Atoms

A WEALTH OF KNOWLEDGE, theoretical and experimental, is encompassed in the ever expanding field of atomic study, one of the oldest branches of modern physics. Quantum electrodynamics, one of the most complete and best verified modern physical theories, has been developed largely through studies in atomic physics and has sustained all experimental challenges of the past 20 years, including precise measurements of the Lamb shift, electron and muon gyromagnetic ratios, and of positronium and muonium hyperfine structure. Yet major questions remain unsolved, such as the divergences in the theory, the calculation of the value of the fine‐structure constant and the relation of the muon and the electron—questions that must be formulated in the broader framework of elementary‐particle physics. Invariance and symmetry principles play an important role in atomic physics; space reflection, time reversal and charge‐conjugation invariances can be tested for the electromagnetic interactions. Atomic structure, atomic collisions, muonic and mesic atoms, quantum electronics and applications in astrophysics and other fields of science are parts of the frontier of modern atomic physics.