Dr Tony Phillips and Patrick L Barry in First Science:
Four hundred years ago – or so the story goes – Galileo Galilei started dropping things off the Leaning Tower of Pisa: Cannon balls, musket balls, gold, silver and wood. He might have expected the heavier objects to fall faster. Not so. They all hit the ground at the same time, and so he made a big discovery: gravity accelerates all objects at the same rate, regardless of their mass or composition.
Nowadays this is called “Universality of Free Fall” or the “Equivalence Principle,” and it is a cornerstone of modern physics. In particular, Einstein crafted his theory of gravity, i.e., the general theory of relativity, assuming the Equivalence Principle is true.
But what if it’s wrong?
“Some modern theories actually suggest that the acceleration of gravity does depend on the material composition of the object in a very subtle way,” says Jim Williams, a physicist at NASA’s Jet Propulsion Laboratory (JPL). If so, the theory of relativity would need re-writing; there would be a revolution in physics.
A group of NASA-supported researchers are going to test the Equivalence Principle by shooting laser beams at the Moon.