When Lin Kailiang, a doctoral student, sent two emails in November 2012 to two titans of 20th-century physics — the 90-year-old Nobel laureate Yang Chen-Ning and the 89-year-old visionary, Freeman Dyson — he didn't expect it would forever alter the trajectory of his life.

His proposition was as simple as it was ambitious — he wished to conduct a comparative study of their scientific styles. Defying his expectations, both men responded, and then, the phone rang. It was Yang himself, inviting the young student to visit him at his office at Tsinghua University in Beijing.

Lin could not have predicted that this brief encounter would blossom into a profound friendship, which 12 years later would lead him to write Guilai Rengshi Shaonian (Always Be Young: A Biography of Yang Chen-Ning), chronicling the Nobel laureate's century-long journey.

Published by CITIC Press Group and Hunan Education Publishing House, the book is a poignant tribute to Yang, who passed away in October, introducing the legend to the younger generation. It is distinguished by being officially authorized by Yang's office, granting Lin access to a treasure trove of historical materials that illuminate the man behind the monumental science.

Lin, a teacher at Northwest A&F University in Xianyang, Shaanxi province, and an author in whom Yang placed his trust, seeks to portray him not just as a colossus of 20th-century physics, but as a perpetual "youth" — a scientist driven by a lifelong passion for truth and beauty, and an intellectual deeply connected to his roots.

"Too many people see him only as a scientific monument, overlooking the warmth and humanity of the ordinary man," Lin tells China Daily in an exclusive interview. "I wanted readers to see that his greatness was rooted in the most genuine qualities — curiosity, perseverance and a deep love for his homeland."

To bridge the gap between Yang's complex world and the minds of young readers, Lin weaves in charming anecdotes from the physicist's early years. He recounts a young Yang engrossed in classic mathematical puzzles like chickens and rabbits in the same cage, and playing the 15 Puzzle with his siblings to hone his logical thinking. These stories are strategic, designed to find common ground with the everyday experiences of his audience.

When confronting Yang's most esoteric contributions, such as the Yang-Baxter equation, Lin turns to a metaphor. He describes it as akin to the thought behind hair braiding, a visual simplification that allows readers to grasp the elegance and symmetry of the theory without the need to navigate its daunting mathematical form.

Zhang Tingting, the book's illustrator from Hunan Education Publishing House, learned hair braiding after reading the book, which made her feel that math was no longer distant from everyday life. "Thanks to the book, I braided my girl's hair for the first time," she says.

"The goal is to make the beauty of his science accessible," Lin says. "You don't need the full equation to appreciate the profound insight."

The book's title, rich with poetic resonance, is a core theme. "Guilai (returned) operates on two levels," Lin elaborates. "It refers to Yang's tangible return to China in his later years, renouncing his foreign citizenship to fulfill a spiritual promise to his father and his homeland. But it also reflects his personal philosophy of life as a 'great circle' — a journey of completion and fulfillment."

At Yang's 80th birthday banquet at Tsinghua University, Yang gave a brief speech, which he ended with the following words: "The Tsinghua campus is where I grew up. My life has now completed a big circle. Thus, this last part of my career will be an especially meaningful act of my life!"

As for shaonian (youth), it is a tribute to Yang's enduring spirit. "Even in advanced age, his mindset remained youthful, his thinking vibrant, and his wonder for the world undimmed," Lin says. A proposed English title for the book by Lin is "Always Be Young", which plays on the phonetic similarity between "young" and "Yang", capturing this very essence. According to Lin, this title was inspired by the physicist Marvin Goldberger, who was also one of Yang's old friends. He once said, "May you always be young (Yang)", at a birthday party for Yang.

Beyond the equations and the Nobel Prize, beyond the moments on the world stage that defined Yang's public life, Lin wants to give a glimpse of Yang's private world, as a son and a father.

A story that touches Lin's heart is Yang's long-awaited family reunion with his father in Geneva in 1957. It was the first time Yang's father, Yang Ko-Chuen, then 61, met his daughter-in-law and his grandson. Yang Ko-Chuen, who required daily insulin injections to manage his diabetes, was initially too weak for much activity.

But after a few weeks in Geneva, his strength slowly returned, enough for a simple, profound ritual: taking his young grandson for walks in a nearby park. There, the two discovered a small wonder — a "secret path "through the bushes lining the park. It became their special adventure. It was the scene of their departure, however, that etched itself permanently into Yang Chen-Ning's memory, a memory he would later share in an article, Father and I, and one that Lin finds especially resonant and has included in his biography.

Yang Chen-Ning wrote: "Every time the two of them were preparing for one of their walks, with Father combing his hair in front of the mirror in the hallway, and Franklin (his grandson) jumping up and down to open the door of the apartment, I was suffused with a sense of fulfillment."

For Lin, the power of this scene grows with time and life experience. "A reader who hasn't reached that stage of life — being sandwiched between the older and younger generations — might find it hard to fully grasp," he says. "When I was younger, I read this and it didn't strike me as deeply. But now, at my current age, as I was writing this very passage in the biography, it touched me profoundly."

This glimpse of the great physicist not as a titan of science, but as a man watching his aging father and his young son find joy in each other, reveals a universal truth. "It shows Yang Chen-Ning as an ordinary man, a real person," Lin says. "It's something in which everyone can find a mirror. These details make the great scientist relatable."

Acknowledging the compressed writing schedule, Lin admits that a more comprehensive version is possible. He was unable to conduct a full archival deep dive and plans a more extensive, sequel volume in the future.

The 2012 meeting was the beginning of a relationship that deeply influenced Lin's career. He recalls Yang Chen-Ning's encouragement of his early work on the physicist Dyson, a pivotal moment that decided his path in science communication, a career he had never previously considered. He says that Yang Chen-Ning was his guide, the one who illuminated the path forward.

"He saw value in making scientific history accessible," Lin says, adding that mentorship is now a driving force behind his own efforts.

Through his work, Lin hopes that the boundless curiosity of Yang Chen-Ning, the boy who never aged, will continue to ignite imaginations for generations to come.

Contact the writer at wangqian@chinadaily.com.cn