© Blayne Heckel.
Researchers at the University of Washington used a torsion pendulum with 42 holes to measure the gravitational interaction between the missing mass of the holes with the missing mass in a disk just below the pendulum. The lower attractor disk rotated at a slow rate. As it did so, the gravitational tug between the pendulum and the attractor holes caused the pendulum to twist. By comparing the observed pendulum twist to the prediction from Newton's law of universal gravitation, Newton's law was shown to be valid at distances down to 5 x 10-5 m. (Unit: 3)