Absolute Zero Temperature

Type of physical science: Classical physics

Field of study: Thermodynamics

Absolute zero is the lowest possible temperature and provides an essential reference to establish a scale of temperature and a scale of disorder, or entropy. At absolute zero, atomic motion and entropy are at the minimum possible value.

Overview

As temperature is decreased, the motion of atoms and molecules becomes less and less, until, at absolute zero, the motion of the atoms and molecules is the minimum possible. There can be no temperature lower than absolute zero because no more heat energy can be removed from a substance at this temperature. The achievement of an actual temperature of absolute zero is a theoretical impossibility, although extremely low temperatures (within a millionth of a degree of absolute zero) have been achieved.

Entropy is a measure of randomness or disorder. As temperature is increased, the entropy, or disorder, increases. Conversely, as temperature is lowered, the entropy decreases. At a temperature of absolute zero, a perfect crystalline substance (a solid with all molecules oriented in the same manner) has an entropy of zero, for there is no disorder present. All regular atomic motion ceases, and the atoms are aligned in a regular repeating pattern. Since there can be no more increase in order, there is no disorder and the entropy is exactly zero.

Imagine a collection of hot gas molecules being cooled to lower temperatures. As the temperature is lowered, the molecules move more and more slowly. They undergo slower translational motion, which is movement from place to place through three dimensions. On average, the cooled molecules also undergo slower rotational and vibrational motion. Rotational motion is a tumbling end-over-end movement, and vibrational motion is the extension and compression of distances between atoms in a molecule.

At the boiling point, a transformation takes place, and the gas or vapor changes to a liquid. At the melting point, another transformation takes place, and the liquid changes to a solid. Motion does not cease just because the molecules are in a solid state. In a solid, the molecules and atoms are locked into a regular but not completely rigid structure; they continue to vibrate around their average positions. As the temperature is lowered, this motion contains less and less energy, until, at absolute zero, no more energy can be removed. Although the entropy is at zero, there may be some residual energy, even though no more heat can be removed from the material.

The direction of heat flow is from hot to cold. It is the temperature and not the heat energy of an object that determines the direction of heat flow. Heat is the energy of random motion, whereas temperature is related to the average random kinetic energy per atom or molecule. Work is organized motion; heat is random motion.

The natural tendency of heat to spread to lower-temperature regions is associated with an increase in disorder, or entropy. Entropy is increased any time a concentrated form of energy is converted to a more dispersed form of energy. The second law of thermodynamics states that spontaneous changes result in increases in the total entropy of the universe.

When heat energy is removed from an object, the object undergoes a lowering of temperature. If an object were to reach a temperature of absolute zero, there would be no random motion of the atoms and thus no heat energy left to be removed. The object would have reached the lowest possible temperature.

Absolute zero is useful to mark the value of minimum entropy and also to mark the value of minimum temperature. The Kelvin scale of temperature recognizes this fact by defining the value of absolute zero as exactly 0 kelvins (-273.15 degrees Celsius). Although it is possible to get small amounts of matter very close to absolute zero, it will never be possible to remove all the heat energy from a sample of matter. Any cooling process results in transformations between two different entropy states that cause a corresponding decrease in temperature. When the temperature of a sample lowers, the two entropy states used to cause cooling tend to get closer together, so that as the temperature approaches absolute zero, the differences in the two entropy states, and thus the further cooling that can be achieved, also approach zero. According to the third law of thermodynamics, it is not possible to reach absolute zero in a finite number of cooling steps.

The universe's tendency toward increasing disorder and entropy is also a tendency toward the maximum dispersal of energy. The natural direction of change is toward the spreading out of energy, not toward its concentration. Atoms and molecules in motion may give up some of their energy to molecules and atoms near them. Atoms in a solid that are vibrating much faster than others near them will transfer their excess energy to the other atoms, causing the original atoms to vibrate more slowly and the adjacent atoms to vibrate more rapidly. This spread of thermal or heat energy is similar to the natural tendency for gas molecules to spread from a higher-pressure region to a lower-pressure region. Just as a gas tends to spread out naturally to fill a container, energy transformations also tend to result in a dispersal or spreading out of energy over more available states. The natural spread of heat energy is associated with the maximum possible arrangements or ways to distribute that energy. Thus, absolute zero provides a convenient reference point for entropy, since at absolute zero the entropy is zero.

