spin

History

The idea of spin arose in the 1920s, when Otto Stern and Walther Gerlach discovered the splitting of a beam of silver atoms in a magnetic field. Later, Wolfgang Pauli and Paul Dirac developed its theoretical description.

How it works

Spin manifests in magnetic properties: for example, in permanent magnets, electrons align their spins in one direction, creating a net magnetic field. It's like many tiny magnets oriented the same way.

History

The idea of spin arose in the 1920s, when Otto Stern and Walther Gerlach discovered the splitting of a beam of silver atoms in a magnetic field. Later, Wolfgang Pauli and Paul Dirac developed its theoretical description.

How it works

Spin manifests in magnetic properties: for example, in permanent magnets, electrons align their spins in one direction, creating a net magnetic field. It's like many tiny magnets oriented the same way.

History

The experimental discovery of spin is linked to the Stern–Gerlach experiment (1922), where a beam of silver atoms split into two components in an inhomogeneous magnetic field, contradicting classical physics. The theoretical explanation was given by George Uhlenbeck and Samuel Goudsmit in 1925, proposing an electron’s own momentum. Later, Pauli introduced spin matrices, and Dirac in 1928 naturally incorporated spin into the relativistic equation for the electron. Subsequently, the concept of spin became key for all quantum field theory and particle physics.

How it works

Spin is described by operators obeying the commutation relations [Ŝ_i, Ŝ_j] = iħ ε_{ijk} Ŝ_k. For spin-1/2 particles, the Pauli representation with matrices σ_x, σ_y, σ_z is often used. The Hamiltonian for interaction with a magnetic field: H = –μ·B, where the magnetic moment μ = g (e/2m) S. Quantum spin correlations (entanglement) are tested in Bell's inequalities, demonstrating the nonlocality of quantum mechanics. Spin is also responsible for the fine structure of spectra and plays a central role in spin electronics (spintronics).

Open problems & fun facts

Nature of spin: is spin a fundamental or composite property?; Problem of quantum gravity: how to describe spin in curved spacetime?; Spin measurement: how does the wavefunction collapse occur when measuring a projection?

💡 Thanks to spin, magnetic resonance imaging (MRI) works: hydrogen nuclei in our body have spin and, placed in a magnetic field, emit signals that the scanner reads.