  Gravitational response of real objects is a fascinating topic. Einstein
formalized the Galileo-Newton ideas of equality of free falls into complete
physical equivalence or the Principle of Equivalence [Albert Einstein, The
meaning of Relativity, 5 ed. Princeton, (1921)]. However, in this article we
point out that in a gravitational field, g, the bulk response of an
electrically neutral but atomistic test mass is model dependant. Depending on
the particular quantum approximation scheme, opposing results for the gravity
induced (electric) polarization P have been reported. For instance, P is small
and oriented anti-parallel to g, if the deformations of the positive background
lattice is neglected But, it is about ~ 100,1000 times larger and opposite in
direction in the elastic lattice approximation. Hence, the elastic model
contradicts 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 consistent
with EP but the elastic system breaks EP. Here the historical literature is
surveyed and some implications are outlined.