A plot of the volume of a gas versus its temperature gives a straight line that, if extrapolated to a volume of zero, gives a lowest temperature of approximately -273 degrees Celsius. Based on this observation, Sir William Thomson (Lord Kelvin) suggested a temperature scale with a minimum value of 0 at the point at which extrapolation of gas behavior suggests that the volume reach zero. Since negative volumes are not possible, the graph implies that such a minimum possible temperature exists. This value is now known as absolute zero, and it has a temperature value of exactly -273.15 degrees Celsius, or 0 kelvins. On the Kelvin scale, water under a pressure of exactly one atmosphere freezes at 273.15 kelvins (0 degrees Celsius) and boils at 373.15 kelvins (100 degrees Celsius).

The vapor pressure of helium can be used to measure temperature down to about one kelvin. At lower temperatures, magnetic properties can be used to measure temperature. The Curie-Weiss law relates how certain magnetic properties change in a known way at extremely low temperatures and therefore can be used to determine temperature. Population of energy levels within atoms can be used to determine temperature because the higher the temperature, the greater the extent to which higher energy states are occupied. Both vibrational and rotational levels in a molecule are affected by temperature and can be probed with electromagnetic radiation such as light.

Extremely low temperatures can be achieved with the use of a technique called adiabatic demagnetization. "Adiabatic" means no flow of heat is allowed, and "demagnetization" refers to removing or turning off a magnetic field. In 1926, William Francis Giauque and Peter J. W. Debye independently suggested this method as a means to achieve temperatures much lower than one kelvin. Their approach involved the use of rare-earth salts such as gadolinium sulfate, in which unpaired electrons act like tiny magnets that can be aligned with or in opposition to an external magnetic field. Normally, the magnetic fields of the unpaired electrons are oriented in a random manner. When an external magnetic field is applied, however, the unpaired electrons tend to orient their magnetic fields in line with the external field; this is a more ordered, or lower entropy, state. If the sample is thermally isolated so that no heat can enter or leave the sample and the magnetic field is removed or turned off, then the sample maintains the same entropy but goes to a lower temperature. This process of adiabatic demagnetization is repeated, with each complete cycle of applying and then removing a magnetic field resulting in a lower temperature.

Even lower temperatures can be obtained by using adiabatic nuclear demagnetization, in which the magnetic fields generated by protons in the nuclei of atoms are used in place of electronic magnetic fields. A temperature of 100 picokelvins, or 0.0000000001 kelvin, was achieved in 1999 by the use of adiabatic nuclear demagnetization.

Applications

Serving as one of the reference points in establishing the temperature scale is one of the important applications of absolute zero. The triple point of water, where its liquid, solid, and vapor states can coexist, is defined at exactly 273.16 kelvins, or 0.01 degree Celsius; in conjunction with absolute zero at exactly 0 kelvins, this sets the absolute value and increment size of the Kelvin scale. On the Kelvin scale, there is no negative temperature.

Although it might seem that a temperature of absolute zero could be reached, this remains an impossible goal. Each step of the cooling process described above lowers the temperature, but the amount by which the temperature is lowered gets smaller with each step. Thus, it would require an infinite number of steps to reach absolute zero. This general statement is true regardless of the specific cooling method that might be used. The third law of thermodynamics is the formal statement of the fact that it is impossible to reach absolute zero.

Nevertheless, the knowledge that there is a lower limit of temperature has important applications. In addition to providing a reference point for the temperature scale, absolute zero sets a goal for seeking temperatures ever closer to absolute zero and the unusual phenomena that occur at these low temperatures.

The third law of thermodynamics is useful because it provides a reference point for measuring entropy. As temperature increases, entropy increases, because more randomness and disorder are created in the form of increased atomic and molecular motion. The connection between absolute zero and entropy is essential because it establishes a reference point for when the entropy of elements and compounds is exactly zero.

Changes tend to occur that cause an increase in the total randomness of the universe. This observation is expressed in the second law of thermodynamics, which states that the entropy of the universe tends to increase with any spontaneous change. An understanding of entropy and the ability to assign numerical values to entropy are important in determining whether a chemical or physical change will occur. Since the entropy of an element or compound is zero at absolute zero, then the entropy at a higher temperature is always determined relative to this reference. This is accomplished by determining the effect that increasing temperature has on the entropy of a substance. The heat capacity, which is a measure of how effectively a solid can absorb heat energy, and the jump in entropy that occurs with phase transitions, such as from solid to liquid or liquid to gas, can be used to establish the entropy of a substance at any temperature.

