Madam Wu – The First Lady of Physics

It was 1942, the tense days of World War II. In the Manhattan Project, uranium and plutonium—the key materials for atomic bombs—were being produced in several secret reactors. In the southeastern region along the Columbia River, the Hanford Site had been built to produce plutonium.

But a problem appeared: after the reactor was started, just as the chain reaction began, it would shut down on its own. This kept happening repeatedly. The project director, Enrico Fermi, grew concerned. Only four years earlier, he had received the Nobel Prize for his work on neutron interactions. He was expected to understand the behavior of neutrons better than anyone.

Fermi realized that once the chain reaction began inside the reactor, some of the fission products being formed were somehow absorbing neutrons—causing the chain reaction to stop. But he could not determine which specific fission product was responsible. None of the other scientists and engineers on the project could answer the question either. Yet they all had the same suggestion: “Ask Miss Wu."

Who was this Miss Wu?

It turned out that this 30-year-old Chinese woman was Chien-Shiung Wu. Just two years earlier, she had completed her PhD under the supervision of Ernest Lawrence, the inventor of the cyclotron. Lawrence, who won the Nobel Prize in Physics in 1939, was one of the leading scientists of the Manhattan Project and the director of the Radiation Laboratory, where plutonium had been discovered.

Enrico Fermi contacted Miss Wu. After analyzing the data sent by Fermi, Wu quickly identified the cause of the Hanford reactor problem. She determined that among the fission products, the isotope Xenon-135 was being produced, and it rapidly absorbed neutrons. As a result, there were not enough neutrons left in the reactor to sustain the chain reaction, causing the reactor to shut down.

Fermi was deeply impressed by Miss Wu’s brilliance and her practical expertise in nuclear physics. With her help, the problem at the Hanford reactor was solved. When Fermi requested the chief scientist of the Manhattan Project, J. Robert Oppenheimer, he readily agreed to appoint Chien-Shiung Wu as an expert scientist on the project.

During those years of World War II, nuclear physics experiments were carried out on a scale that would have been nearly impossible under normal circumstances. The Manhattan Project was so secret that most of the people working in its laboratories did not even know the larger purpose of their work. Between 1942 and 1945, about 130,000 people worked on the project. Of these, only around 1,500 to 2,000 were engineers and scientists, and the core scientific research was led by roughly a hundred scientists—many of them world-renowned physicists exiled from Europe.

Chien-Shiung Wu was the first and only Chinese scientist to work on the Manhattan Project. She was not only a pioneer there; she went on to achieve many historic milestones in physics. For her groundbreaking contributions, she became known as the “First Lady of Physics” and the “Queen of Nuclear Research.” She was often compared to Marie Curie and was even called the “Chinese Marie Curie.”

Chien-Shiung Wu was born on May 31, 1912, in a small village called Liuhe, near Shanghai. Her father, Zhong-Yi Wu, was an engineer who was deeply committed to education and intellectual independence for both boys and girls. To promote girls’ education, he established a school himself, and Wu began her early education there.

In 1923, after completing her primary education at that school, Wu enrolled in the Suzhou School for Girls. She graduated in 1929 from its “Normal School Program,” a teacher-training track designed to produce ideal educators and expand education across China. After completing this program, she spent a year studying at the Shanghai Gong Xue Public School.

In 1930, Wu enrolled at National Central University in Nanjing. She initially studied mathematics but switched to physics in her second year. With the rise of Quantum Mechanics and the growing excitement in fields like nuclear physics, she became deeply fascinated by the subject. In 1934, she graduated with a bachelor’s degree in physics with honors and record-high marks.

During 1935–36, Wu began research in X-ray crystallography. Physics research quickly became her passion. Meanwhile, the tensions in Europe were escalating toward World War II. Many prominent scientists, including Albert Einstein, had already moved to the United States.

With financial support from her uncle, Wu traveled to the United States in 1936. She had planned to enroll in a graduate program at the University of Michigan. Before doing so, she visited the University of California Berkeley. There, she met another Chinese student, Luke Chia-Liu Yuan. While showing her around the physics department, Yuan took her to the office of Professor Ernest Lawrence.

After a brief discussion about recent developments in physics, Lawrence quickly recognized Wu’s exceptional talent. But when he heard that she planned to pursue her PhD at the University of Michigan, he was surprised and told her:

“But Michigan still does not admit female students.”

Chien-Shiung Wu was unaware of this fact. There was no point in going to the University of Michigan anymore. Instead, she enrolled in a PhD program under Ernest Lawrence. Working in Lawrence’s lab alongside Professor Emilio Segrè, Wu developed exceptional expertise in nuclear fission. She earned her PhD in 1940, with her research focusing on the fission products of uranium—skills that would later prove crucial in the Manhattan Project.

After completing her PhD, Wu wished to continue research at the University of California Berkeley. However, at that time, American universities were still reluctant to appoint women or people of Asian or African descent to faculty or permanent research positions.

