In his general theory of relativity, Einstein reworked Newton's 250-year-old theory of gravity. In doing so, he started a revolution in the world of physics. Gravity didn't act on objects instantaneously, as Newton suggested; rather, Einstein said, it travels in waves at precisely the speed of light.

This piece of information -- gravity's speed -- suggested to Einstein the possibility of symmetry between gravity and electromagnetism, as well as a way to unify these two disparate forces.

But there was something Einstein wasn't able to reconcile: that gravity and electromagnetism are exceedingly different in strength. As physicist Brian Greene illustrates so spectacularly in this video segment, electromagnetism is actually billions and billions of times stronger than gravity. Until recently, it seemed that this difference in strength outweighed any similarities between the two forces. Today, some physicists believe that string theory has the power to explain the relationship among all forces and uncover the commonality between gravity and electromagnetism that Einstein couldn't see.