Classical unified field theories

Classical unified field theories sought to describe gravitation and electromagnetism within one field framework, before quantum theory. They modified spacetime geometry or introduced additional fields to combine the interactions.

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Kaluza–Klein theory: A theory that unifies gravity and electromagnetism by adding a compact fifth dimension to general relativity. Its extra dimension makes electromagnetic fields emerge from higher-dimensional geometry.
General relativity: Einstein's theory in which gravitation is described by the curvature of spacetime. Its geometric account of gravity inspired the search for a similarly geometric account of electromagnetism.
Quantum field theory: A framework that combines quantum mechanics with fields whose excitations behave as particles. It replaces the purely classical starting point of these unification programs.
Kaluza–Klein theory in modern physics: Modern uses of extra-dimensional Kaluza–Klein ideas in particle physics and theories beyond the Standard Model. Later theories reused its extra-dimensional mechanism after the original classical program faded.
Weyl's gauge theory: Hermann Weyl's 1918 theory that sought to unify gravity and electromagnetism through scale-changing geometry. It introduced a geometric connection intended to encode electromagnetism alongside gravity.
Maxwell's equations: The classical equations governing electric and magnetic fields and their sources. They provided the electromagnetic theory that unification proposals sought to incorporate.
Electroweak interaction: The unified quantum theory of electromagnetic and weak interactions. It achieved a different, experimentally successful kind of unification using quantum fields.
Gauge theory: A field theory whose descriptions have local transformation freedoms called gauge symmetries. Weyl's proposal helped introduce the gauge principle, later central to modern field theory.
Einstein–Maxwell theory: The coupled classical field equations for general relativity and electromagnetism. It supplied the gravitational and electromagnetic equations these proposals aimed to combine more deeply.
Hermann Weyl: A German mathematician and physicist who made foundational contributions to geometry, relativity, and quantum theory. His 1918 proposal was an early influential geometric unification attempt.