SternGerlachExperiment

The Stern-Gerlach experiment, conducted by Otto Stern and Walther Gerlach in 1922, demonstrated the quantization of angular momentum for atomic particles. In their setup a beam of neutral silver atoms passed through a nonuniform magnetic field produced by a pair of magnets with a strong field gradient. The resulting force, proportional to the magnetic moment and the field gradient, caused the beam to deflect. Rather than forming a single continuous spot, the beam split into two distinct components on a detector screen, revealing discrete orientations of angular momentum along the field axis.

Interpretation of the results showed that atomic angular momentum along the measurement axis is quantized. For silver atoms, the ground-state orbital angular momentum is effectively zero, so the observed splitting reflects the two possible spin states of the unpaired electron: spin up and spin down along the field direction. The experiment thus provided direct evidence of spin as a quantum property and supported the quantum description of angular momentum with discrete eigenvalues.

The Stern-Gerlach experiment is regarded as a foundational demonstration of space quantization and stimulated the development of quantum mechanics, including the concept of measurement and the role of spin in atomic structure. It also introduced the idea that a physical system can be prepared in definite eigenstates of a chosen observable and later manipulated by sequential SG apparatus to explore spin dynamics.

Over the years, SG-type experiments have been performed with different atomic and molecular species, neutrons, and engineered spin systems, and continue to illustrate fundamental aspects of quantum measurement and coherence.