Ever since the work of Isaac Newton in 1687, physicists have understood gravity to be universal: every object exerts a gravitational force proportional to its mass on everything around it. “My resolution was ‘Okay, I am going to not only measure the gravitational field of this chair, but we are going to go small, small, small!’” he recalls. A force on the test mass causes the torsion pendulum to rotate until it is balanced by a restoring force from the twisting of the fiber. “[But] this was not true of Friday or Saturday.”To measure the gravitational force of the source mass, the researchers did not simply place it near the test mass. Aspelmeyer’s efforts to isolate gravitational forces between progressively smaller masses are a critical step toward such a definitive test.