Electroweak Unified Theory
A single theoretical and mathematical framework covering electromagnetism and the weak interaction, despite their apparent differences: the former has an infinite range, while the effect of the latter does not range beyond the atomic nucleus. The apparent difference is understood as the consequence of the mass difference between the mediators : the photon is massless, whereas the W+, W- and Z0 bosons are heavy. The unification becomes visible only at high energies.