Friday, October 18, 2013
channeling ada: chien-shiung wu, courageous hero of physics
This post originally appeared on the Scientific American Guest Blog on October 15, 2013.
Today marks the 5th Ada Lovelace Day, an annual celebration of women who have made important contributions to the fields of science, technology, engineering, and mathematics (STEM). The event is named for Augusta Ada King, Countess of Lovelace, who is often credited as the first computer programmer. Since its inception in 2009, Ada Lovelace Day has grown from a purely blog-based affair to one marked by worldwide events including public lectures and Wikipedia edit-a-thons. This year, the Ada Lovelace Day organizers have also published a book of essays celebrating women in STEM entitled, A Passion For Science: Stories of Discovery and Invention. This blog post presents my chapter of that book. It describes the life and work of Chien-Shiung Wu, one of the most important physicists of the 20th century. Few outside of physics have ever heard of Wu, nor could they name any of her considerable contributions to science. I hope this essay will change that in some small way. –MW
It is the afternoon of May 31, 2012, and the skies above Liuhe in the Chinese province of Jiangsu are overcast but resplendent in silver and grey. A late-spring chill fills the air as a crowd of expectant locals and distinguished guests, including a number of representatives from the People’s Government, gathers in a circular stone-walled courtyard to honor a hometown legend. Scores of women, men and children who have made the journey here huddle in their well-worn jackets and coats as they wait for the memorial ceremony to begin.
Over the next two hours, attendees of this spirited congregation will take turns paying their respects with flowers, speeches, and songs to one of the most decorated and esteemed scientists of the 20th century. She has been dubbed the "First Lady of Physics" and the "Chinese Marie Curie" for her groundbreaking work in nuclear science—some of which, controversially, helped earn her male colleagues, but not her, a Nobel Prize. But here in Liuhe, where she was born exactly 100 years ago (and where she was buried after her death in 1997) she is known simply as Chien-Shiung: "Courageous Hero".
For one who faced so many uphill battles on the road to worldwide recognition and acclaim, physicist Chien-Shiung Wu more than lived up to the moniker her parents conferred upon her the day she came into the world in Liuhe, some 30 miles northwest of the port city of Shanghai. To begin with, Wu was born at a time when her homeland forbade girls from going to school. This was still an era when Chinese girls were expected to bind their feet and grow up to serve their male compatriots.
And yet, only a year before Wu’s birth, the Xinhai Revolution had overthrown the last Chinese dynasty and established the new Republic of China. With that massive uprising came a sea change of attitudes and a new generation of leaders eager to overturn the status quo. One of those leaders was Wu’s father, Zhongyi Wu. An engineer by training who believed strongly in equal rights for women, Zhongyi felt that the best thing he could do to help his daughter and her peers was to start a school for girls — the region’s first. With the aid of his wife, Fan Fuhua, who persuaded other families to let their young ones enroll, Zhongyi Wu opened the Mingde School for Girls and became its principal. And so, young Chien-Shiung, an inquisitive child from the get-go, was one of the first girls to obtain formal education in China.
But her father’s school could only take Wu so far. To continue learning, her only option was to join a girls' boarding facility 50 miles from home. She was all of 10 years old when she began classes at the Suzhou Girls' School, where she quickly came to discover the beauty and intrigue of physical science. It was, of course, not easy for a child so young to be away from her family, but her parents gave her strength. "Ignore the obstacles," her father told her. "Just put your head down and keep walking forward."
With such encouragement, Wu dedicated herself to the goal of studying math and science at the university level. She practically lived at school for seven full years, during which time she worked twice as hard as many of her peers so that she would have the skills required to earn a place in the physics department at the National Central University in Nanjing. Her commitment paid off: In 1930, she completed high school and began at NCU as a math major, transferring later into physics.
Wu graduated from NCU in 1934 as the school’s undisputed top student. But she once again found herself up against a wall: While the world was beginning to unravel the mysteries of the atom, a topic that intrigued her immensely, China had no graduate programs in physics. And so, at the suggestion of a mentor and with the financial backing of an uncle, Wu left for the United States on what she thought would be a brief detour in her journey to a scientific career in China. Little did she know that the course of her life would take a dramatic turn almost as soon as she landed on the California coast — nor that she would never again set eyes on the family she was leaving behind.
