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In physics, certain systems can achieve negative temperature; that is, their thermodynamic temperature can be expressed as a negative quantity on the Kelvin or Rankine scales. In colloquial usage, "negative temperature" may refer to temperatures that are expressed as negative numbers on the more familiar Celsius or Fahrenheit scales, with values that are colder than the zero points of those scales but still warmer than absolute zero; this usage is not treated in this article. A system with a truly negative temperature on the Kelvin scale is ''hotter'' than any system with a positive temperature. If a negative-temperature system and a positive-temperature system come in contact, heat will flow from the negative- to the positive-temperature system. That a system at negative temperature is hotter than any system at positive temperature is paradoxical if absolute temperature is interpreted as an average kinetic energy of the system. The paradox is resolved by understanding temperature through its more rigorous definition as the tradeoff between energy and entropy, with the ''reciprocal'' of the temperature, thermodynamic beta, as the more fundamental quantity. Systems with a positive temperature will increase in entropy as one adds energy to the system. Systems with a negative temperature will decrease in entropy as one adds energy to the system. Most familiar systems cannot achieve negative temperatures, because adding energy always increases their entropy. The possibility of decreasing in entropy with increasing energy requires the system to "saturate" in entropy, with the number of high energy states being small. These kinds of systems, ''bounded'' by a maximum amount of energy, are generally forbidden classically. Thus, negative temperature is a strictly quantum phenomenon. Some systems, however (see the examples below), have a maximum amount of energy that they can hold, and as they approach that maximum energy their entropy actually begins to decrease. == Clarification on different definitions of temperature and entropy == Temperature is defined by the relationship between entropy and energy. The negative temperatures discussed here arise from statistical mechanics using the Boltzmann definition of entropy. Entropy is also independently defined in thermodynamics, and in the thermodynamic limit (i.e. many degrees of freedom) these definitions are generally consistent with each other. However, for small systems and systems where the number of states decreases with energy, the Boltzmann entropy and the thermodynamic entropy are not consistent, and the temperatures derived from these entropies are different. Some theorists have proposed using an alternate definition of entropy originally proposed by Gibbs as a way to resolve these inconsistencies, although this new definition would create other inconsistencies. Regardless of how we choose to define entropy and temperature, the physical behavior of a negative temperature system is the same. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Negative temperature」の詳細全文を読む スポンサード リンク
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