The Eöt-Wash torsion pendulum
In a torsion pendulum the gravitational force between the test masses, which are suspended on a fibre (the pendulum), and the attractor masses causes the fibre to twist by an amount that depends on the force. In the Eöt-Wash 21-fold symmetric torsion pendulum (left), the test masses are actually 21 holes (i.e. they are "missing" masses) in a ring of molybdenum, while the attractor masses are 21 holes in a separate ring beneath. Moreover, the attractor is rotated at a "drive" frequency, ω, so that the gravitational interaction between the masses produces a torque (the "signal") that varies in multiples of 21ω. This decoupling of the drive and signal frequencies greatly suppresses systematic effects associated with, for example, vibrations and magnetic fields. A 10 μm thick conducting shield stretched between the pendulum and attractor (not shown) prevents the pendulum from "seeing" the attractor, and essentially eliminates electrostatic and molecular effects. A laser reflected from a mirror on the pendulum is able to detect twists with a resolution of better than 1 nanoradian (the angular size of a pea in New York as viewed from London!). The torque is measured as a function of vertical separation between the pendulum and the attractor, and compared with the predictions of the inverse-square law. The instrument actually includes a third mass (also containing 21 holes) beneath the attractor that exactly cancels the Newtonian torque for one vertical separation of the other two masses. The location of this "zero-point" is highly sensitive to new physics because anything that modifies the inverse-square law will shift its position.