What are far fields?

The linearized theory of gravitation is based on the presumption that over whole regions of space, at any rate in the vicinity of the sources of the field, the gravitational field is weak, and the metric deviates only slightly from that of a Minkowski space. In nature we often meet a situation in which a distribution of matter (a satellite near the Earth, the Earth, the planetary system, our Galaxy) is surrounded by vacuum, and the closest matter is so far away that the gravitational field is weak in an intermediate region. In the neighbourhood of the sources, however, the field can be strong.

If such an intermediate region exists, and far away sources are not present or their influence can be neglected, then we speak of the far field of the configuration in question (Fig. 28.1). Notice that here, by contrast, for example, to most problems in electrodynamics, we may not always assume an isolated matter distribution which is surrounded only by a vacuum. The assumption of a void (the ‘infinite empty space’) into which waves pass and disappear contradicts the basic conception of General Relativity; also the fact that we orient our local inertial system towards the fixed stars indicates that we must always in principle take into account the existence of the whole Universe whenever we examine the properties of a part of the Universe.