'You're there!" said teacher to student.

The student was Zachary Mowitz, studying cello at the Curtis Institute, and the teacher was Peter Wiley, cellist and faculty member at Curtis.

Twenty-seven minutes before, Mowitz had come in and sat down, and Wiley had said, "Since we're here, why don't you just … play it." And Mowitz launched solo into Samuel Barber's famously difficult Concerto for Violoncello and Orchestra.

Twenty-seven minutes of explosive music later, Wiley said, "Bravo. I applaud you. I do. You're there."

You're there, coming from Wiley, would be magic words for many cellists. "You're so ready, totally ready," he said. Then he paused.

In five days, Mowitz, 21, was scheduled to perform a recital at Field Concert Hall on Locust Street. He'd come a long way to get here, years of study, playing at the highest levels, and now on the threshold of a career. The run-up to the recital would face him with the limits of his ability -- and body.

If you make it to Curtis, you’ve already proved you’re one of the best musicians in the world. (“We have students who come here already under professional management, and some win top orchestra jobs while still in school,” said Jennifer Kallend, managing director of external affairs and patron engagement.) For that reason alone, Curtis student recitals, free to the public, are among the best cultural/entertainment values in Philadelphia: the best music played by the best musicians. Recitals happen two to three times a week October through May, about 100 a year, most in Field Hall but others in places like Gould Rehearsal Hall at Lenfest Hall or (in spring) St. Mark’s Episcopal Church. They’re broadcast on WHYY-TV, and Friday recitals (plus other events) are live-streamed and archived at curtis.edu/CurtisPerforms. True to the Curtis “learn-by-doing” approach, students are encouraged to play as many recitals as they can. By the end of the year, Mowitz will have played in nine of them.

Thorny and knotty, Barber's concerto tests the cellist to the utmost. Barber (once a star student at Curtis himself) wrote it in 1945-46 for famed prodigy Raya Garbousova. "Many of the licks suit her unique virtuosity," Mowitz said before his lesson. "I really love the piece, but it's known as one of the most awkward, uncomfortable pieces to play. Annoying."

Wiley had a suggestion. "You're really stabbing your entries into some of these sections... . But what if we used a rounder bowing technique, easing into the music?" He grabbed his own cello and demonstrated. "Get into mood, color, feeling, imagination."

Curtis Institute instructor Peter Wiley demonstrates some ideas on the cello during a lesson with student Zachary Mowitz.

Mowitz nodded, liked it, tried it out. He shook his left wrist.

Curtis instructor Peter Wiley (left) and student Zachary Mowitz review technique during a lesson.

That was Sunday. "I've got a long week ahead," Mowitz said after the lesson. "The main priority is sleep. I'm going to be working on that."

Cello was Zach Mowitz's first love. Son of composer Ira J. Mowitz, he grew up in Princeton and considers cello "my first and only instrument." He came upon the Barber concerto in spring 2016, and it offered, he said, all the elements of a recital piece: "It's beautiful, it's a major work, and it's a big challenge." For the last year, through hours of work, under the guidance of Wiley and Carter Brey, he's been rising to that challenge. "I've been trying to hit a peak these last two months," he said. Then he charged off -- to a modern music concert and an opera rehearsal.

What kind of kid is Mowitz? Wiley said, "He's changed a lot in just a couple of years at Curtis. He's very musical, in the sense of making true music beyond the technical aspects. He's open to experiment, with posture, the way he holds the cello. It reflects a willingness to change, to figure out what works."

Violinist and educator Ida Kavafian has known him in her roles as teacher, colleague, and employer. Mowitz was part of her Young Artist Program at Music from Angel Fire in New Mexico. "Zach brings the same integrity and intensity to everything that he does," she said. "He is the kind of student that we all hope to have in our teaching careers." With such a student, she knows that "my values will be passed on to future generations. In other words, he gets it."

Three days before the scheduled recital, Mowitz met with pianist Yoni Levyatov in Field Hall.

Accompanist Yoni Levyatov (left) and cellist Zachary Mowitz practice Barber's "Concerto for Violoncello" a few days before Mowitz's recital.

They played the concerto start to finish; it was only their second chance to do so. Beneath the lights, Mowitz's left hand took the stress, especially his index, middle, and ring fingers, as he reached for athletic figures way up high on the neck, then plummeted to frenetic bustle in the lower ranges. All the while, he performed, with passion, perplexity, ecstasy, angst. He shook his left hand when done.

Amazingly, his recital Friday was sandwiched between performances Thursday and Saturday of John Adams' opera Dr. Atomic at the Perelman Theater. Mowitz is in the orchestra. "It's a load of work," he says, "but it's an opportunity you just can't miss."

The day before the recital, he sent an email: "My arm/wrist problems are more serious than I thought, so I've (very hesitantly) decided to cancel my Barber performance tomorrow. With Doctor Atomic on either side of it and my chiropractor's advice, it seemed too risky to put my body under that stress." His recital was delayed from March 3 to March 27.

Repetitive stress injuries are a fact of life in, and a shadow over, the classical musician's world. In some surveys, more than 70 percent of symphony orchestra musicians polled say they have or have had some degree of it. So it was time for Mowitz to rest, relatively speaking, from the Barber.

Relatively speaking. Constantly gigging, practicing, auditioning for grants and jobs, a Curtis student has a busy life. During the hiatus, Mowitz auditioned for and won a prestigious Philadelphia Orchestra Albert M. Greenfield Student Competition award, which comes with $800 and a chance to perform as a soloist with the orchestra in the 2017-2018 season.

But it was a rest, and a good rest. "The wrists (because it was both arms) are much better, and the rest definitely did me good," he wrote by email. "There is still tension in my forearms, but no more risk of anything like tendinitis."

Finally, March 27, the night of the recital. From beneath the stage, a Steinway D concert grand piano rose up through a trapdoor into the stage light of Field Hall. First up was Timothy Chooi, a rising star, already playing professional engagements all over North America. Chooi, relaxed in his tuxedo, set fire to the hall with the Violin Concerto in E minor by Felix Mendelssohn.

Mowitz, dressed down by comparison, was next up. Levyatov began, and from his first entrance, Mowitz made his recital count.

Music is hard to describe, blessedly so; most music, even if we describe it well, escapes us. Yet there was a newly elegant, nuanced quality to Mowitz's performance that night. Perhaps he was simply following his teacher's advice: There was still some stabbing, to be sure, in a piece that often calls for it, but his right-hand work with the bow was fluid, rounder. There was always a sense of singing, of refusing strain. Mowitz's treetop technique freed him to explore the music, release himself to it.

Wiley said afterward that "the recital was the best I've heard him play it. The variety of sonorities he managed to get, the colors, the character. He played with ease, great command, excellence. Very convincing, stylistically."

Afterward, Mowitz's father said, "People say, 'You must be proud.' But he's the one doing all the playing. Things come out of him that absolutely amaze me. He's really got a voice." Father and son had a big hug in the hall.

Told his playing was his most musical yet, Mowitz smiled, relieved, happy, and trying to play it down. "It definitely felt the freest this time," he said. "The rest really helped.

"Now," he said, "for some serious sleep."