  Gravity is the weakest fundamental interaction and the only one that has not
been measured at the particle level. Traditional experimental methods, from
astronomical observations to torsion balances, use macroscopic masses to both
source and probe gravitational fields. Matter wave interferometers have used
neutrons, atoms and molecular clusters as microscopic test particles, but
initially probed the field sourced by the entire earth. Later, the
gravitational field arising from hundreds of kilograms of artificial source
masses was measured with atom interferometry. Miniaturizing the source mass and
moving it into the vacuum chamber could improve positioning accuracy, allow the
use of monocrystalline source masses for improved gravitational measurements,
and test new physics, such as beyond-standard-model ("fifth") forces of nature
and non-classical effects of gravity. In this work, we detect the gravitational
force between freely falling cesium atoms and an in-vacuum, centimeter-sized
source mass using atom interferometry with state-of-the-art sensitivity. The
ability to sense gravitational-strength coupling is conjectured to access a
natural lower bound for fundamental forces, thereby representing an important
milestone in searches for physics beyond the standard model. A local, in-vacuum
source mass is particularly sensitive to a wide class of interactions whose
effects would otherwise be suppressed beyond detectability in regions of high
matter density. For example, our measurement strengthens limits on a number of
cosmologically-motivated scalar field models, such as chameleon and symmetron
fields, by over two orders of magnitude and paves the way toward novel
measurements of Newton's gravitational constant G and the gravitational
Aharonov-Bohm effect
