In a letter written in 1867, James Clerk Maxwell described a
hypothetical creature: a “neat-fingered being” capable of
separating fast molecules from slow ones. Maxwell mused that
such a creature would seem to violate the second law of
thermodynamics, which had recently been enunciated by
Rudolf Clausius and is now a pillar of our understanding of the
natural world. Over the past century and a half, that hypothetical creature – Maxwell’s demon – has wandered through
the thoughts of eminent scientists, has appeared in research
articles and popular cultural references, and in recent years has
been observed in laboratory experiments. Along the way, the
mischievous devil has sharpened our understanding of the
second law of thermodynamics, exposing a deep relationship
between physics and information. I will give an overview of the
questions raised and the lessons learned from contemplating
Maxwell’s demon, and I will summarize our current understanding of this topic. This story highlights the importance of
imagination and whimsy in scientific discovery.