Max Planck

“ Born limits the definition of rigidity to a single electron. Against this we surely cannot object from the outset, but whether it brings us physically further than the general principles of relativity theory, might well escape each test for the time being. ”

“ A body that has an "independent" volume, i.e. independent of external influences, does not exist in all of nature, and even less in relativity theory. At least I think, that the attempt to make the abstraction of the rigid body (which is so important for ordinary mechanics) also useful for the theory of relativity, does not promise any real success [2] .
The task, to determine the deformation of a somehow accelerated body is therefore essentially a problem of elasticity, in relativity theory and in ordinary mechanics. ”

“ As long nothing is known about these influences, as long nothing can be said about the contraction of the volume. Most directly, this can be seen at slightly compressible bodies, as in gases, but it applies equally well to solid bodies. ”