The group, which celebrated its birthday on Friday at Carnegie Hall, changed music with its open-eared and open-minded approach.Late one night in 1973, a young violinist named David Harrington was listening to the radio. He heard some music that was just a few years old: George Crumb’s “Black Angels,” a harsh and eerie, prayerful and screaming piece for amplified string quartet, full of grief and anger about the quagmire in Vietnam.“A lot of people my age,” Harrington recalled in a recent interview, “were desperately trying to find work that felt like it somehow related to what we were experiencing, what our country had been going through.”For him, “Black Angels” was it. “I thought, I don’t have any choice,” he said. “I have to play that piece.”Harrington got three friends together and, with the help of a Greco-Roman mythological dictionary to brainstorm a name, the Kronos Quartet was born with a vision, then rare, of focusing on new and recent compositions.Fifty years, and over 1,000 fresh works and arrangements later — an anniversary and achievement celebrated on Friday with a sold-out concert at Carnegie Hall — the group has changed the music world.When Kronos formed, contemporary music was widely viewed as mathematically rigid and atonal: unlistenable audience poison. Buoyed by dramatic stage lighting, trendy clothes and passionate, eclectic performances and recordings, the quartet showed that a new approach to the new could fill halls and draw young crowds.Kronos proved that composers working in different idioms than standard-issue modernism — like Terry Riley, Philip Glass, Steve Reich, John Adams and Osvaldo Golijov — could become core string quartet material, as could world traditions and collaborators on nonwestern instruments. A quartet could adapt the music of far-afield artists like Thelonious Monk, Bill Evans, Astor Piazzolla and Sigur Rós, and could define the hard-edge soundtracks of films like “Requiem for a Dream.”Kronos and dozens of collaborators ended the quartet’s anniversary concert at Carnegie with a performance of Terry Riley’s “Sunrise of the Planetary Dream Collector.”Stephanie BergerThe group didn’t necessarily shy from modernism and its tough descendants — the likes of Schnittke and Zorn — but it did play that music in welcoming company on its programs, and with populist theatricality. At one 1987 show, a New York Times review noted, the modernist composer Elliott Carter sat next to Sting, which says it all.For all its variety, Kronos had a point of view, an aesthetic, a brand. Few if any ensembles of any size before it had been so flexible, open-eared and open-minded.“I can’t think of a more significant player in terms of contemporary music becoming seen as fun and enjoyable,” said Clive Gillinson, Carnegie’s executive and artistic director. “It’s not a risk. It’s music you’ve never heard before, but you’re going to enjoy it.”Not everyone was convinced. Some sniffed that the group too often tipped into wan crossover. Some found the energy good-natured but the playing a little ragged. Some thought the showy lighting and sound were overwrought. Some rolled their eyes at an arrangement of Jimi Hendrix’s “Purple Haze” that was once a standby for Kronos encores.But playing Hendrix was a sincere gesture, the symbolic move of a quartet seizing the whole of music for its own and boldly crossing traditional genre — as well as racial, national, ethnic and gender — divides. This was, after all, the era of “Graceland,” Paul Simon’s blockbuster 1986 album, and some of Kronos’s defining recordings were in that globe-trotting spirit: “Pieces of Africa” (1992), the omnivorous “Caravan” (2000) and “Nuevo” (2002), which explored Mexican classical, folk and pop.Like the more traditionally minded Emerson String Quartet, also formed in the mid-1970s, Kronos was lucky to come of age during the CD boom — Emerson on the august label Deutsche Grammophon, Kronos on hip Nonesuch.The 1997 album “Early Music” was a surprising dip into medieval repertoire — but typical of Kronos in that it combined arrangements of Machaut, Pérotin and Hildegard with Cage, Schnittke, Pärt, Scandinavian fiddling and Tuvan chant, closing with a minute and a half of bells tolling at a monastery in France.This was a narrative approach to recording, rather than one of just stacking pieces, at a time when projects like that were hardly mainstream in the classical world.“What were thought of as these wacky ideas are very much normal now,” said Andrew Yee, the cellist of the Attacca Quartet. “Everyone — all the young quartets — has at least a small part of Kronos built into their DNA.”The Canadian Inuk vocalist and composer Tanya Tagaq, center, joined the quartet at the concert.Stephanie BergerFriday’s concert embodied the Kronos spirit, with a parade of collaborators from around the world, multimedia elements and sound effects, in works that often had an earnest, liberal political message. In one piece, the writer Ariel Aberg-Riger recited a plain-spoken account of the life of the conservationist Rachel Carson as the quartet underscored her. During another, the Canadian Inuk vocalist and composer Tanya Tagaq roared “You colonizer!” over and over.Laurie Anderson was her usual gnomically witty, poignant presence for part of “Landfall,” her 2012 work with the quartet about climate and loss. Roots Americana was on the program, as was one of Kronos’s Mexican arrangements, Indonesian sinden (a style of gamelan singing) and Bollywood. A longtime collaborator, the pipa virtuoso Wu Man, was featured in an excerpt from her “Two Chinese Paintings.”Dozens of musicians joined for the finale, Terry Riley’s “Sunrise of the Planetary Dream Collector” (1980). An initially minor-key, slightly melancholy, ultimately propulsive jam, it is a wistful counterpart to the composer’s “In C.” Most moving was the spectacle: Many of those onstage hadn’t yet been born when Kronos formed.Laurie Anderson joined Friday’s performance with part of her 2012 work “Landfall.”Stephanie BergerThe evening passed in something of a blur of activity, which is not unusual for the quartet. The group has done — and still does — so much that it can be easy to take it and its impact for granted.“One of our jobs,” Harrington said, “is to make it seem like music just falls out of the sky.”There is so much music, of so many kinds, that if one piece or album doesn’t appeal, the next very well might. “The Kronos does not guarantee profundity,” Bernard Holland wrote in The Times in 2006. “It just likes to keep the conversation going.”Early on, Kronos created a nonprofit arm that let the quartet raise money, sponsor ambitious initiatives and commission music on its own, rather than depending on composers and presenters. The group’s recently completed “50 for the Future” project commissioned dozens of new pieces designed for young players and made them available online for free.This is the work of a quartet with its legacy in mind, but there are no plans for Kronos to disband. An ensemble constantly chasing newness may be less beholden to a given set of players than a more traditional quartet. Harrington, of course, has been with the group from the beginning, and the violinist John Sherba and the violist Hank Dutt since the late ’70s. The cello chair, long held by Joan Jeanrenaud, has had some more turnover; Paul Wiancko, a generation younger than the others, joined earlier this year.At 74, Harrington demurs when retirement — “the R word,” as he called it in a short documentary screened at Carnegie — comes up. “There’s nothing else I’ve seen in life that would be half as interesting as this,” he said in the interview. “The idea of stepping away from it is impossible.”That said, he added: “I can imagine this group continuing on and on. I want it to be the most activist, energetic, energizing ensemble in the universe. If we can make it that way, I don’t think it should be restricted by my own lifetime.” More