With 20-20 hindsight, we all agree that Einstein’s discoveries of 1905 revolutionized nearly every area of fundamental physics. But what did scientists think at the time? One telling source is Poincaré’s 1907 account of the “new crisis” in physics (available here, on the newly released Popular Science archive). Poincaré identifies five fundamental principles he thought were in danger of being overturned:
- Carnot’s principle of heat transfer. Brownian motion was thought to violate Carnot’s principle of heat transfer, since it apparently involved an unlimited source of motion. Poincaré wrote, “to see the world return backward, we no longer have need of the infinitely keen eye of Maxwell’s demon; our microscope suffices.”
- The principle of relativity. Although Einstein had recently defended this principle, Poincaré wasn’t convinced, and in particular worried about the prohibition on superluminal signaling. Anticipating a coming revolution in gravity, he wrote: “are such signals inconceivable, if we admit with Laplace that universal gravitation is transmitted a million times more rapidly than light?”
- Newton’s third law (of action-reaction). Electrodynamics seemed to be suggesting that not every action corresponds to an equal and opposite reaction. In particular, the action of one electric charge on another doesn’t necessarily give rise to a simultaneous reaction.
- Lavoisier’s principle of fixed mass. Alluding to Einstein, Poincaré wrote that electrodynamics suggests a body’s mass might increase with velocity, refuting principle of fixed mass: “And now certain persons think that it seems true to us only because in mechanics merely moderate velocities are considered.”
Mayer’s principle of energy conservation. Finally, the recent discovery of radiation by the Curies suggested to
LaplacePoincaré that radium might be a limitless source of energy, and hence that energy is not locally conserved.
What I find striking about this list is Poincaré’s recognition of the deep and difficult consequences of taking classical electrodynamics seriously — and in particular, of retaining the principle of relativity. Of course, only 3 and 4 were actually overturned, and a version of 4 may still be salvageable (by replacing “mass” with “rest mass”). And it’s somewhat surprising that as late as 1907, Poincaré isn’t mentioning Einstein by name.
But then, I suppose it’s never clear what the revolution will bring until well after it’s over.
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