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But the observer on the disc may regard his disc as a

reference-body which is "at rest"; on the basis of the general

principle of relativity he is justified in doing this. The force

acting on himself, and in fact on all other bodies which are at

rest relative to the disc, he regards as the effect of a

gravitational field. Nevertheless, the space-distribution of this

gravitational field is of a kind that would not be possible on

Newton's theory of gravitation.[18] But since the observer

believes in the general theory of relativity, this does not

disturb him; he is quite in the right when he believes that a

general law of gravitation can be formulated-a law which not only

explains the motion of the stars correctly, but also the field of

force experienced by himself.

[18] The field disappears at the centre of the disc and increases

proportionally to the distance from the centre as we proceed

outwards.