Stern–Gerlach

Plate 36 speaks for itself: “Two spots, where classical physics drew a smear.”

The Physics

Silver atoms passing through a non-uniform magnetic field split into two separate beams instead of a continuous smear: the magnetic moment is quantised.

F_z = μ_z ∂B_z/∂z

• F_z: force along the field gradient, N
• μ_z: magnetic moment component, J T⁻¹
• ∂B_z/∂z: field gradient, T m⁻¹

Classically one smear; the plate: two. Gerlach & Stern, 1922.

Honest caveat: The two spots come from the electron's spin, which was not proposed until 1925.

Sources

• W. Gerlach, O. Stern, Z. Phys. 9, 349 (1922).

On the poster

Plate 036, “Stern–Gerlach”: the pixel-art plate with its slogan “Two spots, where classical physics drew a smear.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.