Unified field theory classical field theory and, two years later, that of Theodor Kaluza, who extended General Relativity to five dimensions. Continuing in this latter direction, Oscar Klein proposed in 1926 that the fourth spatial dimension be curled up into a small, unobserved circle. In Kaluza–Klein theory, the gravitational curvature of the extra spatial direction behaves as an additional force similar to electromagnetism. These and other models of electromagnetism and gravity were pursued by Albert Einstein in his attempts at a classical unified field theory. By 1930 Einstein had already considered the Einstein–Maxwell–Dirac System [Dongen]. This system is (heuristically) the super-classical [Varadarajan] limit
Field (physics) of jet manifolds (covariant classical field theory). In modern physics, the most often studied fields are those that model the four fundamental forces which one day may lead to the Unified Field Theory. A convenient way of classifying a field (classical or quantum) is by the symmetries it possesses. Physical symmetries are usually of two types: Fields are often classified by their behaviour under transformations of spacetime. The terms used in this classification are: Fields may have internal symmetries in addition to spacetime symmetries. In many situations, one needs fields which are a list of space-time scalars: (φ, φ, ...