"It was not until 1920 that the idea of linking electromagnetism and
gravity resurfaced. At that time a new theory of gravitation had been proposed by Albert Einstein (1879-1955), called the general theory of relativity. It was a replacement of Newton's theory, which had stood unchallenged since 1687. Inspired by Einstein's work, a young German mathematician named Theodore Kaluza was seized by a curious idea. The theory of relativity links space an time together to form a four-dimensional space-time continuum. What would happen, mused Kaluza, if general relativity were formulated in five rather than four dimensions? This is what Kaluza did, and to everyone's astonishment it was discovered that five-dimensional gravity obeys the same laws as
four-dimensional gravity as well as Maxwell's laws for the electromagnetic field. In other words, gravitation and electromagnetism are automatically unified in five dimensions, where electromagnetism is merely a component of gravity!"
The only drawback of the theory concerns the extra dimension. Why
don't we see it? An ingenious answer was provided by Oskar Klein. A
hosepipe viewed from afar looks like a wiggly line, i.e. one- dimensional.
However, on closer inspection it can be seen as a narrow tube. It is, in fact,
two-dimensional, and what was taken to be a point on the line is actually a
little circle going around the tube. In the same way, reasoned Klein, what we normally regard as a point in three dimensional space could in reality be a little circle going around a fourth space dimension. Thus Kaluza's extra
dimension might well exist, but be impossible to detect because it is closed
(circular) and rolled up to a very small circumference. In spite of
these bizarre overtones, it seems probable that in future a "theory of everything" will make use of the idea of unseen higher dimensions."
....
"Although nature manifests four distinct forces, physicists believe that
each may be part of a smaller number of more primitive forces. At high energy, the electromagnetic and weak forces appear to merge into a single "electroweak" force. Some "grand unified theories" suggest that a further amalgamation takes place between the electroweak and strong forces at as yet unattained energies. The most ambitious unification schemes envisage an amalgamation of all four forces into a single "superforce" at ultra-high levels of energy."...
"The real burden in the next three centuries will not be the development of fancy mathematics, but the experimental testing of these ambitious theories. All current thinking about total unification assumes that the effects of linking all the forces and particles together will only become manifest at energies that are some trillion times greater than those currently attainable in particle accelerators.Probably we shall never reach such energies directly" ( A Theory of Everything" Volume 21 of "The World ofScience)