Pagina 120 di 212

Thus on our circular disc, or, to make the case more general, in

every gravitational field, a clock will go more quickly or less

quickly, according to the position in which the clock is situated

(at rest). For this reason it is not possible to obtain a

reasonable definition of time with the aid of clocks which are

arranged at rest with respect to the body of reference. A similar

difficulty presents itself when we attempt to apply our earlier

definition of simultaneity in such a case, but I do not wish to

go any farther into this question.