An imaginary creature who is able to sort hot molecules from cold molecules without expending energy, thus bringing about a general decrease in entropy and violating the second law of thermodynamics.
An imaginary creature who is able to sort fast-moving molecules from slow-moving molecules without adding any energy to the system. Such a creature could separate a gas into two containers, one containing hot gas and the other cold gas, bringing about a general decrease in entropy and violating the second law of thermodynamics ; however, such a creature is impossible, since the act of observing the molecules to detect their speed must add energy to the system.
A hypothetical creature created for a thought experiment. Maxwell's demon, if it existed, would act as a sort of gate guard by sorting between higher-energy molecules and lower-energy molecules within an enclosed space. Because such a sorting process would tend to decrease entropy, it would violate the second law of thermodynamics.
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