Absolute zero is the theoretical temperature at which entropy would reach its minimum value. The laws of thermodynamics state that absolute zero cannot be reached because this would require a thermodynamic system to be fully removed from the rest of the universe. A system at absolute zero would still possess quantum mechanical zeropoint energy. While molecular motion would not cease entirely at absolute zero, the system would not have enough energy for transference to other systems. It is therefore correct to say that molecular kinetic energy is minimal at absolute zero.
By international agreement, absolute zero is defined as 0K on the Kelvin scale and as −273.15°C on the Celsius scale.^{[1]} Scientists have achieved temperatures very close to absolute zero, where matter exhibits quantum effects such as superconductivity and superfluidity.
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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. The concept was well known among naturalists of the time. Some contended an absolute minimum temperature occurred within earth (as one of the four socalled 'elements'), others within water, others air, and some more recently within nitre^{[citation needed]}. 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."^{[2]}
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 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 boilingpoint of water was marked at +73 and the meltingpoint of ice at 51, so that the zero of his scale was equivalent to about −240 on the Celsius scale.
This close approximation to the modern value of −273.15 °C^{[1]} for the zero of the airthermometer was further improved upon in 1779 by Johann Heinrich Lambert, who observed that −270 °C might be regarded as absolute cold.^{[3]}
Values of this order for the absolute zero were not, however, universally accepted about this period. PierreSimon Laplace and Antoine Lavoisier, in their 1780 treatise on heat, arrived at values ranging from 1,500 to 3,000 below the freezingpoint 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 −3000 °C as the natural zero of temperature.
After J.P. Joule had determined the mechanical equivalent of heat, 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 airthermometer.^{[4]}
The average temperature of the universe due to cosmic microwave background radiation today is 2.73 K.
Absolute zero cannot be achieved artificially, although it is possible to reach temperatures close to it through the use of cryocoolers. Laser cooling is a technique used to take temperatures to within a billionth of a degree of 0 K.^{[5]} At very low temperatures in the vicinity of absolute zero, matter exhibits many unusual properties including superconductivity, superfluidity, and BoseEinstein condensation. In order to study such phenomena, scientists have worked to obtain ever lower temperatures.
At temperatures near 0 K, nearly all molecular motion ceases and, when entropy = S, ΔS = 0 for any adiabatic process. Pure substances can (ideally) form perfect crystals as T → 0. Max Planck's strong form of the third law of thermodynamics states the entropy of a perfect crystal vanishes at absolute zero. The original Nernst heat theorem makes the weaker and less controversial claim that the entropy change for any isothermal process approaches zero as T → 0:
The implication is that the entropy of a perfect crystal simply approaches a constant value.
The 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 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 structure extends uninterrupted in all directions. The perfect order can be represented by translational symmetry along three (not usually orthogonal) axes. Every lattice element of the structure is in its proper place, whether it is a single atom or a molecular grouping. For 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 Debye model, the specific heat and entropy of a pure crystal are proportional to T^{ 3}, while the enthalpy and chemical potential are proportional to T^{ 4}. (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 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's relations show that various other quantities also vanish. These phenomena were unanticipated.
Since the relation between changes in Gibbs free energy (G), the enthalpy (H) and the entropy is
thus, as T decreases, ΔG and ΔH approach each other (so long as ΔS is bounded). Experimentally, it is found that all spontaneous processes (including chemical reactions) result in a decrease in G as they proceed toward equilbrium. If ΔS and/or T are small, the condition ΔG < 0 may imply that ΔH < 0, which would indicate an exothermic reaction. However, this is not required; endothermic reactions can proceed spontaneously if the TΔS term is large enough.
Moreover, the slopes of the derivatives of ΔG and ΔH converge and are equal to zero at T = 0. This ensures that ΔG and ΔH are nearly the same over a considerable range of temperatures and justifies the approximate empirical Principle of Thomsen and Berthelot, which states 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)
One model that estimates the properties of an electron gas at absolute zero is the fermi gas. But, interestingly, the fermi temperature of electrons, where the lattice temperature is zero, is nonzero. In fact, the electrons have very high velocities. For an isolated system this is probably a violation of conservation of momentum because—as the electrons are interacting with the lattice and the total momentum is zero—the lattice cannot be at zero velocity. Also, if the lattice was precisely at zero temperature its nuclei would have infinite de Broglie wavelength. (If the momentum goes to zero, the wavelength becomes infinite).
