beam to return, scientists have been able to pinpoint the Earth-Moon distance within a fraction of an inch. Led by astrophysicist Tom Murphy of the University of California at San Diego and including Adelberger as one of the team members, researchers hope to use this method to check for subtle differences between the motions of the Earth and Moon in the Sun’s gravitational field. If such discrepancies are found, they could point to minuscule violations of the equivalence principle.

To test the actions of gravity on varying masses, we might wonder why scientists don’t just drop two objects and see if they land simultaneously—as, legend would have it, Galileo did from the Leaning Tower of Pisa. The free fall would need to take place in a total vacuum to prevent air resistance from skewing the results, so the project planned by the ESA (European Space Agency), called MICROSCOPE (MICROSatellite à traînée Compensée pour l’Observation du Principe d’Equivalence), is designed to do just that. Targeted for launch in 2008, it will reconstitute the leaning tower as a floating satellite, orbiting almost 700 miles above Earth. This vehicle will shield two cylinders, made of platinum and titanium, which will be released simultaneously and allowed to move freely inside. Because both masses will be subject to the same gravitational field, namely Earth’s, the equivalence principle predicts that they should follow identical orbits. Each time they deviate from their uniform paths an electrical field will steer them back into place. Therefore, by measuring the electrical signals required to keep both objects moving along the same trajectory, researchers will gain precise information about whether or not the equivalence principle is violated.

In case MICROSCOPE doesn’t constitute proof enough of Einstein’s conjecture, yet another mission is planned after 2011. Known as STEP (Satellite Test of the Equivalence Principle), it is a joint project of NASA and the ESA. Housed within an Earth-orbiting satellite, hollow test cylinders of various masses will be