measured field of force the gravitational field, generalising the expression. This field is not absolute, but always requires that some observer should be specified.
It may avoid some mystification if we state at once that there are certain intricacies in the gravitational influence radiating from heavy matter which are distinctive. A theory which did not admit this would run counter to common sense. What our argument has shown is that the characteristic symptom in a region in the neighbourhood of matter is not the field of force; it must be something more intricate. In due course we shall have to explain the nature of this more complex effect of matter on the condition of the world.
Our previous rule, that the observer perceives an artificial field of force when he deviates from a straight track, must now be superseded. We need rather a rule determining when he perceives a field of force of any kind. Indeed the original rule has become meaningless, because a straight track is no longer an absolute conception. Uniform motion in a straight line is not the same for an observer rotating with the earth as for a non-rotating observer who takes into account the sinuosity of the rotation. We have decided that these two observers are on the same footing and their judgments merit the same respect. A straight-line in space-time is accordingly not an absolute conception, but is only defined relative to some observer.
Now we have seen that so long as the observer and his measuring-appliances are unconstrained (falling freely) the field of force immediately round him vanishes. It is only when he is deflected from his proper track that he finds himself in the midst of a field of force. Leaving on one side the question of the motion of electrically charged bodies, which must be reserved for more profound treatment, the observer can only leave his proper track if he is being disturbed by material impacts, e.g. the molecules of the ground bombarding the soles of his boots. We may say then that a body does not leave its natural track without visible cause; and any field of force round an observer is the result of his leaving his natural track by such cause. There is nothing mysterious about this field of force; it is merely the reflection in the phenomena of the observer's disturbance; just as the flight of the houses and hedgerows past our railway-carriage is the reflection of our motion with the train.
