Absolute Zero and Even Colder?

Abstract

We consider first the absolute zero of temperature and then negative Kelvin temperatures. The unattainability formulation of the Third Law of Thermodynamics is briefly reviewed. It puts limitations on the quest for absolute zero, and in its strongest mode forbids the attainment of absolute zero by any method whatsoever. But typically it is stated principally with respect to thermal-entropy-reduction refrigeration (TSRR). TSRR entails reduction of a refrigerated system's thermal entropy, i.e., its localization in momentum space. The possibility or impossibility of overcoming these limitations via TSRR is considered, with respect to both standard and absorption TSRR. (In standard TSRR, refrigeration is achieved at the expense of work input; in absorption TSRR, at the expense of high-temperature heat input.) We then consider the possibility or impossibility of the attainability of absolute zero temperature via configurational-entropy-reduction refrigeration (CSRR). CSRR entails reduction of a refrigerated system's configurational entropy, i.e., its localization in position space, via positional isolation of entities that happen to be in their ground states. Of course, the Second Law of Thermodynamics requires any decrease in entropy of a refrigerated system to be paid for by a compensating greater (in the limit of perfection, equal) increase in eLtropy. Or, in other words, the Second law of Thermodynamics requires any localization in the total momentum-plus-position phase space of a refrigerated system to be paid for by a compensating greater (in the limit of perfection, equal) delocalization in the total momentum-plus-position phase space of the refrigerated system and/or of its surroundings. We also briefly consider energy-reduction refrigeration (ERR), which entails extraction of energy but not entropy from a refrigerated system, and quantum-control refrigeration (QCR). (S not E denotes entropy in TSRR and CSRR, and E denotes energy in ERR, because S is the standard symbol for entropy, and E for energy.) With respect to both TSRR and CSRR, we consider not only the issue of attainability of absolute zero, but also the separate issues, even if absolute zero can be attained, of maintaining it, and of verifying that it has been attained. Purely dynamic-as opposed to thermodynamic-limitations on the quest for absolute