In addition to the use of absolute zero as a reference for measurement, the quest to get as close to absolute zero as possible has resulted in the discovery of new technologies and scientific phenomena. One important technological application has been the ability to produce and maintain solids and liquids at very low temperatures. Liquid nitrogen, with a boiling point of 77 kelvins, is used for temporary refrigeration during transport. Liquid oxygen, with a boiling point of 90 kelvins, is a convenient means of storing oxygen for industrial, medical, and rocketry applications. Liquid helium, with a boiling point of 4.2 kelvins, is used for cooling superconducting magnets used in magnetic resonance imaging.

The pursuit of absolute zero also led to the discovery of the phenomena of superconductivity and superfluidity. Superconductivity is the disappearance of resistance to the flow of electrical current in metals and metal compounds. A current established in a superconducting material goes on without stopping for as long as the temperature is low enough to maintain the condition. Superconductivity was originally observed in mercury at a temperature of 4.1 kelvins. A number of other metals and metal compounds have since been found to be superconducting, and the temperature of superconducting materials has been pushed above the boiling point of liquid nitrogen, making it easier to maintain the necessary temperature.

Helium remains a liquid at a pressure of one atmosphere but can be converted to a solid at 1 kelvin and twenty-five atmospheres of pressure. Helium has a residual energy content that causes it to remain a liquid, while other elements and compounds change to a solid under a pressure of one atmosphere and low temperature. Below 2.2 kelvins, liquid helium takes on the property of superfluidity; it is sometimes called helium I above this temperature and helium II below it because its properties are so different at the lower temperature. Helium II conducts heat two hundred times better than copper metal, and the temperature remains so constant that the liquid will not boil; rather, whole layers of surface atoms evaporate at the same time. Helium II has no resistance to flow, meaning that a current of flowing helium atoms will go on without stopping, just as an electric current travels in a superconductor without stopping. The liquid tends to flow easily along the walls and up the sides of a glass container.

The superfluidity of helium II is partly due to Bose-Einstein condensation, a process by which bosons (as opposed to fermions) are cooled to very near absolute zero so that they all occupy the lowest quantum state. At this point, the bosons become absolutely identical to each other and coalesce, or condense, into a single entity, one that exhibits quantum phenomena on a macroscopic scale. Helium II is not a true Bose-Einstein condensate, but it does share some of the same properties. The first genuine Bose-Einstein condensate was created in 1995 by Eric Cornell and Carl Wieman at JILA, previously the Joint Institute for Laboratory Astrophysics. Cornell and Wieman used laser cooling and another technique, magnetic evaporative cooling, to lower the temperature of rubidium 87 atoms to around 170 nanokelvins, or 0.00000017 kelvin.

Context

In 1787, Jacques-Alexandre-Cesar Charles observed that the volume of a gas is directly proportional to its temperature if the pressure is held constant. In the next century, Lord Kelvin suggested a temperature scale with a minimum value of zero, which would come to be called absolute zero.

Early attempts at achieving low temperature were based on allowing a liquid to evaporate to its vapor because this evaporation process results in a cooling of the remaining liquid. From 1852 to 1862, Lord Kelvin and James Prescott Joule conducted a series of experiments on the expansion of gases. They discovered that when a gas at high pressure is allowed to expand through a throttle valve or constriction to a lower pressure, it undergoes a cooling, provided the gas is below a certain temperature unique to each gas, called its inversion temperature.

In 1860, Sir William Siemens developed a heat exchanger in which a cooled gas was allowed to cool the next cycle of gas prior to expansion, and so a succession of lower temperatures could be achieved. William Hampson and Karl Paul Gottfried von Linde applied Siemens's idea on a large scale and were able to develop a method of routinely liquefying air. Any gas can form a liquid, provided it is below another temperature unique to each type of gas, called the critical temperature.

In 1898, James Dewar used liquid nitrogen to cool hydrogen gas below its inversion temperature of 193 kelvins and used the Siemens process to cool hydrogen gas successfully until it became liquid hydrogen, which boils at 20 kelvins. In 1908, Heike Kamerlingh Onnes used liquid hydrogen to cool helium below its inversion temperature of 100 kelvins. Successive expansions of helium finally brought it below its critical temperature of 5.2 kelvins, and liquid helium was formed for the first time. Liquid helium boils at 4.2 kelvins. Boiling liquid helium under reduced pressure to allow for evaporative cooling is one way to obtain temperatures less than 1 kelvin. In 1911, Kamerlingh Onnes observed superconductivity in mercury at 4.1 kelvins.

