What goes up, must come down; that's a simple statement for a very complex force that has been a puzzle for millennia.

The ancient Greeks had a simple explanation for gravity: the elements (earth, air, fire, water) always sought their natural place. Objects composed of Earth elements had no choice but to fall, with heavier objects falling faster than lighter ones. The Greeks had a profound influence on science, and their teachings went unchallenged for 1,500 years.

Danish physicist Isaac Beeckmann gave considerable thought to gravitation. In 1613 he suggested the principle of inertia and, five years later, combined that with his law of uniformly accelerating bodies in a vacuum. He determined the distance an object falls was related to the square of the amount of time it was falling. Galileo came up with a novel idea for the time: the use of experimentation to verify hypotheses. Legend has it...