Absolute zero
Absolute zero
Absolute zero, 0 degrees Kelvin (K), –459.67°F, or–273.15°C, is the minimum possible temperature: the state in which all motion of particles in a substance is at a minimum. It is also referred to as the temperature at which pressure is zero. Equivalently, when the entropy of a substance has been reduced to zero, the substance is at absolute zero.
The third law of thermodynamics dictates that absolute zero can never be achieved. The law states that the entropy of a perfect crystal is zero at absolute zero. If the particles comprising a substance are not ordered as a perfect crystal, then their entropy cannot be zero. At any temperature above zero, however, imperfections in the crystal lattice will be present (induced by thermal motion), and to remove them requires compensatory motion, which itself leaves a residue of imperfection. Another way of stating this dilemma is that as the temperature of a substance approaches absolute zero, it becomes increasingly more difficult to remove heat from the substance while decreasing its entropy. Consequently, absolute zero can be approached but never attained.
Although the third law of thermodynamics declares that it is impossible to cool a substance all the way to absolute zero, temperatures of only a few billionths of a degree Kelvin have been achieved in the laboratory. These very low temperatures are referred to as cryogenic temperatures. To reach such low temperatures, procedures use specially insulated vessels called cryostats.
The motions of particles near absolute zero are so slow that their behavior, even in large groups, is governed by quantum-mechanical laws that otherwise tend to be swamped by the chaotic atomic- and molecular-scale motions that are perceived as heat. As a result, various special phenomena (e.g., Bose-Einstein condensation, superfluids such as helium II) can only be observed in materials cooled nearly to absolute zero.
Atoms may be cooled by many methods, but laser cooling and trapping have proved essential to achieving the lowest possible temperatures. A laser beam can cool atoms that are fired in a direction contrary to the beam because when the atoms encounter photons, they absorb them if their energy is at a value acceptable to the atom (atoms can only absorb and emit photons of certain energies). If a photon is absorbed, its momentum is transferred to the atom; if the atom and photon were originally traveling in opposite directions, this slows the atom down, which is equivalent to cooling it.
When atoms have been cooled to within millionths or billionths of a degree of absolute zero, a number of important phenomena appear, such as the creation of Bose-Einstein condensates, so called because they were predicted in 1924 by German physicist Albert Einstein (1879–1955) and Indian physicist Satyendranath Bose (1894–1974). According to Bose and Einstein, bosons—particles having an integral value of the property termed “spin”—are allowed to coexist locally in the same quantum energy state.
KEY TERMS
- Absolute zero
- —Absolute zero is the lowest temperature and is equal to 0K (–459°F [–273°C]).
- Boson
- —A type of subatomic particle that has an integral value of spin and obeys the laws of Bose-Einstein statistics.
- Fermion
- —A type of subatomic particle with fractional spin.
(Fermions, particles that have half-integer spin values, cannot coexist locally in the same energy state; electrons are fermions, and so cannot share electron orbitals in atoms.) At temperatures far above absolute zero, large collections of bosons (e.g., rubidium atoms) are excited by thermal energy to occupy a wide variety of energy states, but near absolute zero, some or all of the bosons will lapse into an identical, low-energy state. A collection of bosons in this condition is a Bose-Einstein condensate. Bose-Einstein condensates were first produced, with the help of laser cooling and trapping, in 1995. Since that time, numerous researchers have produced them and investigated their properties.
A Bose-Einstein condensate can emit “atom lasers,” beams of fast-moving atoms analogous to the beams of photons that comprise conventional lasers. Furthermore, the speed of light in a Bose-Einstein condensate can be controlled by a laser beam. Researchers have succeeded in reducing the speed of light in a Bose-Einstein condensate to 38 mph (61 km/h) and even to zero, effectively stopping a pulse of light for approximately a thousandth of a second and then restarting it. This ability does not contradict the famous statement that nothing can exceed the speed of light in a vacuum, i.e., 186,000 miles per second (300,000 kilometers per second). Light is slowed in any transparent medium, such as water or glass, but its vacuum speed remains the limiting speed everywhere in the Universe.
Temperatures near absolute zero permit the study not only of Bose-Einstein condensates, but of large, fragile molecules that cannot exist at higher temperatures, of superfluids, of the orderly arrangements of electrons termed Wigner crystals, and of other phenomena.
See also Atomic theory; Matter; Physics; Quantum mechanics; Subatomic particles.
Resources
BOOKS
Logan, Chuck. Absolute Zero. New York: HarperCollins, 2002.
