Absolute zero
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In physics, absolute zero is a fundamental lower bound on the temperature of a macroscopic system. In practice it is believed to be unachievable but its existence has been inferred from extrapolation from observed physical phenomena and from kinetic theory. Today it is defined as the temperature at which all motion of particles would theoretically cease.
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 this is −273.15 °C or −459.67 °F.
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History
A state of absolute zero was first proposed by Guillaume Amontons in 1702. Amontons was investigating the relationship between pressure and temperature in gases though he lacked accurate and precise thermometers. Though his results were at best semi-quantitative, he established that the pressure of a gas increases by roughly one-third between the temperatures of "cold" and the boiling point of water. His work led him to speculate that a sufficient reduction in temperature would lead to the disappearance of pressure.
Though absolute zero can be defined in this way, such a definition has practical and conceptual limitations as any real gas will liquefy before attaining a temperature of 0 °R.
In 1848, William Thomson, 1st Baron Kelvin proposed an absolute temperature scale in which equal reduction in measured temperature gave rise to equal reduction in the heat of a body. This freed the concept from the constraints of the gas laws and established an absolute zero as the temperature at which no further heat could be transfered from a body. Absolute zero has never been reached.
Kinetic theory
According to kinetic theory there would be no movement of individual particles at absolute zero, and thus any material at this temperature would be solid. This contradicts experimental evidence. A more practical definition of absolute zero is as the temperature where no further energy may be extracted. For the case of free atoms at temperatures approaching absolute zero, most of the energy is in the form of translational motion and the temperature can be measured in terms of the distribution of this motion, with slower speeds corresponding to lower temperatures.
In fact because of quantum mechanical effects, the speed at absolute zero is not exactly zero, but depends, as does the energy, on the volume within which the atom is confined. At absolute zero, the molecules and atoms in a system are all in the ground state (i.e., the lowest possible energy state) and the system has the least possible amount of kinetic energy allowed by the laws of physics. This minimum energy corresponds to the zero-point energy encountered in the quantum mechanical particle in a box problem. As emphasised above, the lowest possible energy is not necessarily zero energy, owing to the ramifications of quantum theory.
Cryogenics
It can be shown from the laws of thermodynamics that absolute zero can never be achieved, though it is possible to achieve temperatures arbitrarily close to it through the use of cryocoolers. This is the same principle that ensures no system may be 100% efficient.
At very low temperatures in the vicinity of absolute zero, matter exhibits many unusual properties including superconductivity, superfluidity, and Bose-Einstein condensation. In order to study such phenomena, scientists have worked to obtain ever lower temperatures.
- The lowest temperature Bose-Einstein condensate achieved was 450 pK, or 4.5 × 10-10 K. This was performed by Wolfgang Ketterle and colleagues at the Massachusetts Institute of Technology[1].
- The Boomerang Nebula, with a temperature of 1 K, is the coldest place known outside a laboratory. The nebula is 5000 light-years from Earth and is in the constellation Centaurus[2].
- The coldest temperature ever produced was 100 pK during an experiment on nuclear magnetic ordering in the Helsinki University of Technology's Low Temperature Lab[3].
Absolute temperature scales
Absolute or thermodynamic temperature is conventionally measured in kelvins, and now rarely in degrees Rankine.
An object's absolute temperature therefore describes how much warmer the object is than absolute zero. While temperature is a measure of the heat of an object, heat itself is simply a highly abstract consideration of the kinetic energy of the molecular particles of the object. At absolute zero, we have reached the baseline. The absolute temperature measures the movement among the particles of an object by comparing it to the state of an object at absolute zero.
Negative temperatures
- Main article: Negative temperature
Certain situations can give rise to "negative" temperatures, though this phenomenon is very much an artifact of the definition of thermodynamic temperature, rather than a system "colder" than absolute zero.
