  The apparent conflict between general relativity and quantum mechanics
remains one of the unresolved mysteries of the physical world. According to
recent theories, this conflict results in gravity-induced quantum state
reduction of "Schr\"odinger cats", quantum superpositions of macroscopic
observables. In recent years, great progress has been made in cooling
micromechanical resonators towards their quantum mechanical ground state. This
work is an important step towards the creation of Schr\"odinger cats in the
laboratory, and the study of their destruction by decoherence. A direct test of
the gravity-induced state reduction scenario may therefore be within reach.
However, a recent analysis shows that for all systems reported to date, quantum
superpositions are destroyed by environmental decoherence long before
gravitational state reduction takes effect. Here we report optical trapping of
glass microspheres in vacuum with high oscillation frequencies, and cooling of
the center-of-mass motion from room temperature to a minimum temperature of 1.5
mK. This new system eliminates the physical contact inherent to clamped
cantilevers, and can allow ground-state cooling from room temperature. After
cooling, the optical trap can be switched off, allowing a microsphere to
undergo free-fall in vacuum. During free-fall, light scattering and other
sources of environmental decoherence are absent, so this system is ideal for
studying gravitational state reduction. A cooled optically trapped object in
vacuum can also be used to search for non-Newtonian gravity forces at small
scales, measure the impact of a single air molecule, and even produce
Schr\"odinger cats of living organisms.