Rogers, Donald. Einstein’s Other Theory: The Planck-Bose-Einstein Theory of Heat Capacity. Princeton, NJ: Princeton University Press, 2005.
PERIODICALS
Seife, Charles. “Laurels for a New Type of Matter.” Science 294, no. 5542 (October 19, 2001): 503.
Larry Gilman
Absolute Zero
Absolute Zero
In mathematics, there is no smallest number. It is always possible to find a number smaller than any number given. Zero is not the smallest number because any negative number is smaller than zero. The number line extends to infinity in both the positive and negative directions. However, when measuring things, it is often necessary to have a smallest number. If a car is stopped, it cannot go any slower. The temperature scale also has a lowest possible temperature, called "absolute zero." This is somewhat confusing, because temperatures measured on either the Fahrenheit or Celsius temperature scales are often negative. In some countries, temperatures below zero are quite common in the winter. So, before talking about absolute zero, some temperature scales should be explored.
Temperature Scales
In the United States, temperatures are usually reported on the Fahrenheit temperature scale. On this scale, water freezes at 32° F and water boils at 212° F. Temperatures below zero ("negative temperatures") are common, especially in the northern states. Thus 0° F is not the coldest possible temperature.
Scientific measurements of temperature in the United States and most other countries use the Celsius temperature scale. On this scale, water freezes at 0° C and water boils at 100° C. Nonetheless, 0° C is not the coldest possible temperature. In some parts of the United States, days or weeks may go by in winter when the temperature never rises above 0° C. Therefore, 0° C is also not the coldest possible temperature. However, there is a coldest possible temperature on both the Fahrenheit and Celsius scales, called absolute zero.*
*Absolute zero on the Fahrenheit scale is −459 degrees.
Determining Absolute Zero
Suppose an experiment is done to determine if there is a lowest possible temperature. Helium gas is put in a cylinder with a pressure gauge. Helium is chosen because the atoms are very small and the attractive forces between helium atoms are also very small. A gas whose atoms exert no forces on each other and whose atoms have no volume is called an "ideal gas." Ideal gases do not exist, but helium behaves like an ideal gas if the temperature is relatively high (room temperature) and the pressure is low (atmospheric pressure).
The cylinder and gas have an initial temperature of 25° C. The cylinder is placed in a refrigerator and the temperature in the refrigerator is lowered to 15° C. The pressure of the gas in the cylinder also goes down (because the gas molecules are going slower). If the temperature is lowered to 5° C, the pressure will go down even more. This is done several more times and a graph is drawn of the data (see figure).
Notice that all of the data points fall along a straight line. Now it can be asked, at what temperature will the gas have no pressure? Of course a real gas will turn to a liquid or solid at some low temperature, but the temperature at which an ideal gas would have no pressure can be extrapolated. This temperature turns out to be approximately −273° C.
Although we picked helium gas for our thought experiment, the type of gas is not critical. At sufficiently high temperatures and low pressures, most gases behave like ideal gases. Consequently, at appropriately high temperatures and low pressures, all gases behave as if they would have no pressure at −273° C. This temperature is known as absolute zero.
Absolute Temperature Scales. Scientists have defined an absolute temperature scale that starts at absolute zero and uses a unit the same size as the Celsius degree. This absolute temperature scale is called the Kelvin scale. This temperature scale does not use a degree symbol. The unit of temperature is the kelvin. On this scale, water freezes at 273 K (read as 273 kelvin) and boils at 373 K. There is also an absolute temperature scale, called the "Rankine scale," that uses a unit the same size as the Fahrenheit degree. However, it is rarely used any more except in some engineering applications in the United States.
Achieving Absolute Zero. Is it possible to build a refrigerator that will cool an object to absolute zero? Surprisingly, the answer is no. This is not just a matter of building a better refrigerator. There are theoretical reasons why a temperature of absolute zero is impossible to achieve. The impossibility of achieving absolute zero is usually called the third law of thermodynamics. But although it prohibits reaching absolute zero, it does not prevent obtaining temperatures as close to absolute zero as we wish to get.
Low Temperatures in Nature
What are the lowest temperatures that occur in nature? The lowest natural temperature ever recorded on Earth was −89° C (recorded in Vostok, Antarctica on July 21, 1983). Other objects in the solar system can have much lower surface temperatures. Triton, a satellite of Neptune, was observed by Voyager 2 to have a surface temperature of 37 K, making it the coldest known locale in the solar system. It is so cold that nitrogen freezes on the surface, making what looks like a pinkish frost. Triton has nitrogen geysers form when liquid nitrogen below the surface is vaporized by some internal heat source. The surface has lakes of frozen water. Pluto is only slightly warmer than Triton, with a temperature around 40 K to 60 K.
