spin precession

Spin precession is the steady wheeling of a particle's spin around the direction of a magnetic field, much as a tilted spinning top wheels around the vertical under gravity. Because the spin carries a magnetic moment, the field exerts a turning force that does not flip the spin over but instead swings it in a circle, keeping a constant tilt while sweeping steadily around. The rate of this circling is set by the field strength and the particle.

The picture is helped by the fact that quantum averages of spin follow the same equations a classical magnet would. So even though an individual spin is a quantum object with only two measurable outcomes along an axis, the expected direction of the spin really does trace a smooth cone over time. The stronger the field, the faster the precession, in simple proportion.

This humble wheeling is the engine of magnetic resonance. In an MRI scanner the spins of hydrogen nuclei in your body precess in a powerful field, and a radio pulse tuned to their precession rate tips and tracks them, producing the signal that becomes an image. The same effect lets chemists identify molecules and underlies some of the most precise clocks and sensors ever built.