LOS ANGELES — When composer Dylan Mattingly and writer Thomas Bartscherer first conceived the idea that would become “Stranger Love,” it was an ambitious but still reasonably manageable project: an hour-long love story unfolding against a backdrop of the four seasons.

Then it grew. And grew.

By the time “Stranger Love” had its first and possibly only performance 11 years later, the opera had ballooned into a vast six-hour theatrical spectacle, full of sumptuously beautiful music, sinuous choreography, splashy visual effects and a world-encompassing aesthetic ranging from the interpersonal to the cosmic.

The premiere performance, which was given Saturday, May 20, at the Walt Disney Concert Hall, represented a historic triumph of aspiration and persistence over realistic thinking.

There was no reason to think it would come to fruition. There was no reason to suppose any presenting organization would choose to champion it once it was written. There was certainly no reason for anyone — except perhaps the creators themselves — to believe with such unshakable faith that the results would conjure up an entrancing musical and emotional spell.

But there it was onstage, performed by the new music group Contemporaneous under the auspices of the Los Angeles Philharmonic, a glorious marathon that seemed to make the hours stand still.

“The way it plays with time is amazing,” said Chris Herring, 38, of West Hollywood during the dinner break between the opera’s first and second acts. “We’re four hours in, and it feels as if barely an hour has passed.”

To hear Mattingly tell it, none of this was exactly his own choice.

“It didn’t start with me saying, ‘Let’s write a six-hour piece,’ and then figure out how to fill the time,’ ” he told The Chronicle during an eloquent, if nerve-riddled, pre-performance interview. “But as soon as we started percolating the idea for Acts 2 and 3, it became clear that this was going to have to be a bigger piece.

“And once I started writing the music, it became clear that I was following something rather than creating something. There was something there I was looking for, and the piece began to tellus它想要什么。”

Mattingly, 32, whogrew up in Berkeleyand now lives a few miles north in Albany, has undertaken some smaller creative projects along the way. But it’s no exaggeration to say that the past decade and more of his life has been dedicated to “Stranger Love” — first composing it, and then, once the score was completed in 2017, trying to make a performance happen.

For the first few years, he said, the joy of the compositional process was enough to sustain him.

“Whatever else was difficult about this, whatever it meant that I essentially did this instead of creating a career, I always knew that this was the best thing I could ever do on the planet,” he said, “And that outweighed everything else.”

But in 2018, the New York contemporary opera festival Prototype gave a concert performance of the work’s four-hour first act. At that point, Mattingly recalled, he became obsessed with seeing the full work get at least one performance.

That single-minded dedication is one of the things that sets Mattingly apart from most of his peers.

“He’s absolutely unique,” said Berkeley composerJohn Adams,一个朋友and mentor since Mattingly’s days at Berkeley High School. “He’s devoted to his musical vision in a way that I don’t think I’ve ever encountered.

“He lives very modestly. He doesn’t have a day job, and he just has an astonishing ability to concentrate on music.”

What Adams describes as Mattingly’s “spiritual intensity” comes through in every minute of “Stranger Love.” It’s built in massive musical episodes that repeat in hypnotic, almost trancelike structures without losing their kinetic spark. Parts of the score burst with clangorous joy; others slow down to a level of stillness in which only the observer’s breath seems to be relevant.

The work’s most obvious, acknowledged forerunner is “Einstein on the Beach,” the pathbreaking five-hour stage work from 1976 by composer Philip Glass and director Robert Wilson. (Like “Einstein,” “Stranger Love” is dubbed an opera mostly because no other term seems close enough to be useful.)

Mattingly’s musical palette includes strains of Glass’ minimalism, but there are many other elements in the pot as well. The mystical ecstasy of the French composer Olivier Messiaen leaves a mark on the score; so do Balinese gamelan music, American microtonal music, Richard Wagner and David Bowie. Conductor David Bloom presided over the performance with dazzling precision and grace.

On Saturday, the piece’s distended time frame was clearly on the audience’s mind, both for better and worse. There were more empty seats in the hall during the evening’s final two hours following the dinner break than at the outset, and a few patrons straggled out in mid-performance.

On the other hand, the novelty of a six-hour experience seemed to have attracted some attendees. David Freeman of Torrance (Los Angeles County) brought his entire family to the performance, including his 9-year-old daughter, Daphne, drawn by the promise of something so markedly unusual. (Daphne’s verdict: “It was sometimes interesting, and sometimes boring.”)

对艾伦Rothman and Carlos Lerner of Los Angeles, who were there to celebrate their 25th wedding anniversary, the length of the event was part of its appeal.

“It’s a memorable way to celebrate,” Lerner said. “We don’t have too many six-hour operas about love. And for us, it’s like running a marathon — it’s an experience that pushes us beyond the regular cycle of experience.”

The most overtly operatic element of “Stranger Love” is its four-hour first act (split by a brief intermission). That’s where we encounter Tasha (soprano Molly Netter) and Andre (tenor Isaiah Robinson) and watch as they meet, fall in love, and face and overcome various challenges.

The latter two acts grow progressively shorter, more abstract and more condensed. During the one-hour second act, a parallel story is recounted in dance, stripped of any specific individuals. The comparatively brief final act offers a musical depiction of love at its most disembodied, with only the stars and planets to keep us company.

Whether or not the pieces of “Stranger Love” always measure up to Mattingly’s ambition, there’s something thrilling and powerful about the sheer bravado of the undertaking — and about his sense that the task of creating it came to him unbidden.

“I think making this piece was worthwhile,” he said. “I didn’t think anybody else was going to do it, so I had to. And I feel good about having chosen that life.

“But I would never, ever recommend it to anyone else.”

With just one performance after more than a decade in gestation, is there a chance that “Stranger Love” could take the stage again? The answer is unknowable, and Mattingly (who was overcome by emotion during Saturday’s curtain call) said he’d be content simply to have heard it once.

Besides, he’s got other projects in the works — including the immodestly titled “History of Life,” which will graft the Homeric tradition of oral poetry onto Charles Darwin’s travels in the Galapagos Islands.

“As much as I’d like to believe that I could say to myself, ‘OK, you need 10 years to rejuvenate your mental health and enjoy taking hikes and eating burritos,’ I don’t think I can do that,” he said.

“Even once I’m done with ‘History of Life,’ I’ve been having even more wild dreams of what can happen after that. It’s that feeling of knowing that there are things I could do because nobody else is going to.”

Reach Joshua Kosman:jkosman@sfchronicle.com; Twitter:@JoshuaKosman