Spin: The Quantum Property That Should Have Been Impossible

Paul Halpern in Forbes:

In the early 1920s, physicists were first working out the mysteries of the quantum Universe. Particles sometimes behaved as waves, with indeterminate positions, momenta, energies, and other properties. There was an inherent uncertainty to a great many properties that we could measure, and physicists raced to work out the rules.

Amidst this frenzy, a young Dutch researcher named George Uhlenbeck implored Paul Ehrenfest, his research supervisor at the University of Leiden, not to submit the paper he wrote with Samuel “Sam” Goudsmit about a new quantum number called spin. It was not correct, Uhlenbeck told him in a frenzy. Let’s just drop it and start over, he implored.

Uhlenbeck and Goudsmit, both then in their mid-20s, had just showed their joint result to the great Dutch physicist Hendrik Lorentz who had found what seemed like a major error. Electrons, he pointed out, couldn’t possibly rotate fast enough to generate the magnetic moment (interaction strength between a particle and an external magnetic field) that the duo had predicted. The particles would need to whirl faster than the sacred speed limit of light. How could they? The spin paper is unphysical, Uhlenbeck told Ehrenfest, and should not be published.

Ehrenfest’s reply was curt. “It is too late,” he told Uhlenbeck. “I have already submitted the paper. It will be published in two weeks.” Then he added, “Both of you are young and can afford to do something stupid.”

More here.