Number of words: 531

Relativity theory has made the structure of electrodynamics much more elegant by unifying the concepts of both charges and currents and electric and magnetic fields. Since all motion is relative, every charge can also appear as a current-in a frame of reference where it moves with respect to the observer -and consequently, its electric field can also appear as a magnetic field. In the relativistic formulation of electrodynamics, the two fields are thus unified into a single electromagnetic field.

The concept of a field has been associated not only with the electromagnetic force, but also with that other major force in the large-scale world, the force of gravity. Gravitational fields are created and felt by all massive bodies, and the resulting forces are always forces of attraction, contrary to the electromagnetic fields which are felt only by charged bodies and which give rise to attractive and repulsive forces. The proper field theory for the gravitational field is the general theory of relativity, and in this theory the influence of a massive body on the surrounding space is more far-reaching than the corresponding influence of a charged body in electrodynamics. Again, the space around the object is ‘conditioned’ in such a way that another object will feel a force, but this time the conditioning affects the geometry, and thus the very structure of space. 

Matter and empty space-the full and the void-were the two fundamentally distinct concepts on which the atomism of Democritus and of Newton was based. In general relativity, these two concepts can no longer be separated. Wherever there is a massive body, there will also be a gravitational field, and this field will manifest itself as the curvature of the space surrounding that body. We must not think, however, that the field fills the space and ‘curves’ it. The two cannot be distinguished; the field is the curved space! In general relativity, the gravitational field and the structure, or geometry, of space are identical. They are represented in Einstein’s field equations by one and the same mathematical quantity. In Einstein’s theory, then, matter cannot be separated from its field of gravity, and the field of gravity cannot be separated from the curved space. Matter and space are thus seen to be inseparable and interdependent parts of a single whole.

Material objects not only determine the structure of the surrounding space but are, in turn, influenced by their environment in an essential way. According to the physicist and philosopher Ernst Mach, the inertia of a material object-the object’s resistance against being accelerated-is not an intrinsic property of matter, but a measure of its interaction with all the rest of the universe. In Mach’s view, matter only has inertia because there is other matter in the universe. When a body rotates, its inertia produces centrifugal forces (used, for example, in a spin-drier to extract water from wet laundry), but these forces appear only because the body rotates ‘relative to the fixed stars’, as Mach has put it. If those fixed stars were suddenly to disappear, the inertia and the centrifugal forces of the rotating body would disappear with them.

Excerpted from Pages 208-209 of ‘The Tao of Physics’ by Fritjof Capra