It might be thought that empty space is very cold. However, most space is not really empty. The space between the planets is not as cold as the surface of some of the planets. The few atoms and molecules in interplanetary space have a high kinetic energy (because of planetary magnetic fields and the solar wind ), so they have a relatively high temperature. However, this hot gas will not warm an object in space, because there are so few atoms in the vacuum of interplanetary space.
The spaces between stars may also have quite high temperatures for the same reason as interplanetary space. Interstellar space can be filled with hot glowing gas, heated by nearby stars or strong stellar magnetic fields. The interstellar gas can sometimes have a temperature of millions of kelvins. However, the gas atoms are very far apart—farther apart than even in the best laboratory vacuum possible.
Intergalactic space is very cold. The radiation left over from the Big Bang at the beginning of the universe has a temperature of about 2.73 K. However, on Earth we have been able to produce much lower temperatures in laboratories.
Artificial Low Temperatures
Although a refrigerator cannot be built that will reach absolute zero, for nearly 100 years we have known how to build a refrigerator that will produce a temperature lower than the lowest naturally occurring temperature. Helium was first liquefied in 1908 by compressing and cooling the gas. Helium boils at 4.2 K. By rapidly pumping away the helium vapor, it is possible to lower the temperature to about 1.2 K. This happens because the helium atoms with the most energy are the ones that escape as vapor. As a result, pumping removes the higher energy atoms and leaves behind the atoms with lower kinetic energy.
The most common isotope of helium is helium-4. It has two neutrons and two protons in its nucleus. Helium-3 only has one neutron in its nucleus. By liquefying and pumping on a container of helium-3 it is possible to produce a temperature of around 0.3 K. This temperature is incredibly cold, but even lower temperatures have been recorded.
A mixture of helium-3 and helium-4 can produce even lower temperatures. At temperatures below 1.0 K, the mixture will separate into pure helium-3 and a saturated solution of helium-3 dissolved in helium-4. If the helium-3 atoms are pumped away from the mixture, more helium-3 atoms will dissolve from the pure helium-3 layer into the mixture. As might be expected, the helium-3 atoms with the most energy are the ones that will move into the mixture, leaving behind lower energy helium-3 atoms. This technique has produced temperatures as low as 0.002 K.
It is possible to produce even lower temperatures by using magnetic traps and other devices. At these extremely low temperatures, it becomes increasing difficult to state a precise definition of temperature. The nucleus of an atom, the conduction electrons of the atom, and the atom itself might all have different temperatures. The lowest temperature produced for liquid helium is 90 microkelvins.
The attempt to produce ever lower temperatures is not just competition for the sake of competition. The technology developed to produce and to measure low temperatures has broad applications in other fields. Also, the behavior of materials at extremely low temperatures tests the limits of our theoretical understanding of matter itself. For example, in 1996 Eric Cornell and a research team used magnetic trapping to produce a new state of matter, called a "Bose-Einstein condensate." The existence of this state of matter confirmed our understanding of the quantum mechanical properties of matter.
see also Superconductivity; Temperature, Measurement of.
Elliot Richmond
Bibliography
Arons, Arnold B. Development of Concepts of Physics. Reading, MA: Addison-Wesley Publishing Company, 1965.
Eisberg, Robert M., and Lerner, L. S. (1981). Physics: Foundations and Applications (Vol. II). New York: McGraw-Hill Book Company.
Epstein, Lewis Carroll. Thinking Physics. San Francisco: Insight Press, 1990.
Hewitt, Paul G. Conceptual Physics 2nd edition. Menlo Park CA: Addison-Wesley Publishing Company, 1992.
MEASURING LOW TEMPERATURES
The temperature of liquid helium cannot be measured by using an ordinary thermometer. Among other problems, the mercury would freeze solid! So special methods must be used.
The most common method is to measure the resistance of certain types of wire. Platinum wire can be used at temperatures down to a few kelvin. In the liquid helium range, a phosphor-bronze wire with a small amount of lead added is used. At temperatures around 1.0 K, carbon resistance thermometers can be used. Below this temperature, various types of magnetic thermometers are used. These thermometers measure the magnetic properties of certain materials (such as magnetic susceptibility) that vary with temperature.