A BoseEinstein condensate is an unusual state of matter that only exists at temperatures lower than a few billionths of a degree above absolute zero.^{[12]}
Absolute, or thermodynamic, temperature is conventionally measured in kelvins (Celsiusscaled increments) and in the Rankine scale (Fahrenheitscaled increments) with increasing rarity. 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_{2}/T_{1}, are the same in all scales. The most transparent definition of this standard comes from the MaxwellBoltzmann distribution. It can also be found in FermiDirac statistics (for particles of halfinteger spin) and BoseEinstein statistics (for particles of integer spin). All of these define the relative numbers of particles in a system as decreasing exponential functions of energy (at the particle level) over kT, with k representing the Boltzmann constant and T representing the temperature observed at the macroscopic level.
Certain semiisolated systems, such as a system of noninteracting 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.
Absolute zero is the theoretical temperature at which entropy would reach its minimum value. The laws of thermodynamics state that absolute zero cannot be reached because this would require a thermodynamic system to be fully removed from the rest of the universe.
A system at absolute zero would still possess quantum mechanical zeropoint energy. While molecular motion would not cease entirely at absolute zero, the system would not have enough energy for transference to other systems. It is therefore correct to say that molecular kinetic energy is minimal at absolute zero.
By international agreement, absolute zero is defined as 0K on the Kelvin scale and as −273.15°C on the Celsius scale.^{[1]} This equates to −459.67°F on the Fahrenheit scale. Scientists have achieved temperatures very close to absolute zero, where matter exhibits quantum effects such as superconductivity and superfluidity.
Contents 
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. The concept was well known among naturalists of the time. Some contended an absolute minimum temperature occurred within earth (as one of the four socalled 'elements'), others within water, others air, and some more recently within nitre^{[citation needed]}. 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."^{[2]}
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 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 boilingpoint of water was marked at +73 and the meltingpoint of ice at 51, so that the zero of his scale was equivalent to about −240 on the Celsius scale.
This close approximation to the modern value of −273.15 °C^{[1]} for the zero of the airthermometer was further improved upon in 1779 by Johann Heinrich Lambert, who observed that −270 °C might be regarded as absolute cold.^{[3]}
Values of this order for the absolute zero were not, however, universally accepted about this period. PierreSimon Laplace and Antoine Lavoisier, in their 1780 treatise on heat, arrived at values ranging from 1,500 to 3,000 below the freezingpoint 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 −3000 °C as the natural zero of temperature.
After J.P. Joule had determined the mechanical equivalent of heat, 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 airthermometer.^{[4]}
causes the lowest observed temperature outside a laboratory.]]
The average temperature of the universe due to cosmic microwave background radiation today is 2.73 K.
Absolute zero cannot be achieved artificially, although it is possible to reach temperatures close to it through the use of cryocoolers. Laser cooling is a technique used to take temperatures to within a billionth of a degree of 0 K.^{[5]} At very low temperatures in the vicinity of absolute zero, matter exhibits many unusual properties including superconductivity, superfluidity, and BoseEinstein condensation. In order to study such phenomena, scientists have worked to obtain ever lower temperatures.
At temperatures near 0 K, nearly all molecular motion ceases and, when entropy = S, ΔS = 0 for any adiabatic process. Pure substances can (ideally) form perfect crystals as T → 0. Max Planck's strong form of the third law of thermodynamics states the entropy of a perfect crystal vanishes at absolute zero. The original Nernst heat theorem makes the weaker and less controversial claim that the entropy change for any isothermal process approaches zero as T → 0:
The implication is that the entropy of a perfect crystal simply approaches a constant value.
The 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 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 structure extends uninterrupted in all directions. The perfect order can be represented by translational symmetry along three (not usually orthogonal) axes. Every lattice element of the structure is in its proper place, whether it is a single atom or a molecular grouping. For 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 Debye model, the specific heat and entropy of a pure crystal are proportional to T^{ 3}, while the enthalpy and chemical potential are proportional to T^{ 4}. (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 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's relations show that various other quantities also vanish. These phenomena were unanticipated.
Since the relation between changes in Gibbs free energy (G), the enthalpy (H) and the entropy is
thus, as T decreases, ΔG and ΔH approach each other (so long as ΔS is bounded). Experimentally, it is found that all spontaneous processes (including chemical reactions) result in a decrease in G as they proceed toward equilbrium. If ΔS and/or T are small, the condition ΔG < 0 may imply that ΔH < 0, which would indicate an exothermic reaction. However, this is not required; endothermic reactions can proceed spontaneously if the TΔS term is large enough.
