[2] I shall define “interval” in a moment.
[3] So long as he has no considerable acceleration. The treatment of acceleration belongs to the general theory of relativity.
[4] This does not mean that its velocity is increasing, but that it is changing its direction. The only sort of motion which is called “unaccelerated” is motion with uniform velocity in a straight line.
[5] See his Space, Time, Matter, Methuen, 1922.
[6] Although “force” is no longer to be regarded as one of the fundamental concepts of dynamics, but only as a convenient way of speaking, it can still be employed, like “sunrise” and “sunset,” provided we realize what we mean. Often it would require very roundabout expressions to avoid the term “force.”
[7] See Eddington, The Mathematical Theory of Relativity, Cambridge University Press, 2d edition, p. 128.
[8] This is subject to the explanations given below as regards conservation of energy.
[9] Mathematical Theory of Relativity, p. 135.
[10] On this subject, see the present author’s A.B.C. of Atoms, chaps. VI and XIII.
[11] Op. cit. § 60.