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 relativity. But in what follows we shall soon see
that this conclusion cannot be maintained.
XIX.
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
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