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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.