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In gravitational fields there are no such things as rigid bodies

with Euclidean properties; thus the fictitious rigid body of

reference is of no avail in the general theory of relativity. The

motion of clocks is also influenced by gravitational fields, and

in such a way that a physical definition of time which is made

directly with the aid of clocks has by no means the same degree

of plausibility as in the special theory of relativity.