Electroweak Theory

The electroweak theory is the unified quantum field theory of the electromagnetic and weak nuclear forces, one of the pillars of the Standard Model of particle physics. Developed by Sheldon Glashow, Abdus Salam, and Steven Weinberg in the 1960s, it describes how at high energies the electromagnetic force and the weak force are manifestations of a single electroweak interaction governed by the gauge group SU(2) × U(1). At low energies, this symmetry is spontaneously broken by the Higgs mechanism, splitting the unified force into the long-range electromagnetic force (mediated by the massless photon) and the short-range weak force (mediated by the massive W and Z bosons).

The electroweak theory is not merely a unification but a demonstration of how effective field theory operates across scales. Above the electroweak symmetry-breaking scale (~246 GeV), the full SU(2) × U(1) symmetry is manifest, and the theory contains four massless gauge bosons. Below this scale, the appropriate description is no longer the full electroweak theory but Fermi's four-fermion theory of weak interactions — an even lower-energy EFT in which the W and Z bosons have been integrated out entirely. The electroweak theory thus sits in the middle of a hierarchy of effective descriptions, each valid in its own energy window.

The theory predicted the existence and masses of the W and Z bosons before their discovery at CERN in 1983, and it correctly predicts the weak mixing angle that relates the strengths of the electromagnetic and weak couplings. Its success is often cited as the paradigm case of gauge symmetry as a predictive engine in physics. But the electroweak scale also marks the boundary where the Standard Model as an effective field theory becomes most suspect: the hierarchy problem — the enormous separation between the electroweak scale and the Planck scale — is the EFT's own diagnostic telling us that something is missing.

The electroweak unification is frequently celebrated as a triumph of theoretical physics. Less frequently noted is that it is also a confession: the universe does not present itself in unified form. It must be decoded. The gauge symmetry is not a feature of nature at all energies; it is a feature of nature at energies we have learned to access.