  The principle of linear superposition is a hallmark of quantum theory. It has
been confirmed experimentally for photons, electrons, neutrons, atoms, for
molecules having masses up to ten thousand amu, and also in collective states
such as SQUIDs and Bose-Einstein condensates. However, the principle does not
seem to hold for positions of large objects! Why for instance, a table is never
found to be in two places at the same time? One possible explanation for the
absence of macroscopic superpositions is that quantum theory is an
approximation to a stochastic nonlinear theory. This hypothesis may have its
fundamental origins in gravitational physics, and is being put to test by
modern ongoing experiments on matter wave interferometry.