Absolute Zero
Absolute zero
Absolute zero , 0 Kelvin, -459.67° Fahrenheit, or -273.15° Celsius, is the minimum possible temperature : the state in which all motion of the particles in a substance has minimum motion. Equivalently, when the entropy of a substance has been reduced to zero, the substance is at absolute zero. Although the third law of thermodynamics declares that it is impossible to cool a substance all the way to absolute zero, temperatures of only a few billionths of a degree Kelvin have been achieved in the laboratory in the last few years.
The motions of particles near absolute zero are so slow that their behavior, even in large groups, is governed by quantum-mechanical laws that otherwise tend to be swamped by the chaotic atomic- and molecular-scale motions that are perceive as heat . As a result, various special phenomena (e.g., Bose-Einstein condensation, superfluids such as helium II) can only be observed in materials cooled nearly to absolute zero.
Atoms may be cooled by many methods, but laser cooling and trapping have proved essential achieving the lowest possible temperatures. A laser beam can cool atoms that are fired in a direction contrary to the beam because when the atoms encounter photons, they absorb them if their energy is at a value acceptable to the atom (atoms can only absorb and emit photons of certain energies). If a photon is absorbed, its momentum is transferred to the atom; if the atom and photon were originally traveling in opposite directions, this slows the atom down, which is equivalent to cooling it.
The third law of thermodynamics, however, dictates that absolute zero can never be achieved. The third states that the entropy of a perfect crystal is zero at absolute zero. If the particles comprising a substance are not ordered as a perfect crystal, then their entropy cannot be zero. At any temperature above zero, however, imperfections in the crystal lattice will be present (induced by thermal motion), and to remove them requires compensatory motion, which itself leaves a residue of imperfection. Another way of stating this dilemma is that as the temperature of a substance approaches absolute zero, it becomes increasingly more difficult to remove heat from the substance while decreasing its entropy. Consequently, absolute zero can be approached but never attained.
When atoms have been cooled to within millionths or billionths of a degree of absolute zero, a number of important phenomena appear, such as the creation of Bose-Einstein condensates, so called because they were predicted in 1924 by German physicist Albert Einstein (1879–1955) and Indian physicist Satyendranath Bose (1894–1974). According to Bose and Einstein, bosons—particles having an integral value of the property termed "spin"—are allowed to coexist locally in the same quantum energy state. (Fermions, particles that have half-integer spin values, cannot coexist locally in the same energy state; electrons are fermions, and so cannot share electron orbitals in atoms.) At temperatures far above absolute zero, large collections of bosons (e.g., rubidium atoms) are excited by thermal energy to occupy a wide variety of energy states, but near absolute zero, some or all of the bosons will lapse into an identical, low-energy state. A collection of bosons in this condition is a Bose-Einstein condensate. Bose-Einstein condensates were first produced, with the help of laser cooling and trapping, in 1995. Since that time, numerous researchers have produced them and investigated their properties.
A Bose-Einstein condensate can emit "atom lasers," beams of fast-moving atoms analogous to the beams of photons that comprise conventional lasers. Furthermore, the speed of light in a Bose-Einstein condensate can be controlled by a laser beam. Researchers have succeeded in reducing the speed of light in a Bose-Einstein condensate to 38 MPH (61 km/h) and even to zero, effectively stopping a pulse of light for approximately a thousandth of a second and then restarting it. This does not contradict the famous statement that nothing can exceed the speed of light in a vacuum, i.e., 186,000 MPH [300,000 km/h]. Light is slowed in any transparent medium, such as water or glass , but its vacuum speed remains the limiting speed everywhere in the Universe.
Temperatures near absolute zero permit the study not only of Bose-Einstein condensates, but of large, fragile molecules that cannot exist at higher temperatures, of superfluids, of the orderly arrangements of electrons termed Wigner crystals, and of other phenomena.
See also Atomic theory; Matter; Physics; Quantum mechanics; Subatomic particles.
Resources
books
schachtman, tom. absolute zero and the conquest of cold.new york: houghton mifflin, 1999.
periodicals
Glanz, James, "The Subtle Flirtation of Ultracold Atoms." Science. 5361 (April 10, 1998): 200–201.
Seife, Charles, "Laurels for a New Type of Matter." Science. 5542 (October 19, 2001): 503.
Larry Gilman
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Absolute zero
—Absolute zero is the lowest temperature possible. It is equal to 0K (-459°F [-273°C]).
- Boson
—A type of subatomic particle that has an integral value of spin and obeys the laws of Bose-Einstein statistics.
- Fermion
—A type of subatomic particle with fractional spin.