Chien-Shiung Wu

 How Chien-Shiung Wu changed the laws of physics

This Chinese-American pioneer, known as the First Lady of Physics, transformed nuclear science through her work on the Manhattan Project and other groundbreaking experiments.

BREAKING THE LAW OF PHYSICS

Chien-Shiung Wu assembles an electrostatic generator at Smith College physics laboratory, circa 1942.

SCIENCE SOURCE/AGE FOTOSTOCK

BYERIN BLAKEMORE

PUBLISHED MAY 4, 2023

• 10 MIN READ

In 1957 a group of the titans of physics gathered in a lecture hall at Princeton University to be addressed by a diminutive Chinese American woman. As she told the crowd about her recent experiment and its results, men like J. Robert Oppenheimer and George T. Reynolds listened in silence. Then she asked the audience if there were any questions. “The response was dead silence for two minutes,” an observer recalled afterward. “Then thunderous applause and a standing ovation.”

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The woman was Chien-Shiung Wu, and her experiment had just demolished a long-standing pillar of her field—a concept crucial to scientists’ understanding of the world around them. Her results would skyrocket her from an already prominent career into the history books as one of the founding mothers of science. But today she is just as famous for what didn’t happen next as what did.

From China to America

Born in 1912 near Shanghai, China, Wu was influenced by her father, an engineer, and her mother, an educator. Unlike many Chinese women of her time, she received a formal education. As a young teacher in training, she excelled in school. But she was captivated by mathematics and the sciences and began studying them at night. Fascinated by new discoveries and the story of women scientists like Marie Curie, she entered National Central University as a math major in 1930. Soon she transferred to the school’s physics department and began her scientific studies in earnest.

It was a time of rapid change in both the field of physics and China. Domestic unrest and a deteriorating relationship with Japan made life at home uneasy. With the help of an uncle and her academic mentors in China, she decided to immigrate to the United States for graduate school. She would not return to China for 36 years—or ever see her parents again.

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Wu planned to go to the University of Michigan, but a tour of the University of California, Berkeley—and word that a student center at Michigan forbade female students from entering through the front door—changed her mind. This first taste of sexism was just a preview. At the time, physics was dominated by men, and women were largely shut out of the field or shunted into supporting roles.

At Berkeley Wu was visible for her gender and race, and from the start her male colleagues commented as much on her physical appearance as her keen mind. She was “the belle of Berkeley,” historian Sharon Bertsch McGrayne writes, and Wu’s advisor, nuclear physicist and Nobel laureate Emilio Segrè, recalled in his memoir that “when she walked on campus, she was often followed by a swarm of admirers, like a queen.”

When [Wu] walked on campus, she was often followed by a swarm of admirers, like a queen.

EMILIO SEGRÈ

But Wu could hold her own in the laboratory, too. She quickly gained a reputation as a formidable experimentalist—and an expert in the newly discovered phenomenon of nuclear fission. Despite a well-received thesis and ongoing postdoctoral research into fission, though, Berkeley would not hire her after her 1940 Ph.D. Instead she headed to Smith College to teach women physics. The decision to head east with her new husband, physicist Luke Chia-Liu Yuan, was at least partially influenced by growing anti-Asian sentiment sparked by World War II.

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Top secret

The war would provide Wu, and other female scientists, with unprecedented opportunities. As men went to fight, universities reluctantly began hiring women to teach in the sciences. Wu headed to Princeton University, where she became the physics department’s first female instructor. But shortly after her arrival, her career trajectory took an unexpected turn in 1944, when, at the insistence of physicist Enrico Fermi, War Department officials at Columbia University questioned her about aspects of her graduate dissertation on fission. She ended up joining the staff there on a top-secret research program now known as the Manhattan Project.

NEW YORK, NEW YORK

Wu conducted many of her studies at Columbia University in New York City, an important site in the Manhattan Project. Photographed in 1939, two professors stand near the university’s cyclotron.​

BETTMANN/GETTY IMAGES

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“There was an immense need for skilled scientific labor,”says nuclear historian Alex Wellerstein, a professor at the Stevens Institute of Technology. “There was a tremendous sense of urgency.” Wu quickly set to work developing radiation detectors and contributing to research on gaseous diffusion, a process used to enrich uranium that is still used today.

“Gaseous diffusion was absolutely essential for the bomb on Hiroshima,” says Wellerstein. It became the primary enrichment technology for the U.S. during the Cold War—and, says Wellerstein, the technology was so useful that the details are still largely classified today.

