Physicists measure the gravitational force between the smallest masses yet
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. The research effort born on that day has now produced its first result: a measurement of the gravitational force between two tiny gold spheres, each about the size of a sesame seed and weighing as much as four grains of rice-the smallest masses whose gravity has been measured to date. The central challenge facing Aspelmeyer's team was to design a detector exquisitely sensitive to this gravitational force yet totally insensitive to much larger background forces pushing and pulling on the test mass from all sides. 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. To measure the gravitational force of the source mass, the researchers did not simply place it near the test mass. The team compared these measurements to Newton's famous inverse square law of gravity, which describes how the gravitational force between two objects depends on their separation: the data were consistent with Newton's law to within 10 percent. Aspelmeyer's efforts to isolate gravitational forces between progressively smaller masses are a critical step toward such a definitive test.