Everyone knows what gravity is. A baby at three months will express surprise if a box does not topple as expected; a one-year-old knows whether a precarious object will fall or not depending on its shape. Scientists came to think of gravity as a pull to Earth and later, in a more generalized way, as a force of attraction between any two masses.

Then came Albert Einstein. In 1915 he revealed in his general theory of relativity that gravity is not a force so much as the by-product of a curving universe. In other words, what we think we know about gravity from everyday experience is wrong.

The publication of “Die Feldgleichungen der Gravitation” (“The Field Equations of Gravitation”) on December 2, 1915, at first got little notice beyond academe. A few years later a solar eclipse expedition led by Sir Arthur Eddington made an observation that vaulted the theory to fame overnight. Starlight, as Einstein predicted, appeared to bend twice as much as Newtonian physics predicted as it passed the sun, and Eddington confirmed this amount of bending firsthand.* The New York Times famously declared: “Men of Science More or Less Agog over Results of Eclipse Observations.”

They were right to be agog. It would be hard to overstate how disruptive the idea of general relativity was a century ago to prevailing notions of the universe and our physical world. All of a sudden, space and time were no longer a mere backdrop to the real action of the cosmos. Spacetime, rather, had its own geometry, and its curvature dictated the movements of the heavenly bodies and kept our feet planted firmly on the ground. Even light, the theory suggested, had to follow its contours.

The relativity revolution went on to shape much of the 20th century. It influenced philosophy, art, politics and pop culture. Its inventor's name became synonymous with genius and inaugurated Einstein as the world's greatest scientific celebrity. He used his stature to play a major role in global events—he famously advocated for the development of the atomic bomb and then spent decades bemoaning the mistake. He lobbied for the protection of the Jewish people and was an outspoken critic of racism and an activist for civil rights. Even more, the fame surrounding Einstein and his great idea marked a turning point in the public perception of science, establishing the 20th century as the scientific age and ushering in a technological transformation that we are still living through.

The 100th anniversary of general relativity provides an opportunity to survey the incredible pace of science and its effect on society. In the following pages we look back at what we learned from Einstein's achievement and forward to the secrets it may yet reveal. An illuminating graphic shows the multitude of new fields of research the theory has spawned. We examine the first moment of inspiration that led the genius on the path to relativity and celebrate his ability to expose such truths through the power of pure thought. Even Einstein's mistakes often proved fruitful, and we see that what is commonly thought to be one of his greatest faults—his perceived intolerance of quantum mechanics—is misunderstood. And we explore our obsession with genius by investigating the misguided attempts to locate the source of Einstein's brilliance in his brain anatomy.

The passage of 100 years of general relativity is also important because of what the idea still has not done: unite with the other forces of nature to build a unified theory of everything. Einstein spent his last years questing for a deeper set of rules that would reign not just over the realm of the cosmos—general relativity's domain—but also the world inside the atom, where quantum mechanics rules. He thought this dream was in reach, but a century of toil by generations of physicists has not accomplished a single theory of nature. Relativity and quantum mechanics are just as incompatible as they ever were.

Lately scientists have begun to take a new tack, probing some of the mysteries of the universe that have popped up since Einstein's age, such as dark matter and dark energy, in hopes that these paths will eventually lead to the realization of Einstein's dream. Other researchers are attempting to poke holes in general relativity by testing it in the extreme realm of black holes. And one of relativity's weirdest consequences—the possibilities it introduces for time travel—may also offer an avenue for uncovering deeper secrets of nature.

Ultimately it is clear that no other scientific theory has been more important in shaping the course of 20th-century physics, and no other scientist's legacy looms larger over the 21st century than Einstein's. At this milestone anniversary, physics is waiting for the next general relativity. We could use another Einstein.

*Editor's Note (11/4/15): This sentence from the print article was edited after posting. The original erroneously stated that Einstein predicted light would bend, rather than how much it would bend, and that Eddington had confirmed the former phenomenon rather than the latter.