Chances are, you are reading these words in some room or other. Build a wall down the middle of it, and in the middle of the wall, insert a tiny shutter. Conjure up a tiny critter who can open and shut this little door at will. When a hotter-than-average air molecule careers towards it, the creature opens the shutter and lets the molecule pass to the other side of the room. Colder-than-average molecules are allowed to pass in the other direction. Soon enough, one half of the room is sweltering, while the other half is distinctly nippy. Magic?
Not yet. A fridge performs exactly this trick, pumping heat out of its interior through that dinky radiator at the back. The refrigerator only becomes magical if it carries on running after you unplug it. Fighting entropy is expensive. It takes energy. Us living types cock a spectacular snook at the forces of entropy, but we too are in a sense plugged in. Snuff out the sun and you snuff out all life.
This ‘finite being’ of James Clerk Maxwell’s imagination — the ‘demon’ conjured up in a typically charming and flippant letter to his friend Peter Tait in 1867 — needs no power source, because it exists to demonstrate that there is no actual mechanical reason why the air in one half of an enclosed space should not grow hotter than the air in the other half. It could happen, but luckily for us it’s unimaginably unlikely. Our existence depends, moment to moment, on the innate conservatism of big numbers.
Brian Clegg would have it that his short, charming scientific biography of the Victorian Scottish physicist James Clerk Maxwell is not all his own work, but a collaboration with Maxwell’s demon.
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