The Constant: A History of Getting Things Wrong Perpetuum Mobile Redux
Mar 10, 2026
A playful tour of the Brownian ratchet thought experiment and why tiny molecular jiggles cannot power forever-moving machines. Historical tales of overbalanced wheels, sealed-room demonstrations, and dramatic claims that gripped inventors and nations. Explanations of thermodynamics’ role in extinguishing perpetual motion dreams and how clever hoaxes and honest mistakes prolonged the fascination.
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Lippmann's Impossible Brownian Ratchet
- Gabriel Lippmann imagined a Brownian ratchet that would convert molecular motion into continuous rotation.
- He also immediately knew it couldn't work because it violated thermodynamic principles, yet he couldn't identify the specific failure mode.
Fish Tank Thought Experiment Shows Apparent Engine
- The episode describes a fish-tank thought experiment: two divided tanks linked by a rod with paddles on one side and a ratchet on the other.
- With fish or current the ratchet turns, but when the fish are removed the still-water paddle stops and so does the engine.
Brownian Motion Replaces Fish As Driver
- Shrinking the fish-tank device to molecular scale makes Brownian motion the driver, so paddles would be jostled by atoms.
- Lippmann predicted it would turn from molecular knocks, yet thermodynamics implied impossibility.