Was Galileo Wrong? - Galileo Galilei

Using lasers to ping the Moon, researchers are testing a fundamental assumption of modern physics.

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.

"Lunar laser ranging is one of the most important tools we have for searching for flaws in Einstein's general theory of relativity," says Slava Turyshev, a research scientist at JPL who works with Jim Williams and others on the project.

Their experiment is possible because, more than 30 years ago, Apollo astronauts put mirrors on the Moon - small arrays of retro reflectors that can intercept laser beams from Earth and bounce them straight back. Using lasers and mirrors, researchers can "ping" the Moon and precisely monitor its motion around Earth.

more A sketch of Galileo Galilei's legendary experiment.

It's a modern version of the Leaning Tower of Pisa experiment. Instead of dropping balls to the ground, the researchers will watch the Earth and Moon drop toward the Sun. Like musket balls and cannon balls dropped from the Tower, the Earth and Moon are made of a different mix of elements, and they have different masses. Are they accelerated toward the Sun at the same rate? If yes, the Equivalence Principle holds. If not, let the revolution begin.