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Gravitational response of real objects is a fascinating topic. Einsteinformalized the Galileo-Newton ideas of equality of free falls into completephysical equivalence or the Principle of Equivalence [Albert Einstein, Themeaning of Relativity, 5 ed. Princeton, (1921)]. However, in this article wepoint out that in a gravitational field, g, the bulk response of anelectrically neutral but atomistic test mass is model dependant. Depending onthe particular quantum approximation scheme, opposing results for the gravityinduced (electric) polarization P have been reported. For instance, P is smalland oriented anti-parallel to g, if the deformations of the positive backgroundlattice is neglected But, it is about ~ 100,1000 times larger and opposite indirection in the elastic lattice approximation. Hence, the elastic modelcontradicts reports of polarization in accelerated metals [Richard C. Tolman &T.Dale Stewart, Phys Rev 28, 794 (1926); G. F. Moorhead & G. I. Opat, Class.Quant. Grav, 13, 3129 (1996)]. Surprisingly, the rigid system is consistentwith EP but the elastic system breaks EP. Here the historical literature issurveyed and some implications are outlined.

Do Quantum Systems Break The Equivalence Principle?