Moreover, the slopes of the derivatives of ΔG and ΔH converge and are equal to zero at T = 0. This ensures that ΔG and ΔH are nearly the same over a considerable range of temperatures and justifies the approximate empirical Principle of Thomsen and Berthelot, which states 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)
One model that estimates the properties of an electron gas at absolute zero is the fermi gas. But, interestingly, the fermi temperature of electrons, where the lattice temperature is zero, is nonzero. In fact, the electrons have very high velocities. For an isolated system this is probably a violation of conservation of momentum because—as the electrons are interacting with the lattice and the total momentum is zero—the lattice cannot be at zero velocity. Also, if the lattice was precisely at zero temperature its nuclei would have infinite de Broglie wavelength. (If the momentum goes to zero, the wavelength becomes infinite).
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.]]
A Bose–Einstein condensate (BEC) is a state of matter of a dilute gas of weakly interacting bosons confined in an external potential and cooled to temperatures very near to absolute zero. Under such conditions, a large fraction of the bosons occupy the lowest quantum state of the external potential, at which point quantum effects become apparent on a macroscopic scale.^{[12]}
This state of matter was first predicted by Satyendra Nath Bose and Albert Einstein in 1924–25. Bose first sent a paper to Einstein on the quantum statistics of light quanta (now called photons). Einstein was impressed, translated the paper himself from English to German and submitted it for Bose to the Zeitschrift für Physik which published it. Einstein then extended Bose's ideas to material particles (or matter) in two other papers.^{[13]}
Seventy years later, the first gaseous condensate was produced by Eric Cornell and Carl Wieman in 1995 at the University of Colorado at Boulder NISTJILA lab, using a gas of rubidium atoms cooled to 170 nanokelvin (nK) ^{[14]} (1.7×10^{−7} K).^{[15]}
Absolute, or thermodynamic, temperature is conventionally measured in kelvins (Celsiusscaled increments) and in the Rankine scale (Fahrenheitscaled increments) with increasing rarity. 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_{2}/T_{1}, are the same in all scales. The most transparent definition of this standard comes from the MaxwellBoltzmann distribution. It can also be found in FermiDirac statistics (for particles of halfinteger spin) and BoseEinstein statistics (for particles of integer spin). All of these define the relative numbers of particles in a system as decreasing exponential functions of energy (at the particle level) over kT, with k representing the Boltzmann constant and T representing the temperature observed at the macroscopic level.^{[1]}
Temperatures that are expressed as negative numbers on the familiar Celsius or Fahrenheit scales are simply colder than the zero points of those scales. Certain systems can achieve truly negative temperatures; that is, their thermodynamic temperature (expressed in kelvin) can be of a negative quantity. A system with a truly negative temperature is not colder than absolute zero. Rather, a system with a negative temperature is hotter than any system with a positive temperature in the sense that if a negativetemperature system and a positivetemperature system come in contact, heat will flow from the negative to the positivetemperature system.^{[16]}
Most familiar systems cannot achieve negative temperatures because adding energy always increases their entropy. However, some systems have a maximum amount of energy that they can hold, and as they approach that maximum energy their entropy actually begins to decrease. Because temperature is defined by the relationship between energy and entropy, such a system's temperature becomes negative, even though energy is being added (the Heat capacity is negative).^{[16]}
In theory, absolute zero is the temperature where the particles of matter stop moving. Absolute zero is impossible to achieve, because all particles move, even if it's just a small vibration. Some people have gotten very close to absolute zero, but the record temperature was 100 PK (Picokelvin) above absolute zero.^{[1]}
The Kelvin and Rankine temperature scales are defined so that absolute zero is 0 kelvins (K) or 0 degrees Rankine (°R). The Celsius and Fahrenheit scales are defined so that absolute zero is −273.15 °C or −459.67 °F.^{[2]}
At this stage the pressure of the particles is zero. If we plot a graph to it, we can see that the temperature of the particles is zero. The temperature cannot go down any further. And no, the particles cannot move in "reverse" either because as the movement of particles is vibration, vibrating in reverse would be nothing but simply vibrating again. The closer the temperature of an object gets to absolute zero, the less resistive the material is to electricity therefore it will conduct electricity almost perfectly, with no measurable resistance.
