Thus far we have considered the gravitational field, or space-time curvature, produced by matter, but it should be apparent that owing to the identification of mass and energy, space-time curvature is a necessary consequence of the presence of energy. The electromagnetic field should thus be susceptible of giving rise to a gravitational field. Briefly, we may present the problem as follows:
Minkowski and Einstein had succeeded in establishing that electric and magnetic forces were the components of the same tensor
. From this tensor it was possible to build up another tensor
, representing the energy and characteristics of the electromagnetic field, just as
represented the energy and characteristics of matter. These two types of energy, which are not identical, are designated as loose energy and bound energy, respectively. If, now, we wish to discover the precise type of space-time curvature, or gravitational field, which the presence of an electromagnetic field would produce, all we have to do is to replace
by