Thermal exploration in engine design

摘要

The air we breathe is filled with molecules that can be represented mathematically in a Boltzmann probability curve as a descending exponential distribution of low- to high-energy states. In this context, the average ambient temperature (20℃), for example, is more likely to reflect a state of many more molecules with low energy than high energy and a temperature with a positive sign. Such an exponential distribution has a lower bound of zero energy but no upper bound. However, systems can be designed to have an upper bound in energy such that higher energies are more likely than lower energies. In this case, although we "feel" the same ambient temperature of 20℃, it bears a negative sign to account for exponential growth in energy toward this bound. On page 1019 of this issue, Marques Muniz et al. (1) demonstrate just such a system, with interacting photons instead of molecules. The findings of Marques Muniz et al. contribute to an expanding landscape of thermal possibilities for device design. This space now includes negative temperatures observed in contexts as varied as magnetic spins (2,3), cold atoms in optical lattices (4), and vortices in two-dimensional hydrodynamics (5). It also includes the many "temperatures" needed to fit thermal distributions that are studied in quantum simulators and computers (6) and the conversion of information to energy. The latter is leading to a new understanding of classical and quantum nanothermodynamics as well as the second law of thermodynamics (7).