This award supports preliminary research on a new method to test Newton's law of gravity at length scales shorter than 100 micrometers. Recent theoretical suggestions including unification scenarios, extra dimensions, and cosmological considerations involving dark energy all involve the possibility of a breakdown of Newtonian gravity for short ranges. Previously published results by the PI and others described upper limits to such a breakdown at around 100 micrometers using a torsion pendulum with an array of holes. The new method uses a different true null-experiment with a simple, yet highly efficient geometry designed to detect violations of Newtonian gravity at much shorter distances. A vertical plate torsion pendulum will hang parallel to an attractor plate which generates a uniform gravitational field. As this plate is moved toward and away from the pendulum, the forces on the pendulum will not change if Newtonian gravity holds. To maximize the pendulum's sensitivity to violations of Newtonian gravity, it is made from a plate with a step in its density profile, so that one half of the plate's mass is closer to the attractor than the other. To avoid forces due to electrostatics and residual gas, a thin metallic membrane is placed in the space between the attractor and the pendulum. This experiment is an alternative approach to test using existing torsion balances, with entirely different systematic uncertainties and the potential to test gravity at length scales as small as 20 micrometers. This award will support graduate students to build a scientific instrument and conduct a very challenging measurement, providing excellent training in all the aspects of experimental physics and scientific research. The new method involves many technical innovations that may find applications in other areas such as gravity-wave detection.
