Philip Glass: 'I think I'm built for this kind of life. I train like an athlete'

When I told a friend I was interviewing Philip Glass, he gave me some advice. "Just ask him," he said, "the same question, again and again and again." It's a view that once gave rise to a New Yorker cartoon. Two men in pith helmets are standing in a jungle. "That," says one to the other, ear cocked to a distant rumble, "is either a dissonant, repetitive sound or a new Philip Glass piece."

After several weeks washing up to Philip Glass, and cooking to him, and going to sleep to him, I can testify that those repetitions do strange things to the brain. Famously hypnotic, the shimmering scales and arpeggios are also quietly relentless, veering between the beauty of a waterfall, or of waves crashing on a beach and – well, you wouldn't want to talk about water torture, but let's just say that sometimes you long for a nice little tune. The most prolific composer on the planet, and one of the most successful, creator of more than 20 operas (all with pretty weird titles), eight symphonies, numerous concertos for violin, piano and timpani, and more than 30 film scores (including, most famously, The Hours) sure as hell knows how to mess with your mind.

"So why do they send a poet, rather than a musician to meet me?" he asks, when I volunteer, perhaps unwisely, my woeful lack of musical knowledge. I'm not a poet, alas, but we've been talking about Rumi, one of a number of poets Glass has set to music. Others include Allen Ginsberg and his friend, and fellow Buddhist, Leonard Cohen. For a moment, I fear that I've unleashed some rather un-Buddhist irritation, but then Glass softens. "If someone's not interested in it academically," he says kindly, "you could be better off. People get distracted by all sorts of non-essential things. Schools of music and historical values. It doesn't really explain what you're listening to."

We're sitting in the basement bar of The Boundary, a super-hip new "project" in a Victorian warehouse in the East End. With its stripped brick walls, black and white photos and funky leather armchairs, it has something of the feel of a New York SoHo loft, the kind in which, you imagine, Philip Glass might have spent happy evenings knocking up genre-bending works with Laurie Anderson, say, or David Byrne, or Brian Eno. Because Glass, more than any other contemporary composer, has worked across the art-forms – in theatre, dance, opera, the visual artists and rock. He has pushed the boundaries of music, pushed the boundaries of cross-arts collaboration, pushed the boundaries of opera and, at 72, is still pushing boundaries now.

He has, as always, been up since six. Last night, as usual, he went to bed at one. And after our strict hour, he's dashing off to catch a train. How does he keep going? "Well, you know," he says, "I think I'm built for this kind of life. It doesn't bother me at all. I train," he adds, "like an athlete, in terms of exercise, diet and sleep. I do yoga. I also do Qigong. I've been doing it since I was 20." And is he, as I've read, a vegetarian? "Since I was 20." And does he drink? "Just a glass of wine, maybe three times a week."

Gosh. This friend of David Bowie and Lou Reed, who lived in Paris in the Swinging Sixties, and discovered Ravi Shankar before the Beatles, clearly lives like Gwyneth Paltrow. "You say I'm disciplined," he says, as I mentally tot up my own weekly tally of units, "but I think I'm not disciplined. My discipline is that I'm afraid of not being disciplined." Er, right.

Maybe it's an American thing. Americans, I say, are good at doing things like going to the gym before work. "My girlfriend does," he volunteers. "She runs and does pilates." Is his girlfriend Wendy Sutter? "Yeah," he says, "but she doesn't... she likes to be called my girlfriend, but she is also an artist in her own right." Wendy Sutter (whose date of birth is coyly absent from all her publicity material but who looks several decades younger than Glass) is indeed an artist in her own right. She is the cellist who inspired Glass's Songs and Poems for Solo Cello, some of which Glass will perform with her next week, in an evening of chamber music at the Barbican.

The Songs and Poems were hailed by The Washington Post as "not merely pleasant, but gripping" and by Bloomberg.com as "the first major solo cello work of the 21st century". And they are exquisite – plangent, fierce, passionate, full of yearning and aching with what could be joy or pain. They reminded the San Francisco Classical Voice (and me) of the Bach cello suites. Were they a conscious homage to Bach? "Oh yeah," says Glass. "The cello is often remarked as the instrument that's closest to the human voice, in that it's the instrument that mimics the range. It kind of plunges you into the interior of what an individual psyche must be like."

Well, if all that aching beauty has anything to do with spirituality, or personal pain, or his feelings about Sutter, Glass (who has been married four times) is clearly not about to give anything away. So I ask about his father, who set the whole thing off, in his radio repair and record shop in Baltimore in the Forties. "Some paper in England said my father was an immigrant," says Glass. "He'd have been shocked. He never left the country. His parents did come from Russia or Belarus, or some place like that." "Some place like that"? If his parents were, as Glass says, "totally Americanised", so, it seems, is their son, who appears not to have bothered even to find out where his grandparents came from.