A life atomic
The United States of the 1930s saw the dawn of a new era in scientific inquiry. Atomic physics in particular took a major step forward in 1931, when future Nobel Prize-winner Ernest Lawrence, with the help of graduate student M Stanley Livingston, built the first cyclotron, a particle accelerator that uses magnetic fields to speed up and smash together atomic bits so that their interactions can be studied precisely.
Lawrence and his cyclotron were based at the University of California at Berkeley, which was fast becoming the world’s leading hotspot for the study of the atom. It was also a stone’s throw from San Francisco, the city where Chien-Shiung Wu landed in the late summer of 1936 after her ship had crossed the vast and turbulent Pacific on her way to graduate school. Wu’s ultimate destination was the University of Michigan, where she planned to study for her PhD, but with some down time before classes began, she decided to pay a visit to the Berkeley campus and its world-class physics department.
Only a few days into her California sojourn, Wu’s plans changed completely. For starters, she made the acquaintance of a fellow Chinese physics student named Luke Yuan, who would go on to become a permanent fixture in her life. Furthermore, after meeting with an obviously impressed Professor Lawrence, she was invited to pursue her graduate work at Berkeley. An opportunity to study under some of the legends of nuclear physics — which included not only Lawrence but also future Manhattan Project director Robert Oppenheimer — was a dream come true for Wu, who desperately wanted to learn as much as she could about the fundamental nature of matter. In an abrupt and daring move, she abandoned her plans to enroll at Michigan.
As a graduate student, "Miss Wu" was quite popular with her peers. She also became notorious for an unwavering work ethic that saw her toiling in the lab well into the small hours of morning on many a night. It was a reputation that would follow her for her entire professional career. "I have always felt," she later explained, "that in physics, and probably in other endeavors, too, you must have total commitment. It is not just a job, it is a way of life."
The truth is, however, that Wu had something of a difficult time adapting to American culture. English was a tricky language to master, and she would spend her adult life fumbling with certain pronunciations and grammatical rules. What’s more, she missed Chinese food and preferred the Chinese style of dress — so much so that she would continue to wear traditional high-necked qipao dresses well into her old age, oftentimes underneath a white lab coat.
Not quite a year after Wu’s arrival in California, international headlines reported devastating news: Japan had invaded China. Since landing in the U.S., Wu had remained in close contact with her parents, brothers and sister, but after the invasion, she wouldn’t hear another word from her family for eight long years. It was a trying time, as horrific updates from the front trickled overseas: By the end of 1937, some 42,000 civilians in her home province of Nanjing alone had been raped or murdered by Japanese troops. Four years later, the conflict would officially merge with World War II after Japan surprised the United States with its attack on Pearl Harbor.
With nothing she could do to help her loved ones, Wu attempted to tune out the war and focus instead on her work. She pursued her thesis under Lawrence and his assistant, another future Nobelist, Emilio Segrè. By 1940, Wu had completed her PhD and was considered an expert — "the authority," according to Robert Oppenheimer — in the new science of nuclear fission, the splitting of large atomic nuclei either by an induced nuclear reaction or by natural radioactive decay.
Ask Miss Wu
Wu stayed on at Berkeley as a research assistant for two years, solidifying her reputation as one of the most capable experimental physicists in the country. It was during this time that scientists led by physics icon Enrico Fermi were attempting, unsuccessfully, to produce the first large-scale, self-sustaining plutonium chain reaction at a research facility in Hanford, Washington. Fermi’s reactions to that point would run for a few hours but then sputter out without explanation.
Legend has it that someone suggested to Fermi that he "ask Miss Wu" for advice. He did, and Wu swiftly deduced that the problem was the buildup of xenon, a plutonium fission by-product. Xenon is an inert noble gas, but it turned out that the particular isotope produced in Fermi’s chain reaction had a tendency to capture stray neutrons.
In 1942, Wu and her new husband, Luke Yuan, moved to the East Coast. While many of her colleagues at Berkeley had been recruited for the war effort, Wu was not asked to join, despite her considerable knowledge of atomic physics. Neither was she asked to remain on at Berkeley in a more permanent role. It was an unfortunate reality that Wu encountered discrimination for being female at a time when most of the top American universities still refused to accept women, either as students or professors. During wartime, she also faced significant ethnic racism.
When Yuan obtained a position at RCA Laboratories in Princeton, New Jersey to work on the development of radar, Wu accepted an assistant professorship at Smith College, a women’s school in Northampton, Massachusetts. The scenario was far from ideal. The newlyweds, living 200 miles apart, only saw each other on weekends in New York City. And while Wu enjoyed teaching upstart female scientists like she had once been, she had very few opportunities to do what she relished most: solve problems in the lab.
