Maxwell Zombies (MZ) are a class of thermodynamic paradoxes closely related to the famous 19th-century thought experiment, Maxwell's Demon (MD), which tests the foundations of the second law of thermodynamics. MZs differ from MDs insofar as they are not prone to the same physical shortcomings that ultimately foil the demons.
The original Maxwell's demon is an imaginary creature that sorts gas molecules on an individual basis, creating a thermodynamic disequilibrium (either in pressure or temperature), whereby work can be performed on a perpetual basis, thereby subverting the second law. MDs fail for a number of reasons. First, the demon is microscopic in size like the molecules it sorts, hence is subject to the same kind of random thermal fluctuations, making it shaky and ineffective. Second, the MD would have difficulty 'seeing' the molecules it is trying to sort. Third, by thinking and doing the computations necessary to sort molecules, the MD generates sufficient disorder (entropy), when periodically clearing out its registers (i.e., forgetting), to satisfy the second law.
The Maxwell zombies sidestep these shortcoming. First, they are macroscopic in size, rather than microscopic, hence are not crippled by thermal fluctuations. Moreover, they operate on vast numbers of molecules simultaneously, rather than one at a time, therefore they are much more efficient. Second, they have no need to 'see' molecules; instead, they operate on them as they come along. Third, they do not perform computations. Thus, unlike Maxwell's demon, which is a small, intelligent, perceiving creature, Maxwell zombies are big, dumb, and blind devices.
More than two dozen Maxwell zombies have appeared in mainstream scientific literature and international conferences. Many remain unresolved in favor of the second law. The most theoretically grounded and experimentally verified of these is Duncan's paradox.