Historical perspective on testing the Equivalence Principle

Few facts in science are more surprising and none has had a longer history than the apparent equivalence of the two kinds of mass in physics, gravitational and inertial. From Galileo and Newton to Eötvös and Einstein, it has been a compelling issue both theoretically and experimentally. Ground-based tests have now a precision of about 1 part in 10 ¹². Even with this extraordinary agreement, there are profound theoretical reasons for carrying the measurements further. Our generation has the unique oppurtunity to make an advance of a factor of a million in testing the Equivalence Principle in space.