It wasn’t long before Wu began to feel unhappy at Smith. When she vented her frustrations to her former advisor, Ernest Lawrence, he recommended her to a number of institutions in need of professors to pick up the slack while many of their staff members were on leave to help with the war. In short order, Wu was offered positions at eight prestigious universities, three of which still barred women from matriculating. She chose Princeton to be near Yuan and, in so doing, became that institution’s first female professor.
The Manhattan Project
Within a few months, Wu was recruited to join the Manhattan Project, the United States' cloak-and-dagger war research and development program. Many of her former professors and colleagues had already spent years working in secret to develop an atomic bomb. Now, Wu would apply her expertise in support of this goal at a New York City warehouse owned by Columbia University.
Contrary to public perception, a fair number of women — many hundreds, certainly, and possibly thousands — were involved in the technical reaches of the Manhattan Project. They were chemists, technicians, doctors, mathematicians, and more. But Wu was one of the very few women who contributed at the highest levels of physics research for this critical war effort.
Aside from her earlier help on Fermi’s plutonium problem, Wu’s work dealt mainly with the enrichment of uranium, the conversion of that element’s most abundant isotope, 238U, which is not fissionable, into the much rarer 235U, which is. In addition, she made major improvements to the Geiger counter, a device that any student of high school physics will recognize today as a common radiation detector.
On August 6, 1945, the work of Wu and thousands of others became known to the world when a uranium bomb was dropped on Hiroshima, Japan, with devastating results. The use of nuclear power, both for international arsenals and for peaceful electricity production, was only getting started. But World War II was about to become history.
The end of the war brought happy news and the turning of several new leaves, both professional and personal, for Chien-Shiung Wu. For starters, after not hearing from her family for eight agonizing years, she finally received word that everyone back home in China was well. Her father was even regarded as a war hero: He had engineered the Burma Road, a crucial transportation route used by the Allies to send supplies to Chinese troops.
Wu was also thrilled to learn that Columbia University wanted her to stay on as a senior researcher. The Morningside Heights neighborhood of Manhattan would in fact become her professional home for the next quarter of a century. It would soon become her personal home as well. After the birth of their son, Vincent, in 1947, Wu and Yuan moved to an apartment just a few blocks from Columbia’s physics building, Pupin Hall.
By this point in her career, Wu had earned a solid reputation as a highly skilled experimental physicist. With the war behind her, she needed a new problem to focus on. Wu chose wisely: Her investigations of beta decay — a mysterious type of radioactivity in which a large atomic nucleus emits energy and morphs into a new element — would help her reshape the world’s understanding of several fundamental atomic processes.
At the time, no one really understood how beta decay worked. Back in 1933, Enrico Fermi had devised what seemed like a viable theory for how an atom’s nucleus, composed of protons and neutrons, could shoot off an electron along with a neutrino and change into a completely different element in the process. But a number of physicists had tried to support Fermi’s theory with experimental data, and their results were muddled at best.
If there was one thing for which Chien-Shiung Wu was known, it was going the extra mile to design experiments in a way that unequivocally elucidated the mechanisms of a system. "She had a very, very strong sense that things had to be done right," Wu’s former graduate student, Leon Lidofsky, told author Sharon McGrayne. "If it was done sloppily, it wasn’t worth doing because the results weren’t reliable."
Wu continued to work on beta decay and related problems for the next decade. Somewhat incredulously, she was overlooked year after year for membership to the Columbia faculty because she hadn’t been assigned to teach. It wasn’t until 1952, eight years after she began her research for the Manhattan Project, that she was asked to join officially.
Two years later, following a lengthy naturalization process, Wu and Yuan became U.S. citizens. It was a decision they’d made after China had become a Communist state in 1949. Unfortunately, due to ongoing tensions between the U.S. and Chinese governments during the Cold War, Wu would not be able to visit her homeland again until the 1970s, by which time most of her immediate family members had died.
Meanwhile, her son, Vincent, was growing up fast. As in her Berkeley days, Wu continued to be a workaholic, so she relied heavily on a nanny for childcare needs. "If my mother was overly busy in her lab, I didn’t feel deprived," said Vincent, who went on to become a successful atomic physicist himself. "I spent most of my time in the company of friends, on school work, or interests that lots of kids of school age have. I always like to figure things out for myself, so it wasn’t like I needed my parents to do my homework for me."
