The musicians will move like the parts of a mobile, Lewis explained by phone: "You go up on stage and take a card, which has a task assigned to it. You have to walk around without playing in order to do it, and that regulates time."

St. Paul contemporary chamber quartet Zeitgeist, abetted by more than a dozen local improvisers, will perform "Calder" and other Lewis compositions in an event designed to showcase his catalytic impact on music.

"The main thing about these kinds of pieces is that I want people to take responsibility for the musical environment they are creating," Lewis said. "They come together and create sort of momentary community."

Lewis, who will turn 70 in July, is among an elite group of experimental composers alongside John Cage, John Zorn and perhaps a dozen others.

He first came to prominence as a trombonist, but has focused on composing and teaching in recent years. A pioneer in interactive music involving computers, Lewis built his own machines as far back as the mid-1970s, and developed a program in which the computer acts as a member of the ensemble, responding almost intuitively to musical elements such as pitch or timbre.

It would be fair to refer to Lewis as a renaissance man.

"I prefer to call him an African man," countered multi-instrumentalist Douglas Ewart, who will be part of Saturday's cohort.

Ewart first met Lewis in 1971 when the latter was a teenager taking a semester off from Yale and beginning his engagement with Chicago's Association for the Advancement of Creative Musicians (AACM). He remembers the two of them riding the bus, then the train, then the bus again from the South Side up to Evanston to play in saxophonist Fred Anderson's band.

Lewis calls Ewart "my oldest closest friend."

Ewart, for his part, said "I wish everybody could have a friend like George — my love for him is forever. He is also a brilliant man who is always learning, a hard worker who had a number of other brilliant people invest in his development."

What is it like to play Lewis' work? "Challenging," Ewart replied. "There is always the notion of involving people in a way that makes them consider what they are doing, as individuals and as a community. It gives the music a freshness and elasticity that isn't common."

But Lewis doesn't always cede control to the players. "Interregnum," a work co-commissioned by Zeitgeist that the quartet will perform Saturday, purposefully has less leeway.

The dictionary definition of interregnum is "a period when normal government is suspended." Lewis said that has been a theme in his work for the past decade or so.

"Around the world, people are making unprecedented demands of institutions for structural change and real change, not just paper change or virtue-signaling," he said. "In one sense it is a good thing — no one is in charge and you can get things done. But it is also very dangerous. It is a space where new pathways can emerge. I'm looking at it from both standpoints — the danger and the promise.

"The way the piece is organized there is no central driving force," though the composer's guiding hand is firm. "You have to play the notes that are there."

With "Interregnum," the musical community is expanded beyond the players, to directly encompass the audience.

"Certain structural changes need to happen," Lewis said. "Minneapolis has been one of the major flashpoints in the entire world for this kind of thing, right? That's an astonishing position to be in. It's a scary position to be in. But it means that on some level people are facing this reality.

"People will come out of the Walker after the concert, and they'll say, 'Hmm, that was really different.' And they might wonder what else could be different, and put that critical consciousness on the rest of what they're experiencing. That's what I want to do — make people think about their situation. And if they do, then maybe they take action."

The Music of George Lewis
With: Lewis plus Zeitgeist and Friends.
When: 8 p.m. Sat.
Where: Walker Art Center, Mpls.
Tickets: $28.50. walkerart.org