Since the introduction of the special principle of relativity has been justified, every intellect which strives after generalisation must feel the temptation to venture the step towards the general principle of relativity.  But a simple and apparently quite reliable consideration seems to suggest that, for the present at any rate, there is little hope of success in such an attempt; Let us imagine ourselves transferred to our old friend the railway carriage, which is travelling at a uniform rate.  As long as it is moving unifromly, the occupant of the carriage is not sensible of its motion, and it is for this reason that he can without reluctance interpret the facts of the case as indicating that the carriage is at rest, but the embankment in motion.  Moreover, according to the special principle of relativity, this interpretation is quite justified also from a physical point of view.

If the motion of the carriage is now changed into a non-uniform motion, as for instance by a powerful application of the brakes, then the occupant of the carriage experiences a correspondingly powerful jerk forwards.  The retarded motion is manifested in the mechanical behaviour of bodies relative to the person in the railway carriage.  The mechanical behaviour is different from that of the case previously considered, and for this reason it would appear to be impossible that the same mechanical laws hold relatively to the non-uniformly moving carriage, as hold with reference to the carriage when at rest or in uniform motion.  At all events it is clear that the Galileian law does not hold with respect to the non-uniformly moving carriage.  Because of this, we feel compelled at the present juncture to grant a kind of absolute physical reality to non-uniform motion, in opposition to the general principle of relatvity.  But in what follows we shall soon see that this conclusion cannot be maintained.

THE GRAVITATIONAL FIELD

“If we pick up a stone and then let it go, why does it fall to the ground ?” The usual answer to this question is:  “Because it is attracted by the earth.”  Modern physics formulates the answer rather differently for the following reason.  As a result of the more careful study of electromagnetic phenomena, we have come to regard action at a distance as a process impossible without the intervention of some intermediary medium.  If, for instance, a magnet attracts a piece of iron, we cannot be content to regard this as meaning that the magnet acts directly on the iron through the intermediate empty space, but we are constrained to imagine —­ after the manner of Faraday —­ that the magnet always calls into being something physically real in the space around it, that something being what we call a “magnetic field.”  In its turn this magnetic field operates on the piece of iron, so that the latter strives to move towards the magnet.  We shall not discuss here the justification for this incidental conception, which is indeed a somewhat arbitrary one.  We shall only mention that with its aid electromagnetic phenomena can be theoretically represented much more satisfactorily than without it, and this applies particularly to the transmission of electromagnetic waves.  The effects of gravitation also are regarded in an analogous manner.