# Kaluza-Klein theory

Kaluza-Klein theory is a model that seeks to unite classical gravity and electromagnetism by resorting
to higher dimensions. In 1919
the German mathematician Theodor Kaluza (1885–1954) pointed out that
if general relativity theory is extended
to a five-dimensional spacetime, the equations
can be separated out into ordinary four-dimensional gravitation plus an
extra set, which is equivalent to Maxwell's
equations for the electromagnetic field, plus an extra field known as
the **dilaton**. Thus electromagnetism is explained as a manifestation
of curvature in a fourth
dimension of physical space, in the same
way that gravitation is explained in Einstein's theory as a manifestation
of curvature in the first three.

In 1926 the Swedish physicist Oskar Klein (1894–1977) proposed that the reason the extra spatial dimension goes unseen is that it is compact – curled up like a ball with a fantastically small radius. In the 1980s and 1990s, Kaluza-Klein theory experienced a big revival and can now be seen as a precursor of string theory.