ABSTRACT
The spacetime in GR is a Riemannian four-dimensional continuum V 4. In accordance with the principle of equivalence, gravity is determined by the geometry of spacetime. Very soon after the discovery and experimental verification of GR, Kaluza (1921) proposed that the four-dimensional spacetime be supplemented with a fifth dimension, in order to give a unified account of the gravitational and electromagnetic interactions, the only two basic interactions known at the time. Although the physical effects of gravity and electromagnetism are seemingly very distinct (all particles are subject to gravity, but only charged particles are subject to electromagnetism), Kaluza showed that both can emerge as different manifestations of a five-dimensional GR. In this theory point particles move along geodesic lines in a five-dimensional Riemann spacetime V 5, and these trajectories are seen in V 4 as trajectories of particles subject to both gravitational and electromagnetic forces.