In the early 1930s Giauque carried out adiabatic demagnetization on a rare-earth salt, gadolinium sulfate, to achieve much lower temperatures than could be achieved by evaporating helium. By 1957, a temperature of 0.00002 kelvin had been obtained by a paramagnetic salt, which is a salt capable of being magnetized. Similar processes have been used based on nuclear magnetic moments, where nuclei of certain atoms generate tiny magnetic fields that can line up with or oppose an external magnetic field. This technique, called adiabatic nuclear demagnetization, has resulted in temperatures of 0.0000000001 kelvin.

Another method of cooling material to near absolute zero is laser cooling, specifically Doppler cooling, in which a laser's frequency is tuned to below the electronic transition level of an atom. The laser is then aimed at the atoms to be cooled, causing the atoms to move toward it, absorbing more photons and losing momentum.

Principal terms

ABSOLUTE ZERO: the lowest possible temperature, which corresponds to a value of 0 kelvins (-273.15 degrees Celsius)

ADIABATIC DEMAGNETIZATION: a technique used to cool a solid by aligning the magnetic field of sample atoms with an external magnetic field and then turning off the external field

CRITICAL TEMPERATURE: the temperature above which a gas cannot be converted to a liquid

HEAT: the energy resulting from random motion of molecules or atoms

INVERSION TEMPERATURE: the temperature below which the expansion of a gas from high pressure to low pressure results in a cooling of the gas

KELVIN: the temperature unit in the Kelvin scale, in which water freezes at 273.15 kelvins

SUPERCONDUCTIVITY: a phenomenon observed in certain metals and metal compounds in which all resistance to the flow of electric current disappears at low temperatures

SUPERFLUIDITY: a phenomenon in which cooling helium to below 2.2 kelvins causes all resistance to its flow to disappear

TEMPERATURE: the average energy resulting from the random motion of atoms or molecules

Bibliography

Anderson, M. H., et al. "Observation of Bose-Einstein Condensation in a Dilute Atomic Vapor." Science 269.5221 (1995): 198–201. Print.

Aron, Jacob. "Atoms Go beyond Absolute Zero." New Scientist 12 Jan. 2012: 12. Print.

Asimov, Isaac. Asimov's New Guide to Science. New York: Basic, 1984. Print. An overview of all the biological and physical sciences that discusses the historical development and contributions of individual scientists in great detail. Has a good section on the history of low-temperature developments.

Atkins, P. W. The Second Law. New York: Freeman, 1984. Print. Discusses the second law of thermodynamics and the intellectual origins and implications of the tendency toward randomness and disorder. Includes connections between temperature, heat, and absolute zero. Excellent color drawings.

Fenn, John B. Engines, Entropy, and Energy. New York: Freeman, 1982. Print. Features a cartoon caveman that appears throughout the chapters to help explain energy and entropy and how these ideas affect real processes occurring in machines or in nature. Accessible to those without a science or mathematics background.

Gavroglu, Kostas, ed. History of Artificial Cold, Scientific, Technological and Cultural Issues. Dordrecht: Springer, 2014. Print.

Haldar, Pradeep, and Pier Abetti. "Absolute Zero, As the Name Suggests, Is As Cold As It Gets." IEEE Spectrum 48.3 (2011): 50–60. Print.

Knuuttila, Tauno. "World Record in Low Temperatures." O. V. Lounasmaa Laboratory. Aalto University, 13 Sept. 2010. Web. 16 Dec. 2013.

MacDonald, D. K. C. Near Zero: An Introduction to Low Temperature Physics. Garden City: Doubleday, 1961. Print. One of the books in the classic Science Study Series, written in a conversational style that makes the concepts of superconductivity, superfluidity, heat, temperature, and methods of approaching absolute zero quite understandable.

Mendelssohn, Kurt. The Quest for Absolute Zero: The Meaning of Low Temperature Physics. 2nd ed. London: Taylor, 1977. Print. An extremely detailed and fascinating account of the personalities and developments involved in low temperatures and absolute zero. Discusses early technological development and phenomena of superconductivity and superfluidity.

Spielberg, Nathan, and Bryon D. Anderson. Seven Ideas That Shook the Universe. New York: Wiley, 1987. Print. An examination of the key ideas in physics. Includes a section on energy that discusses how heat, motion, and temperature are related and a section on entropy that discusses the second and third law of thermodynamics, temperature, and absolute zero.

Temperature scales compared

Liquefaction of Gases

Thermal Properties of Solids