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.