Conservation of parity
In 1956, Wu would once again demonstrate her experimental mojo by achieving something very few people ever have: She disproved a fundamental "law" of nature. Many in the physics community believe she should have shared in the Nobel Prize that was later given for this most significant result of her career, but it did not play out that way.
The law in question is known as the conservation of parity, and it held sway in the physics community for nearly 40 years. Simply put, parity states that nature does not favor right or left. If you watch a girl throw a baseball through a mirror, the laws of physics will be the same both for the girl and for her mirror image.
As physicists in the mid-20th century began to discover a zoo of new subatomic particles, two of these, the theta meson and the tau meson, gave them fits. The theta and the tau shared a number of the same properties, including mass — a result that suggested they might actually be two forms of the same particle. But measurements also showed them decaying into two different parity states, one positive and one negative. If they were in fact the same particle, this would have to mean conservation of parity is not upheld in all cases. It was a troubling concept. At the time, parity was a bedrock law of physics; based on mathematical proofs, it was as well accepted as the laws of gravity. But had it really been proven?
At a scientific conference in April, 1956, renowned theoretical physicist Richard Feynman floated the idea to his colleagues: What if the parity rule were wrong? Fellow theoreticians Tsung Dao Lee of Columbia and Chen Ning Yang of the Advanced Institute for Study in Princeton began to wrestle with this problem. They soon came to believe it possible that parity might not be conserved in some nuclear reactions—specifically, those involving beta decay. But how to test it?
Lee approached Wu, an expert in beta decay, for advice. She suggested a specific approach using an isotope of the element cobalt as the best choice to test the hypothesis. After scouring the literature further, Lee and Yang published a paper stating that conservation of parity had not actually been proven in all cases, and suggesting some experiments to see what was really going on.
Wu immediately got to work. She was uniquely qualified to design and carry out this test, and she wanted to be the first to do it. "Nobody believed it would happen and, because it was so difficult, they wouldn’t tackle it," Yang later told McGrayne. "Wu had the perception that right-left symmetry was so basic and fundamental that it should be tested."
Wu dropped everything for six months — including sleep, meals, and a long-planned trip to China with her husband — to pursue the parity experiment. Even before Lee and Yang’s article was published, she had lined up a team of physicists to assist in carrying it out using special, super-cooling equipment at the National Bureau of Standards (NBS) in Washington, DC. Wu began commuting back and forth between New York and Washington to check on the experiment, while the NBS team worked around the clock to prepare it for its first trials.
Finally, two days after Christmas, the team was ready. Whatever the outcome, Wu and her colleagues knew their results would mark an important moment in the history of nuclear physics. They flipped a few switches, and the experiment was officially underway.
The key factor the team was looking for was the direction in which electrons flung themselves from cobalt nuclei as the nuclei went through beta decay. If conservation of parity were conserved, they would see electrons ejected symmetrically in multiple directions. But if parity were not conserved, the electrons would fly off primarily in one direction. The team’s first results were clear: Electrons were not ejecting symmetrically. In the top left corner of the notepad where they’d jotted their data, team member Ralph Hudson wrote, with triumphant emphasis, "PARITY NOT CONSERVED!"
Wu and her colleagues checked and re-checked their results many times over the next fortnight. At last, around 2 a.m. on January 9, 1957, the team broke out a bottle of champagne. The tau meson and the theta meson were the same particle — now known as the K meson — after all. As Wu later told McGrayne, "These are moments of exaltation and ecstasy. A glimpse of this wonder can be the reward of a lifetime."
The next day, The New York Times heralded the "shattering of a fundamental concept of nuclear physics" on its front page. It was an unforgettable moment for Wu, but also a stark reminder that what we consider "laws" of nature are not necessarily irrefutable in the eyes of science. As fellow physicist Richard Feynman once famously quipped, "If it disagrees with experiment, it’s wrong. In that simple statement is the key to science."
Many honors, but no Nobel
The parity results were so spectacular that they garnered a Nobel Prize that very same year, but not for Wu. In October 1957, the Nobel Committee announced that Lee and Yang had won the physics prize "for their penetrating investigation of the so-called parity laws which has led to important discoveries regarding the elementary particles."
Wu was bitterly disappointed. It was not the first time theorists would win a Nobel while a key experimentalist who did the crucial work to back them up did not. When Wu’s own thesis advisor, Ernest Lawrence, won in 1939 for the invention of the cyclotron, his graduate student, M Stanley Livingston, who did much of the labor translating Lawrence’s vision into a physical, working machine, got nothing.
