The fact is that Big Changes don’t happen very often, and when they do they are generally noticed only many years later, mainly by historians, who get paid to spot them. In 1517, when the citizens of Wittenberg saw Martin Luther’s 95 Theses nailed to the church door, do you think they exclaimed, ““Glory be, it’s the Protestant Reformation!’’? That had to wait for the invention of college survey courses.

There’s another reason to be skeptical. This century has already had a major flirtation with apocalypse, and anyone who lived through it is still too exhausted to do it all over again. It was called the ’60s.

Life seemed more vivid in those days. The colors were Day-Glo. The music was inspired. The drugs were ““mind-blowing.’’ The politics were intense. Everything seemed to matter, and the pace was very, very fast.

Two separate strands of behavior seemed to wind through the ’60s. One of them was violent, the other benign, but they were twisted together like a double helix. For some people they seemed to point toward nothing less than a transformation of American life, and so the language of those days tended to the millennial. Giddy academics wrote of the birth of a new ““consciousness.’’ Young rebels plotted revolution.

Days of rage: It began with the murder of President Kennedy in 1963. The violence seemed to escalate from that day on. Young American men were sent off to Vietnam, to kill and be killed in what became staggering numbers. The civil- rights movement burst into civil-rights riots. College campuses were paralyzed as students took over administration offices.

Consider the events of the single, awful year of 1968. Martin Luther King Jr. and Robert Kennedy were shot dead; assassination seemed to be replacing the ballot box in American political life. In Paris, students flung up street barricades; for a while there was serious talk of another French Revolution. Soviet troops invaded Czechoslovakia. And in Chicago, the Democrats held a convention that pretty much epitomized the year: in the streets, youth demonstrations brutally put down by what would later be called a ““police riot’’; on the floor, bitter denunciations that opened wounds from which the party has never fully recovered.

Feelin’ groovy: Meanwhile, something else entirely was going on–a gentle, rather antic movement of young dropouts who renounced conventional society and donned the brightly colored vestments of hippiedom. Enraptured by rock music, wafted on clouds of marijuana smoke, liberated from sexual strictures (the birth-control pill was another innovation of the ’60s), they professed a simple code of ““peace and love.’’ The blissful Woodstock festival of August 1969 was the high point of the movement. Not long afterward, things began to fall apart. In 1970 the Beatles broke up and Jimi Hendrix died of a drug overdose. The show was over.

What have the ’60s left behind? A continuing suspicion of government, though now it comes more from the right than from the left (yesterday’s hippies are today’s libertarians, and the revolutionary Weathermen seem to have joined militias). A vision of the simple life, without the utopian overtones, that has never gone away. A widening of American social boundaries, to include blacks, women, gays and young people in more ample ways. And a sobering sense that the world and human nature are not easily transformed, even in years that happen to end in a lot of zeros.