Prior to 1905, space and time were comfortable absolutes. Over 250 years of practical experience and experimentation had firmly established the supremacy of the physics of Sir Isaac Newton. The picture of the universe painted by Newton was one of amazing clarity and practical value. The motions of projectiles, pendulums, steam engines, and even distant planets could be readily explained using the brilliant 17th century physicist's theories. However, certain phenomena, including how light was able to travel in a vacuum and the exact nature of gravity continued to elude satisfactory scientific explanation. While searching for a solution to the failure of the now famous Michelson-Morey experiment, Albert Einstein( who left Germany when the Nazis took power, and became an American citizen in 1940 ) discovered his Special Theory of Relativity. A few years later, he expanded his theory into an all-encompassing grand view of the universe. His General Theory of Relativity was the first to describe the nature of gravity. The General Theory describes a four dimensional universe in which the three spatial dimensions are coupled with a fourth, time. Any object in the universe with mass is described as causing a warp, or curve into the very structure of spacetime itself. Gravity is shown to be a result, not of some unforeseen, mysterious force, but as a function of the curvature of space itself. All matter, from the tiniest sub-atomic particle to the most massive of galaxies, will induce this curvature. This idea is frequently explained by describing space as a rubber sheet, and a body such as the sun as a bowling hall. If the hall is placed on the rubber sheet, the sheet will bend under the weight of the ball, forming a gravity well. Thus, the orbits of the planets can be seen to result from them 'rolling' around the mouth of the sun's gravity well. Of course, this analogy is but a shadow of the tree nature of space. In reality, this gravity well is a four dimensional structure. Shortly after publication of the General Theory, physicists began to explore this strange new world. What does the passage mainly discuss?