Lowest temperature recorded on Earth|coldest temperatures achieved on Earth]] at the time and his achievement earned him the [[Nobel Prize]] in 1913.<ref name="nobel">{{Cite web |title=The Nobel Prize in Physics 1913: Heike Kamerlingh Onnes |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1913/onnes-bio.html |access-date=24 April 2012 |publisher=Nobel Media AB}}</ref> Kamerlingh Onnes would continue to study the properties of materials at temperatures near absolute zero, describing [[superconductivity]] and [[superfluids]] for the first time.

==Negative temperatures==
{{Main|Negative temperature}}

Temperatures below zero on the Celsius or Fahrenheit scales are simply colder than the zero points of those scales. In contrast, certain isolated systems can achieve [[negative thermodynamic temperature]]s (in kelvins), which are not colder than absolute zero, but paradoxically hotter than any positive temperature. If a negative-temperature system and a positive-temperature system come in contact, heat flows from the negative to the positive-temperature system.<ref name="Chase">{{Cite web |last=Chase |first=Scott |title=Below Absolute Zero -What Does Negative Temperature Mean? |url=http://www.phys.ncku.edu.tw/mirrors/physicsfaq/ParticleAndNuclear/neg_temperature.html |url-status=dead |archive-url=https://web.archive.org/web/20110815144418/http://www.phys.ncku.edu.tw/mirrors/physicsfaq/ParticleAndNuclear/neg_temperature.html |archive-date=15 August 2011 |access-date=2 July 2010 |website=The Physics and Relativity FAQ}}</ref><ref name=":1">{{cite book |last1=Kittel |first1=C. |author1-link=Charles Kittel |title=Thermal Physics |last2=Kroemer |first2=H. |author2-link=Herbert Kroemer |publisher=[[W. H. Freeman]] |year=1980 |isbn=978-0-7167-1088-2 |edition=2nd |pages=460–463}}</ref>

Negative temperatures can only occur in systems that have an upper limit to the energy they can contain. In these cases, adding energy can decrease [[Entropy (statistical thermodynamics)|entropy]], reversing the usual relationship between energy and temperature. This leads to a negative thermodynamic temperature. However, such conditions only arise in specialized, quasi-equilibrium systems such as collections of [[Spin (physics)|spins]] in a magnetic field. In contrast, ordinary systems with translational or vibrational motion have no upper energy limit, so their temperatures are always positive.<ref name="Chase" /><ref name=":1" />

==Very low temperatures==
[[File:Boomerang nebula.jpg|thumb|right|The rapid expansion of gases leaving the [[Boomerang Nebula]], a bi-polar, filamentary, likely proto-planetary nebula in Centaurus, has a temperature of 1&nbsp;K, the lowest observed outside of a laboratory.]]
[[File:Bose Einstein condensate.png|right|thumb|Velocity-distribution data of a gas of [[rubidium]] atoms at a temperature within a few billionths of a degree above absolute zero. Left: just before the appearance of a Bose–Einstein condensate. Center: just after the appearance of the condensate. Right: after further evaporation, leaving a sample of nearly pure condensate.]]

The average temperature of the universe today is approximately {{convert|2.73|K|C F|abbr=on}}, based on measurements of [[cosmic microwave background]] radiation.<ref>{{Cite web |last=Kruszelnicki, Karl S. |date=25 September 2003 |title=Coldest Place in the Universe 1 |url=http://www.abc.net.au/science/articles/2003/09/25/947116.htm |access-date=24 September 2012 |publisher=Australian Broadcasting Corporation}}</ref><ref>{{Cite web |date=3 August 2004 |title=What's the temperature of space? |url=http://www.straightdope.com/columns/read/2172/whats-the-temperature-of-space |access-date=24 September 2012 |publisher=The Straight Dope}}</ref> Standard models of the [[future of an expanding universe|future expansion of the universe]] predict that the average temperature of the universe is decreasing over time.<ref>{{Cite journal |last=John |first=Anslyn J. |date=25 August 2021 |title=The building blocks of the universe |journal=HTS Teologiese Studies/Theological Studies |volume=77 |issue=3 |doi=10.4102/hts.v77i3.6831 |s2cid=238730757 |doi-access=free}}</ref> This temperature is calculated as the mean density of energy in space; it should not be confused with the mean [[electron temperature]] (total energy divided by particle count) which has increased over time.<ref>{{Cite news |date=10 November 2020 |title=History of temperature changes in the Universe revealed—First measurement using the Sunyaev-Zeldovich effect |url=https://www.ipmu.jp/en/20201110-CosmicThermal_History |language=en |agency=Kavli Institute for the Physics and Mathematics of the Universe}}</ref>

