The Equivalence Principle
May 18, 2007: Standing on the Moon in 1971, Apollo 15 astronaut Dave Scott held his hands out at shoulder height, a hammer in one hand and a feather in the other. And as the world looked on via live television, he let go.
It was an odd sight: the feather didn't drift to the ground, it plummeted, falling just as fast as the hammer. Without air resistance to slow the feather, the two objects hit moondust at the same instant.
"What do you know!" exclaimed Scott. "Mr. Galileo was right."
Scott was referring to a famous experiment of the 16th century. Depending on who tells the story, Galileo Galilei either dropped balls from the top of the Leaning Tower of Pisa or he rolled balls down slopes at home. Either way, the result was the same: Although the balls were made of different materials, they all reached bottom at the same time.
Today, this is known as "the equivalence principle." Gravity accelerates all objects equally regardless of their masses or the materials from which they are made. It's a cornerstone of modern physics.
But what if the equivalence principle (EP) is wrong?
Galileo's experiments were only accurate to about 1%, leaving room for doubt, and skeptical physicists have been "testing EP" ever since. The best modern limits, based on, e.g., laser ranging of the Moon to measure how fast it falls around Earth, show that EP holds within a few parts in a trillion (1012). This is fantastically accurate, yet the possibility remains that the equivalence principle could fail at some more subtle level.
"It's a possibility we must investigate," says physicist Clifford Will of Washington University in St. Louis, Missouri. "Discovering even the slightest difference in how gravity acts on objects of different materials would have enormous implications."
In fact, it could provide the first real evidence for string theory. String theory elegantly explains fundamental particles as different vibrations of infinitesimal strings, and in doing so solves many lingering problems of modern physics. But string theory is highly controversial, in part because most of its predictions are virtually impossible to verify with experiments. If it's not testable, it's not science.
Go to source: NASA - The Equivalence Principle