Still. At the age of eight, inspired by his father's love of music, Glass started learning the violin and the flute. "I had to take a streetcar across town by myself," he says, "and I had to walk home in the dark. But I was the youngest one at the conservatoire and they looked after me." At home with his parents, he listened to music "all the time. "One of the great things about my father," he says, "was that he had to know all about music because his customers liked all different kinds. He preferred chamber music, central European and he liked modern music: Bartók, Shostakovich. He never told me that one was better than another. In fact, there was a kind of enforced democracy."

High, low, classical, modern, chamber, orchestral – he sounds like a bit of an early postmodernist. He sounds, in fact, like someone whose son might also end up embracing an astonishing range of musical genres, and art-forms, and producing work (operas, symphonies, songs for cello) that might be classified as high art and work (film scores, jingles for commercials) that might be classified as low art. Does he have this father to thank for this? "Yeah," he says. "He didn't know much about world music, but no one did. On the other hand, I learned a lot about jazz when I was a kid, and I was going to school in Chicago by the time I was 15. The Fifties in Chicago was a great time for jazz."

At 15, Glass fails to mention, he was actually an undergraduate at the University of Chicago, studying mathematics and philosophy. It was there he started learning the piano and there, too, that he started composing, because he wanted to find out "where music came from". At 20, he went to study composition at the Juilliard School of Music (where Steve Reich was a fellow student) and then enrolled in a composer-in-residence scheme for two years in Pittsburgh schools. In 1964, he moved to Paris to study with the composer Nadia Boulanger and in 1967 moved back to New York where he founded the Philip Glass Ensemble. Looking at his CV from this time on, it would be tempting to say that the rest is history, but artistic output, as so often in life, did not automatically translate into money.

Glass didn't earn a living from his music, in fact, until he was 42. Until then, he drove cabs, shifted furniture and worked as a plumber. "I was careful," he explains, "to take a job that couldn't have any possible meaning for me." Stories of famous-composer-actually-working-man-shock from that period abound. The art critic Robert Hughes was astonished to find the avant garde composer mending his dishwasher. On another occasion, a woman tapped on the side of his cab and told him that he had the same name as a "very famous composer".

Gradually, the commissions trickled in and, by the time Glass was 44, he realised that the cab driver's licence that he'd renewed as a precaution might not be needed. Einstein on the Beach, his music theatre collaboration with Robert Wilson, performed at the Met in 1976, was followed by Satyagraha, an opera which drew on the early life of Mahatma Gandhi, and then by Akhnaten, a vocal and orchestral composition sung in Akkadian, Biblical Hebrew and Ancient Egyptian. Clearly, we are not talking populist. On the other hand, Glass was soon reaching a mass audience with his film scores. His first, for Koyaanisqatsi, directed by Godfrey Reggio, is still regarded as one of his best. Others include Kundun, The Truman Show and Notes on a Scandal.

What effect has the film work had on him? "It has made me very happy," says Glass with a rare burst of laughter. "The high-art music and commercial, they're different metiers, they're different languages. I'm like the painter," he adds with a grin, "who does abstract painting at home but likes to go to the sidewalk and do sketches of people in the street."

Among the many artists Glass has worked with – ranging from Doris Lessing to Patti Smith and Woody Allen – it's one he didn't actually meet who has, perhaps, had the greatest influence on his work. In 1963, when he was living in Paris, Samuel Beckett was "in the neighbourhood". Glass "didn't have the pleasure of knowing him" but wrote music for his work. "To me," he says, "he represented the quintessential postmodernist. He took away the meaning of a work and made it meaningful. It's a perfect John Cage strategy, in that he denied the independent, inherent existence of the artwork. It had no existence independent of the person that listened to it. The idea that artwork consisted of a transaction, that's the essence of postmodernism. Before that, we thought of string quartets as existing in some sort of platonic realm."

"It was extremely encouraging for me," adds this super-disciplined, super-serious, super-productive model of can-do Americanism, "because it meant that the community of critics and historians, it left them out of the picture. I knew from the beginning that what they had to say would have very little impact on what I did." And then Philip Glass, high priest of postmodernism, looks at his watch. "Now," he says, "I have to catch my train."

Philip Glass: Chamber Music is at the Barbican, London EC2, on 26 May at 8pm (www.barbican.org.uk). The first Prom devoted to his work is on 12 August