"As an experimentalist, my natural tendency is to think it a shame that the experimental team was not included in the prize," Wu’s son, whose work at the Los Alamos National Laboratory focuses on neutron physics, admitted recently. "Beyond that, it would be presumptive to have a specific reaction without knowing the internal reasoning of the award committee. I personally think that if she had been included, it wouldn’t have been undeserved. But I don’t harbor any resentment, as she won many other awards for her work."
Wu did indeed rack up an enviable list of honors, awards, and firsts, even before her official retirement from Columbia in 1981. Perhaps this was because she did not slow down after her momentous feat on conservation of parity. Quite to the contrary, over the following two decades, she would carry out many additional ground-breaking investigations, not only in the area of beta decay but also in the fields of short-lived "exotic" atoms and even the biophysics of sickle cell anemia.
Among Wu’s most distinguished honors were: The Comstock Award of the National Academy of Sciences in 1964; the Tom Bonner Prize of the American Physical Society in 1974 (the same year she was named the society’s first female president); the U.S. National Medal of Science in 1975; the Wolf Prize in Physics in 1978; selection as Italy’s Woman of the Year in 1981; and induction into the United States' National Women’s Hall of Fame in 1998. In 1990 she even became the first living scientist to have an asteroid named after her: Asteroid 2752 Wu Chien-Shiung.
Wu’s final lasting contribution came about after her retirement, when she took time to travel the world and speak to audiences of her successes in the lab and of being a woman in a male-dominated field. Just as her father had been many years before, Wu was a champion of women through-and-through. She was not afraid to speak her mind about the miles yet to go before women would achieve any semblance of equal representation in math and the physical sciences. And she fervently hoped that the impressionable girls and young women she spoke to on her travels might take inspiration from her life story and go on to pursue careers in the STEM fields.
That remarkable story came to an end on February 16, 1997, when Wu died of a stroke at the age of 84. In addition to her husband, her son, and a granddaughter, she left behind an enormous legacy. William Havens, a longtime colleague at Columbia, remarked: "She was the world’s distinguished woman physicist of her time." Tsung-Dao Lee, with whom she remained friendly until the end, spelled it plainly: "CS Wu was one of the giants of physics."
Legacy of a courageous hero
Chien-Shiung Wu made a life and a name and for herself in the United States, but it is here, in her hometown of Liuhe, that Wu chose to be buried alongside her husband, Luke, who died in 2003. The circular courtyard where their remains now rest is part of the Mingde School that Wu’s father began nearly a century ago so that his daughter could begin a proper education. It is heart-warming to imagine how proud he would have been to witness the rows and rows of children who now stand in silence, a single yellow flower in hand, as they honor Madame Wu, one of the most influential nuclear physicists of the 20th century, on what would have been her 100th birthday.
Some 160 miles west of here, on the campus of Nanjing University (formerly National Central University), a wonderful museum invites visitors to learn about the incomparable Chien-Shiung Wu. Lining the walls are annotated framed photos of Wu with dignitaries, with colleagues in the lab, and joking around with friends. Thanks to the careful planning of Luke Yuan, who donated many of his wife’s possessions after her death, the gallery feels like a presidential library, with physical awards, honorary degrees, and even a re-created office space with Wu’s books giving visitors a genuine feel for her life and accomplishments.
In a quiet corner of the museum, the words of one Courageous Hero appear as a final remembrance of her lasting legacy: "Science is not static but is dynamic and ever-improving. It is the courage to doubt what has long been believed and the incessant search for verification and proof that pushes the wheels of science forward." ∞
Images: Linocut of Chien-Shiung Wu by Ele Willoughby (used with permission). LEGO minifigure and photo by the author. All other photos are in the public domain, as posted by The Smithsonian on Flickr.
Benczer-Koller, N (2009), Chien-Shiung Wu 1912 – 1997, National Academy of Sciences.
Cooperman, SH (2004), Chien-Shiung Wu: Pioneering Physicist and Atomic Researcher, New York, NY: Rosen Central.
Hammond, R (2010), Chien-Shiung Wu: Pioneering Nuclear Physicist, New York, NY: Chelsea House.
McGrayne, SB (1998), Nobel Prize Women in Science: Their Lives, Struggles, and Momentous Discoveries, Washington, DC: Joseph Henry Press.
Take a virtual tour of the Wu museum at Nanjing University.