With the discrepancy solved between electromagnetism and Newton's theory, a new problem arose. Special relativity claimed that nothing can go faster than light, but according to Newton's universal theory of gravity, the effects of gravity are instantaneous. Newton's theory said that if the sun were to blow up, the earth would be flung out of orbit instantly, but we wouldn't see the light from the explosion until eight minutes afterward.

Newton's theory of gravity said the effect of gravity depends on two things: the mass of objects and the distance between them. Specifically, he found that the product of the masses of two bodies is directly proportional to the gravitational force between the bodies. Also, the square of the distance between the bodies is inversely proportional to the gravitational force between them. These finding were supported by a host of experimental evidence. The one problem with the theory was that it gave no explanation for why gravity exists or how it works.

In 1907, Einstein made a realization that would lead to the answer and the creation of general relativity. In special relativity, many relationships were established between things in relative motion at a constant velocity, but what about accelerated velocity? Think about when you are on a carnival ride that spins in circles. If you get to going fast enough, you get pushed toward the outside. Einstein became aware that this force you feel while in accelerated motion is undistinguishable from the force of gravity. This is called the equivalence principle. In special relativity, the principle of relativity stated that all observers have the right to claim that they are stationary as long as they are moving at a constant velocity. This principle was updated to the following: all observers, no matter their state of motion, have the right to claim that they are stationary and everything else is moving as long as they include a description of a gravitational field around them.