[Physics] Why Is an Inhomogenous Magnetic Field Used in the Stern Gerlach Experiment

magnetic fieldsmagnetic-momentquantum mechanicsquantum-spin

The apparatus used in the Stern Gerlach experiment is somewhat like the one shown below with magnetic pole pieces of different shapes producing an inhomogeneous magnetic field.

But what would have happened if we had used symmetrical pole peices which produced a near-homogeneous magnetic field? Basically, why not use a homogeneous field?

Stern Gerlach Diagram

Best Answer

The simple answer is that the spins are magnetic dipoles, not monopoles. A monopole will feel a force in any field. But a dipole needs a spatially varying field, because otherwise each pole of the dipole would feel an equal and opposite force (and the net force would be zero).