Everything's relative. Speed, mass, space and time are all subjective. Nor are age, motion or the wanderings of the planets measures that humans can agree on anymore; they can be judged only by the whim of the observer. Light has weight. Space has curves. And coiled within a pound of matter, any matter, is the explosive power of 14 million tons of TNT. We know all this, we are set adrift in this way at the end of the 20th century, because of Albert Einstein.
We tend not to blame Einstein for the bomb, any more than we blame Nobel for dynamite. It wasn't the gentle theorist but the generals of the world who forged e=mc2 into the most terrible dagger in human history, and hoisted that Damoclean blade irretrievably over our heads in 1946. By then, the world had already iconized him: the greatest seer since Newton; science's poetic soul. Genius, in person. In a few thunderclaps of elegance he contained our world and the cosmos in the same equation, and changed forever the way the rest of saw the heavens and ourselves.

The light came on in 1905. Pushed to the fringe of physics by his prickly pacifism and an academic career that seemed designed to annoy his professors, the future emblem of genius was, at the time — the very words have become an Algeresque cliché — just a Swiss patent clerk. Preternaturally confident and suitably unkempt, the 26-year-old Einstein sent three papers, papers scrawled in his spare time, to the premier journal, "Annalen der Physik," to be published "if there is room." They all made the same issue, and they did exactly what he imagined they would: change the world. One was an update of Max Planck's quantum theory of radiation; light, declared Einstein, travels as both a wave and as particles called quanta, mostly because it has to. Another concerned Brownian motion, an until-then unexplained phenomenon involving bouncing molecules. (The patent clerk explained it.) The third, wrote Einstein matter-of-factly in a letter to a friend, "modifies the theory of space and time." Its import: Everything's relative. He could have retired right then and still been the savior of science in the 20th century.

Physics is built on the basic and rather wistful hypothesis that Mother Nature doesn't know much math. Remainders and constants are men's crumbs, not hers -- to a theoretical physicist, the Ten Commandments are too numerous by nine. By 1905, Newton's three were showing cracks under the scrutiny of stronger telescopes and better astronomy; the ether, an omnipresent invisible jello, was supposed to spackle Newton's world smooth again. To Einstein, the ether was just a remainder, and he got rid of it. Nothing can move faster than light, he said, and matter and energy are equivalent: E=mc2. The physicist Louis de Broglie called Einstein's contributions that year "blazing rockets which in the dark of the night suddenly cast a brief but powerful illumination over an immense unknown region." The new view was breathtaking.

Einstein himself, though, would remain in that unknown a while longer. In 1916, he folded special relativity into general relativity: Light had mass, and space and time were simply space-time. Oh, and the universe was quite possibly shaped like a saddle. World-shaking stuff. But war, seemingly Einstein's constant companion, obscured him three more years until British astronomer Arthur Eddington got out and proved it during a solar eclipse: He spotted a star that should have been hidden behind the sun. Light had turned a corner, and so had we. No one really understood what Einstein was talking about — which is only a slight exaggeration even today — but it sure sounded great. Order in the cosmos, even if only one man could see it, was an appetizingly lofty prospect after the all-too-earthbound carnage of World War I. And this fellow Einstein, with his halo of unruly hair and Labrador eyes, was just the gentle genius we were looking for.
Celebrity annoyed Einstein — he would once list his occupation as "artist's model" — but while his theory made the rounds at cocktail parties, the physicist himself discovered that Americans wanted desperately to hear what else he had to say. So he spoke up. His gave speeches and met with heads of state, made enemies of Hitler (and later McCarthy) with his ardent blend of pacifism, Zionism and Communism. (Eventually the sound of his voice got too loud for him; with so much made of every trip, Einstein never left the U.S. after 1935.) His every bon mot was duly recorded for posterity, and his personal quirks (such as very rarely wearing socks) were eagerly added to the fast-growing legend. Not Einstein the physicist anymore; Einstein, the Einstein.

Did the man have flaws? Eager excavators have found that he was unkind to his first wife, Serbian physicist Mileva Maric, and distant at best with his second wife, Elsa, and their son. The famous absentmindedness, so jolly in his later years, was not so benign when it came to human contact. Should we be surprised at this from a man who did not speak until age 3, slouched his way through school, and grew up to find a universe that no others had? Surprised that he was more than a little aloof?

In 1929, TIME noted in a cover story on the physicist that "Albert Einstein's theories have altered human existence not at all." That would not last — the fields of electronics, quantum physics and space travel all bear his fingerprint now (though we're still waiting to see those twins in spaceships wearing watches). But that the atom could be split and its power unleashed — that one was the first to leap off the theoretician's blackboard. In 1939, America's most celebrated pacifist warned Franklin Delano Roosevelt in a letter that the Germans were nearing the nuclear age. America — this the physicist knew from experience from his days in Germany — had better get there first. It did. By 1946 Einstein's epiphany and the Manhattan Project would wreak, in the name of good, the most horrible destruction of our age in Hiroshima and Nagasaki. Einstein knew what he and his visions had done; after the war he made a tearful apology to visiting Japanese physicist Hideki Yukawa. Pacifist, deep-thinking Einstein, who loved children, was the father of the bomb.
At Princeton, he was more like a kindly uncle. When he arrived in 1935, and was asked what he would require for his study, he replied: "A desk, some pads and a pencil, and a large wastebasket — to hold all of my mistakes." His salary request had to be raised by Princeton administrators to avoid embarrassment. He played the violin, helped children with their homework, and did indeed, as the story goes, have some trouble remembering his address. He spent the balance of his life there, carving out a quiet spot within his legend and grappling with another chilling science that he had fathered but could not love: quantum physics.

Einstein, though not religious, was a believer. "I want to know how God created this world... I want to know his thoughts; the rest are details." And he had a good idea of what those thoughts were. Subtle but not malicious, non-interventionist but certainly present, Einstein's God didn't "play dice with the universe." Quantum physics, guided by Heisenberg's uncertainty principle, held that matter lived only as a probability, an approximation, an illusion of order in a chaotic universe. This Einstein could not bear, and he resisted the colder world bitterly until he became, in his own words, "a fossil" among his colleagues. "Stop telling God what to do," Niels Bohr told him, but Einstein couldn't. He spent his last two decades wrestling vainly for a "Unified Field Theory" — the final theory — a cause that Steven Weinberg, among others, has taken up today, so far without success.

Do we see too little beauty in the universe, or did Einstein imagine too much? ("It didn't pan out," he once told a colleague, two weeks after casually mentioning he was on the verge of his "greatest discovery ever.") A half-century after his death, we have his eyes in a jar in New Jersey and his brain (minus a few bits chipped off for analysis) in another jar in Lawrence, Kansas. We have the advances he left us, which have touched nearly every branch of the sciences, and we have the the bomb. But probably above all, in our heads we keep his vision (however vaguely) — the rhyming world is the one we keep on rooting for. Einstein got us closer to nature's truths than anyone had before, and he knew how much he had left unsolved. Once, Uncle Einstein sent this reply, along with a page full of diagrams, to a 15-year-old girl who had written for help on a homework assignment: "Do not worry about your difficulties in mathematics; I can assure you that mine are much greater." Everything's relative.

Frank Pellegrini writes for TIME Daily