If any word defines the 20th-century cultural and intellectual ethos, it is "relativity." The century began with Albert Einstein proposing theories that were understood both as showing that the cosmos can be deeply puzzling (a typical Einsteinian finding: moving faster makes your weight increase while your dimensions shrink, but you age more slowly) and that there is no "correct" view of physical reality. Taking off from this, thinkers have since proposed that everything is relative. Morality, art, literature, systems of government, faith--nothing is clearly good or bad, no one thing is true while other things are wrong. It's all relative. Many who maintain that relativity rules ground their assertions in Einstein. After all, didn't history's most brilliant scientist prove that nothing is definite?

Yet Einstein's theories do not say this, at least not in the way many people think. And certainly, Einstein's ideas of relativity have nothing to do with whether we should believe in God, as many people seem to think, assuming Einstein to have been the ultimate pure-science atheist. Einstein was a theist, believing that a creator is present but rejecting organized religion. He penned such thoughts as, "God reveals himself in the orderly harmony of what exists." Had Einstein chosen only slightly different words for his work, he might have sent the century an entirely different message.

In 1905, Einstein published his first big idea: the Special Theory of Relativity. This postulate held that physical laws are the same for all observers regardless of position and speed; that the speed of light is always the same from all points of view; and that matter and energy are equivalent (that's the E=MC2 part). For techno-reasons we can skip here, applying these three assertions overturned classical, or "Newtonian," physics, which assumed that the universe is simply a gigantic version of the same effects seen on Earth. Under the Special Theory of Relativity, for example, there is no fixed length for an object. To an observer standing on Earth, a 10-foot pole is 10 feet long; to an observer in a spacecraft at high velocity, it may appear five feet long. Neither is the "correct" dimension; both are equally valid relative to perspective. Under the Special Theory, time can "dilate," seeming to move at different rates from different perspectives. In unusual circumstances, even the order of events may become unclear. That is, two different observers might have opposite opinions on whether A came before B or B came before A.

Hearing such things expounded by the world of science, many thinkers jumped to the conclusion that ideas about truth or falsity had been disproved--if we can't even know if a 10-foot pole is 10 feet long, how can we know what ethics are right, or if there is a God? But read again the above paragraph. The first two points of the Special Theory of Relativity were that "physical laws are the same for all observers regardless of position and speed, and that the speed of light is always the same from all points of view." Einstein's theory says that physical laws and the speed of light are reliable absolutes. (His third point, the equivalence of matter and energy, is a pure-physics issue that has no bearing on how we perceive truth.) We do not live in a universe in which anything could happen because physical laws are not constant; we live in a universe in which the speed of light is always the same and physical laws are always consistent from all perspectives.

Clearly, Einstein believed that his physical laws that always applied from all perspectives could lead to mind-bending results, such as time dilation. But most of the mind-bending results Einstein foresaw had to do with "relativistic" velocities for matter or with extremely dense amounts of mass, conditions that are rare even at the cosmic scale. Under typical conditions, even typical conditions for stars and supernovas, the assumptions of the Special Theory help us understand why the universe is stable and comprehensible, rather than pandemonium. Considering his premises that the speed of light is always the same and physical laws are always reliable and predictable, Einstein might as easily have called his first work the Special Theory of the Cosmic Absolutes.

Then there is his second great achievement, the General Theory of Relativity, published in 1915. In this theory, Einstein struggled with the question of why the universe does not fly apart; he came to the conclusion that gravity orders the heavens by giving an unseen but powerful shape to space-time. (Collectively, Einstein's two hypotheses are spoken of as a single theory.) Parts of the General Theory of Relativity are spooky and hard to grasp. Einstein supposed, for example, that space appears flat to us but would appear curved from the perspective of someone outside the universe. In turn, he thought that gravity functions by bending local space-time, such that when an apple falls, what it's doing is rolling down a curvature we cannot perceive.