Absolute zero cannot be achieved, although it is possible to reach temperatures close to it through the use of [[Evaporative cooling (atomic physics)|evaporative cooling]], [[cryocooler]]s, [[dilution refrigerator]]s,<ref>{{Cite journal |last=Zu |first=H. |last2=Dai |first2=W. |last3=de Waele |first3=A. T. A. M. |year=2022 |title=Development of Dilution refrigerators – A review |journal=Cryogenics |volume=121 |doi=10.1016/j.cryogenics.2021.103390 |issn=0011-2275 |s2cid=244005391}}</ref> and [[Magnetic refrigeration#Nuclear demagnetization|nuclear adiabatic demagnetization]]. The use of [[laser cooling]] has produced temperatures of less than a billionth of a kelvin.<ref>{{Cite web |last=Catchpole, Heather |date=4 September 2008 |title=Cosmos Online – Verging on absolute zero |url=http://www.cosmosmagazine.com/features/online/2176/verging-absolute-zero |url-status=dead |archive-url=https://web.archive.org/web/20081122144155/http://www.cosmosmagazine.com/features/online/2176/verging-absolute-zero |archive-date=22 November 2008}}</ref> At very low temperatures in the vicinity of absolute zero, matter exhibits many unusual properties, including [[superconductivity]], [[superfluidity]], and [[Bose–Einstein condensate|Bose–Einstein condensation]]. To study such [[phenomena]], scientists have worked to obtain even lower temperatures.
* In November 2000, [[nuclear spin]] temperatures below {{nowrap|100 picokelvin}} were reported for an experiment at the [[Helsinki University of Technology]]'s Low Temperature Lab in [[Espoo]], [[Finland]]. However, this was the temperature of one particular [[Degrees of freedom (physics and chemistry)|degree of freedom]]—a [[quantum]] property called nuclear spin—not the overall average [[thermodynamic temperature]] for all possible degrees in freedom.<ref>{{Cite book |last=Knuuttila |first=Tauno |url=http://www.hut.fi/Yksikot/Kirjasto/Diss/2000/isbn9512252147 |title=Nuclear Magnetism and Superconductivity in Rhodium |publisher=Helsinki University of Technology |year=2000 |isbn=978-951-22-5208-4 |location=Espoo, Finland |access-date=11 February 2008 |archive-url=https://web.archive.org/web/20010428173229/http://www.hut.fi/Yksikot/Kirjasto/Diss/2000/isbn9512252147/ |archive-date=28 April 2001 |url-status=dead}}</ref><ref>{{Cite press release |title=Low Temperature World Record |date=8 December 2000 |publisher=Low Temperature Laboratory, Teknillinen Korkeakoulu |url=http://ltl.hut.fi/Low-Temp-Record.html |access-date=11 February 2008 |url-status=live |archive-url=https://web.archive.org/web/20080218053521/http://ltl.hut.fi/Low-Temp-Record.html |archive-date=18 February 2008}}</ref>
* In February 2003, the [[Boomerang Nebula]] was observed to have been releasing gases at a speed of {{Convert|500000|km/h|abbr=on}} for the last 1,500 years. This has cooled it down to approximately 1&nbsp;K, as deduced by astronomical observation, which is the lowest natural temperature ever recorded.<ref>{{Cite journal |last=Sahai |first=Raghvendra |last2=Nyman, Lars-Åke |year=1997 |title=The Boomerang Nebula: The Coldest Region of the Universe? |journal=The Astrophysical Journal |volume=487 |issue=2 |pages=L155–L159 |bibcode=1997ApJ...487L.155S |doi=10.1086/310897 |s2cid=121465475 |doi-access=free |hdl=2014/22450}}</ref>
* In November 2003, [[90377 Sedna]] was discovered and is one of the coldest known objects in the Solar System, with an average surface temperature of {{cvt|-240|C|K F|sigfig=2}},<ref>{{Cite web |title=Mysterious Sedna {{!}} Science Mission Directorate |url=https://science.nasa.gov/science-news/science-at-nasa/2004/16mar_sedna/#:~:text=NASA%27s%20new%20Spitzer%20Space%20Telescope%20also%20looked%20for,minus%20240%20degrees%20Celsius%20(minus%20400%20degrees%20Fahrenheit). |access-date=25 November 2022 |website=science.nasa.gov}}</ref> due to its extremely far orbit of 903 [[astronomical unit]]s.
* In May 2005, the [[European Space Agency]] proposed research in space to achieve [[femto-|femtokelvin]] temperatures.<ref>{{Cite web |title=Scientific Perspectives for ESA's Future Programme in Life and Physical sciences in Space |url=http://www.esf.org/fileadmin/Public_documents/Publications/Scientific_Perspectives_for_ESA_s_Future_Programme_in_Life_and_Physical_Sciences_in_Space.pdf |url-status=dead |archive-url=https://web.archive.org/web/20141006024523/http://www.esf.org/fileadmin/Public_documents/Publications/Scientific_Perspectives_for_ESA_s_Future_Programme_in_Life_and_Physical_Sciences_in_Space.pdf |archive-date=6 October 2014 |access-date=28 March 2014 |website=esf.org}}</ref>
