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“In gravitational fields there are no such thing as rigid bodies with Euclidean properties; thus the fictitious rigid body of reference is of no avail in the general theory of relativity. ...
For this reason non-rigid reference-bodies are used, which are, as a whole, not only moving in any way whatsoever, but which also suffer alterations in form ad lib. during their motion...
This non-rigid reference-body, ... might appropriately be termed a "reference mollusc,"...”