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.
Schachtman, Tom. Absolute Zero and the Conquest of Cold. New York: Houghton Mifflin, 1999.
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.
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