* In May 2006, the Institute of Quantum Optics at the [[University of Hannover]] gave details of technologies and benefits of femtokelvin research in space.<ref>{{Cite web |title=Atomic Quantum Sensors in Space |url=http://www.physics.ucla.edu/quantum_to_cosmos/q2c06/Ertmer.pdf |url-status=live |archive-url=https://ghostarchive.org/archive/20221009/http://www.physics.ucla.edu/quantum_to_cosmos/q2c06/Ertmer.pdf |archive-date=9 October 2022 |website=University of California, Los Angeles}}</ref>
* In January 2013, physicist Ulrich Schneider of the [[University of Munich]] in Germany reported to have achieved temperatures formally below absolute zero ("[[negative temperature]]") in gases. The gas is artificially forced out of equilibrium into a high potential energy state, which is, however, cold. When it then emits radiation it approaches the equilibrium, and can continue emitting despite reaching formal absolute zero; thus, the temperature is formally negative.<ref>{{Cite web |date=3 January 2013 |title=Atoms Reach Record Temperature, Colder than Absolute Zero |url=http://www.livescience.com/25959-atoms-colder-than-absolute-zero.html |website=livescience.com}}</ref>
* In September 2014, scientists in the [[CUORE]] collaboration at the [[Laboratori Nazionali del Gran Sasso]] in Italy cooled a copper vessel with a volume of one cubic meter to {{cvt|0.006|K|C F|sigfig=6}} for 15 days, setting a record for the lowest temperature in the known universe over such a large contiguous volume.<ref>{{Cite news |title=CUORE: The Coldest Heart in the Known Universe. |url=http://www.interactions.org/cms/?pid=1034217 |access-date=21 October 2014 |publisher=INFN Press Release}}</ref>
* In June 2015, experimental physicists at [[MIT]] cooled molecules in a gas of sodium potassium to a temperature of 500 nanokelvin, and it is expected to exhibit an exotic state of matter by cooling these molecules somewhat further.<ref>{{Cite web |title=MIT team creates ultracold molecules |url=https://newsoffice.mit.edu/2015/ultracold-molecules-0610 |url-status=dead |archive-url=https://web.archive.org/web/20150818112454/http://newsoffice.mit.edu/2015/ultracold-molecules-0610 |archive-date=18 August 2015 |access-date=10 June 2015 |website=Massachusetts Institute of Technology, Massachusetts, Cambridge}}</ref>
* In 2017, [[Cold Atom Laboratory]] (CAL), an experimental instrument was developed for launch to the [[International Space Station]] (ISS) in 2018.<ref>{{Cite news |date=5 September 2017 |title=Coolest science ever headed to the space station |url=https://www.science.org/content/article/coolest-science-ever-headed-space-station |access-date=24 September 2017 |work=Science {{!}} AAAS |language=en}}</ref> The instrument has created extremely cold conditions in the [[microgravity]] environment of the ISS leading to the formation of [[Bose–Einstein condensate]]s. In this space-based laboratory, temperatures as low as {{nowrap|1 picokelvin}} are projected to be achievable, and it could further the exploration of unknown [[Quantum mechanics|quantum mechanical]] phenomena and test some of the most fundamental [[laws of physics]].<ref name="NASA Cold Atom Laboratory Mission">{{Cite web |date=2017 |title=Cold Atom Laboratory Mission |url=http://coldatomlab.jpl.nasa.gov/mission/ |url-status=dead |archive-url=https://web.archive.org/web/20130329092843/http://coldatomlab.jpl.nasa.gov/mission/ |archive-date=29 March 2013 |access-date=22 December 2016 |website=Jet Propulsion Laboratory |publisher=NASA}}</ref><ref name="CALnasa">{{Cite web |date=26 September 2014 |title=Cold Atom Laboratory Creates Atomic Dance |url=http://www.nasa.gov/mission_pages/station/research/news/cold_atom_lab/ |url-status=dead |archive-url=https://web.archive.org/web/20210708201720/https://www.nasa.gov/mission_pages/station/research/news/cold_atom_lab/ |archive-date=8 July 2021 |access-date=21 May 2015 |website=NASA News}}</ref>
* The current world record for effective temperatures was set in 2021 at {{nowrap|38 picokelvin}} through matter-wave lensing of rubidium [[Bose–Einstein condensate]]s.<ref name=":0">{{Cite journal |last=Deppner |first=Christian |last2=Herr |first2=Waldemar |last3=Cornelius |first3=Merle |last4=Stromberger |first4=Peter |last5=Sternke |first5=Tammo |last6=Grzeschik |first6=Christoph |last7=Grote |first7=Alexander |last8=Rudolph |first8=Jan |last9=Herrmann |first9=Sven |last10=Krutzik |first10=Markus |last11=Wenzlawski |first11=André |date=30 August 2021 |title=Collective-Mode Enhanced Matter-Wave Optics |url=https://link.aps.org/doi/10.1103/PhysRevLett.127.100401 |journal=Physical Review Letters |language=en |volume=127 |issue=10 |pages=100401 |bibcode=2021PhRvL.127j0401D |doi=10.1103/PhysRevLett.127.100401 |issn=0031-9007 |pmid=34533345 |s2cid=237396804}}</ref>

