General Relativity is hailed by many as one of the greatest achievements of human thought of all time. Einstein's theory of space, time, and gravity threw out the old Newtonian stage of a fixed Euclidean space with a universal march of time; the new stage on which events play out is spacetime, a bendable and dynamic fabric which tells matter how to move. This theory perhaps holds the key to unlocking H. G. Wells time machine into the past; according to Kip Throne, it will pave the way towards the next generation of ultra-powerful telescopes which rely on gravitational waves; and it also perhaps holds the key to breaking the cosmic speed limit and colonizing the Milky Way galaxy and beyond in a comparatively short period of time.

When Einstein first realized that someone falling in an elevator near Earth's surface would experience all the same effects as another person riding in a rocket ship accelerating at 9.8 meters per second, he described it as "the happiest thought of his life." He realized that all the laws of physics and any physical experiment done in either reference frame would be identical and completely indistinguishable. This is because the effects of gravity in a constant gravitational field are identical to the effects of constant acceleration. This lead Einstein to postulate that gravity and acceleration are equivalent. Analogous to how all the physical consequences of special relativity could be derived from the postulation of the constancy of light speed and the sameness of physical laws in all inertial reference frames, all of the physical consequences of general relativity are derived from the postulate that acceleration and gravity are equivalent and that the laws of physics are the same in all reference frames. The former has been called the Einstein Equivalence Principle. There are various different forms that this statement can take, but in this lesson we shall describe the strong version of the Einstein Equivalence Principle.