SternGerlach

The Stern-Gerlach experiment is a landmark 1922 experiment conducted by Otto Stern and Walther Gerlach that demonstrated space quantization of atomic angular momentum by sending a beam of neutral atoms through an inhomogeneous magnetic field. In the apparatus, a beam of silver atoms travels through a nonuniform magnetic field produced by a specially shaped magnet. The interaction between the atom’s magnetic moment and the field gradient exerts a force that deflects the atoms. Because the magnetic moment is quantized, the deflection occurs in discrete directions, yielding distinct spatial components.

For silver atoms, which have a single unpaired electron, the total angular momentum is effectively J = 1/2, so the magnetic moment can take two possible projections along the field, m_J = ±1/2. The experiment therefore produces two separate beams corresponding to the two spin states, providing direct evidence that angular momentum is quantized and that electron spin is a quantum degree of freedom. Although the term “spin” was not yet used at the time, the results strongly supported the concept of intrinsic angular momentum.

The Stern-Gerlach experiment had a lasting impact on quantum mechanics, reinforcing the probabilistic interpretation and prompting further studies of spin and measurement. It remains a foundational demonstration in quantum physics and has influenced modern spin-based technologies and educational demonstrations, with many variants and extensions exploring measurements in different directions and with various particles.