  Gunnar Nordstrom constructed the first relativistic theory of gravitation
formulated in terms of interactions with a scalar field. It was an important
precursor to Einstein's general theory of relativity a couple of years later.
He was also the first to introduce an extra dimension to our spacetime so that
gravitation would be just an aspect of electromagnetic interactions in five
dimensions. His scalar theory and generalizations thereof are here presented in
a bit more modern setting. Extra dimensions are of great interest in physics
today and can give scalar gravitational interactions with similar properties as
in Nordstrom's original theory.
