  The accelerated expansion of the universe motivates a wide class of scalar
field theories that modify gravity on large scales. In regions where the weak
field limit of General Relativity has been confirmed by experiment, such
theories need a screening mechanism to suppress the new force. We have measured
the acceleration of an atom toward a macroscopic test mass inside a high vacuum
chamber, where the new force is unscreened in some theories. Our measurement,
made using atom interferometry, shows that the attraction between atoms and the
test mass does not differ appreciably from Newtonian gravity. This result
places stringent limits on the free parameters in chameleon and symmetron
theories of modified gravity.