Though Wu was almost certainly aware of the end goal of her work on the project, she was reticent to discuss the nuclear bomb and its devastating effects later in life. Instead, in the years that followed, the scientist turned to other avenues in physics—and ended up helping overturn one of its most closely held principles.

FEMALE INFLUENCE

Labor shortages during World War II led to women working on the Manhattan Project. These technicians worked in the Y-12 Plant at Oak Ridge, Tennessee.​

CBW/ALAMY

Wu wasn't the only woman whose career was launched by the Manhattan Project, the top-secret attempt to build the first atom bomb. The war effort was paramount, and government officials “didn’t have the luxury of being as sexist or racist as they otherwise would have been,” says Wellerstein. They were desperate to add respected scientists to develop an atom bomb. Wu was one of just a handful of esteemed female physicists during World War II. “Women worked on every aspect of the Manhattan Project, except combat operations,” write Ruth H. Howes and Caroline L. Herzenberg. Though the total number of women who contributed remains unclear, their numbers exceeded those of the women who contributed to the Apollo program during the 1960s.

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Rule breaker

As a researcher at Columbia after the war (it would take until 1952 for her to become the physics department’s first tenured woman professor), she dove into the particulars of radioactive decay and began looking into the properties of subatomic particles. She worked with Tsung-Dao Lee of Columbia and Chen-Ning Yang of Princeton, two physicists who suspected that the newly discovered K-meson, also known as a kaon, violated the long-standing theories of parity and symmetry.

Most particles decayed symmetrically, emitting electrons from both ends. But the K-meson behaved unexpectedly, and Lee and Yang thought further experimentation might help solve the question of whether the law of parity conservation, which stated that physical systems act symmetrically and behave the same even when they are mirrored, was actually valid.

BREAKING THE LAWS OF PHYSICS

Chien-Shiung Wu works in the lab in an undated photograph.

SCIENCE SOURCE/AGE FOTOSTOCK

In 1956 Wu made history with an experiment that toppled the assumption that physical reactions don’t always demonstrate mirrored symmetry. The Wu Experiment, as it is now known, took place amid a scientific scramble to prove that a radioactive isotope can emit electrons asymmetrically while it decays. Wu and her colleagues did the experiment at low-temperature facilities at the National Bureau of Standards in Washington, D.C. It was “a golden opportunity,” Wu later recalled—one that involved growing her own cobalt crystals, boring them with a dentist’s drill, freezing them, and aligning them in a magnetic field. Wu had just a 15-minute window to conduct the experiment, but those 15 minutes opened the door for a more modern understanding of particle physics. That was enough time for her.

It would be Wu who suggested an experiment with the radioactive isotope cobalt-60 and who carried that experiment out. Working with experimentalists at the National Institute of Standards and sleeping just four hours a night, she planned an ambitious experiment that would cool radioactive cobalt crystals near absolute zero, then align their nuclei with a powerful magnet to study whether their emissions happened on both right and left sides.

After months of experimentation, Wu finally confirmed that the nuclei emitted electrons on one side but not the other, successfully negating the law of parity. It was a groundbreaking discovery, and a celebrated one. “These are moments of exaltation and ecstasy,” Wu later said. “A glimpse of this wonder can be the reward of a lifetime.”

“These are moments of exaltation and ecstasy,” Wu later said. “A glimpse of this wonder can be the reward of a lifetime.”

Although Wu and her colleagues were toasted, and Wu became a scientific celebrity outside the field, her contribution was overlooked by the 1957 Nobel Prize committee, which awarded the theorists Lee and Yang the prize in physics for the discovery. Wu had designed and carried out an experiment that fundamentally changed the way physicists think about the universe, setting the stage for the further development of the standard model that governs particle physics. But the jury’s still out on whether sexism, political wrangling, or some other factor affected what many see as her exclusion from the prize. “Until historians are able to view the deliberations of the committee itself, the answer isn’t clear,” says Wellerstein.

To those looking back at Wu’s legacy as a founding mother of physics, though, her influence is impossible to deny. Until her retirement in 1981, she was one of the world’s most respected experimentalists, helping design other influential experiments in everything from magnetism to nuclear fluoroscopy. In 1975 she received the National Medal of Science, and in 1978, by unanimous vote, she became the first recipient of her field’s other most prestigious award, the Wolf Prize. She died in 1997, a titan of the field in her own right.