==See also==
{{Portal|Physics}}
{{Div col|colwidth=22em}}
* [[Degenerate matter]]
* [[Kelvin]] (unit of temperature)
* [[Charles's law]]
* [[Heat]]
* [[International Temperature Scale of 1990]]
* [[Orders of magnitude (temperature)]]
* [[Thermodynamic temperature]]
* [[Triple point]]
* [[Ultracold atom]]
* [[Kinetic energy]]
* [[Entropy]]
* [[Planck temperature]] and [[Hagedorn temperature]], hypothetical upper limits to the thermodynamic temperature scale
{{colend}}

==References==
{{Reflist|30em}}

==Further reading==
* {{Cite book |last=Herbert B. Callen |url=https://archive.org/details/thermodynamicsin0000call |title=Thermodynamics |publisher=John Wiley & Sons |year=1960 |isbn=978-0-471-13035-2 |location=New York |chapter=Chapter 10 |oclc=535083 |chapter-url=https://archive.org/details/thermodynamicsin00call |url-access=registration |chapter-url-access=registration}}
* {{Cite book |last=Herbert B. Callen |title=Thermodynamics and an Introduction to Thermostatistics |publisher=John Wiley & Sons |year=1985 |isbn=978-0-471-86256-7 |edition=Second |location=New York}}
* {{Cite book |last=E.A. Guggenheim |title=Thermodynamics: An Advanced Treatment for Chemists and Physicists |publisher=North Holland Publishing |year=1967 |isbn=978-0-444-86951-7 |edition=Fifth |location=Amsterdam |oclc=324553}}
* {{Cite book |last=George Stanley Rushbrooke |url=https://archive.org/details/in.ernet.dli.2015.476050 |title=Introduction to Statistical Mechanics |publisher=Clarendon Press |year=1949 |location=Oxford |oclc=531928}}
* [https://www.bipm.org/en/search?p_p_id=search_portlet&p_p_lifecycle=2&p_p_state=normal&p_p_mode=view&p_p_resource_id=%2Fdownload%2Fpublication&p_p_cacheability=cacheLevelPage&_search_portlet_dlFileId=41507086&p_p_lifecycle=1&_search_portlet_javax.portlet.action=search&_search_portlet_formDate=1644345579131&_search_portlet_query=absolute+zero&_search_portlet_source=BIPM BIPM Mise en pratique - Kelvin - Appendix 2 - SI Brochure].

==External links==
* [https://www.pbs.org/wgbh/nova/zero/ "Absolute zero"]: a two part ''[[Nova (American TV series)|NOVA]]'' episode [[List of NOVA episodes#Season 35: 2007–2008|originally aired January 2008]]
* [https://web.archive.org/web/20080509100512/http://www.pa.msu.edu/~sciencet/ask_st/012992.html "What is absolute zero?"] ''Lansing State Journal''
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