By 1942, during World War II, most physicists were engaged in war-related projects. As a result, universities began hiring talented individuals from diverse backgrounds for teaching roles. Wu secured a position at Smith College and moved from California to Massachusetts.

This move also had a personal advantage. The Chinese student she had first met in America, Luke Chia-Liu Yuan, had by then become her partner. After completing his PhD at California Institute of Technology in 1937, he had taken a position at Princeton University, working on radar research. In 1942, Chien-Shiung Wu married Luke Chia-Liu Yuan.

Wu had no difficulty teaching at Smith College, but the lack of research opportunities there frustrated her. So, in 1943, when she was offered a research position at Princeton University, she left Smith College and joined Princeton.

Chien-Shiung Wu became the first female faculty member at Princeton University—before her, no woman had ever been appointed to a teaching position there.

Recognizing Chien-Shiung Wu’s exceptional talent, Enrico Fermi and J. Robert Oppenheimer appointed her as a senior scientist in the Manhattan Project in March 1944.

At Columbia University, she supervised work on the gas diffusion process for separating uranium isotopes and conducted research on neutron interactions. At the same time, she also contributed to the development of improved Geiger counter devices for measuring nuclear radiation.

After the war ended, in 1946, Wu joined Columbia University as a research associate. She was promoted to associate professor in 1952 and became a full professor in 1958. She spent the rest of her distinguished career at Columbia University.

At Columbia University, Wu had a close professional relationship with the Chinese theoretical physicist Tsung-Dao Lee. In the mid-1950s, Lee and another Chinese theoretical physicist, Chen-Ning Yang, began questioning a fundamental principle in particle physics—the Parity Conservation.

From experimental evidence, they knew that parity conservation held true for electromagnetic interactions and the Strong Nuclear Force. However, it had never been tested for the Weak Nuclear Force. Their theoretical analysis suggested that parity might not be conserved in weak interactions—that is, parity violation could occur. But without experimental proof, their idea would remain speculative.

Lee and Yang designed several experimental setups on paper to test parity conservation in the laboratory. Lee then sought Wu’s collaboration to implement one of these experiments—selecting the apparatus, constructing it, and determining the experimental procedure.

Wu designed an experiment using a sample of the radioactive isotope Cobalt-60. The sample was cooled to extremely low (cryogenic) temperatures using liquid gases. Cobalt-60 undergoes beta decay, emitting beta particles. The ultra-low temperature was necessary to reduce the thermal motion of the atoms nearly to zero. At the same time, Wu applied a constant magnetic field to the sample so that the nuclear spins of the atoms would align in the same direction.

This cryogenic experiment required highly specialized facilities for handling liquid gases, which were available at the National Bureau of Standards (NBS). Therefore, Wu transported her apparatus to the NBS headquarters in Maryland, where she successfully carried out the experiment.

According to the theoretical calculations of Tsung-Dao Lee and Chen-Ning Yang, if the law of Parity Conservation were not valid, then the beta particles emitted from Cobalt-60 would be emitted asymmetrically. Wu’s experimental results showed that this was indeed the case—parity is not conserved in the Weak Nuclear Force. Later, her colleagues at Columbia University confirmed the result through independent experiments.

The findings were published in the same issue of Physical Review through two separate papers, and were soon verified by laboratories around the world. This discovery of parity violation became a landmark contribution to high-energy physics and the development of the Standard Model.

For their theoretical work, Lee and Yang were awarded the Nobel Prize in Physics in 1957. However, for reasons that remain controversial, Chien-Shiung Wu was not included, even though she provided the crucial experimental proof of their theory. Despite being nominated 23 times, Wu never received the Nobel Prize—an omission often regarded as a significant blemish in the prize’s history.

To honor scientists who made Nobel-worthy contributions but were overlooked, the Wolf Prize was established in 1978. Professor Wu became the first recipient of the Wolf Prize in Physics.

Wu also provided the most convincing experimental verification of Enrico Fermi’s theory of Beta Decay. Her book Beta Decay remains one of the most authoritative references in nuclear physics.

Building on the famous EPR Paradox proposed by Albert Einstein, Boris Podolsky, and Nathan Rosen, Wu experimentally confirmed the correlation between the polarizations of two oppositely moving photons. This became one of the earliest experimental validations of quantum predictions.

In the world of physics, she became widely known as “Madam Wu.” In 1958, she was elected a fellow of the National Academy of Sciences. She was also the first woman in the history of Princeton University to receive a Doctor of Science (DSc) degree. In 1975, she was elected president of the American Physical Society—becoming its first female president. That same year, she received the National Medal of Science.

Chien-Shiung Wu passed away on February 16, 1997. Throughout her life, she actively advocated for equal rights for women in academia and beyond.

In 2021, the United States Postal Service honored her by issuing a “Forever USA” stamp in her name. Only a handful of physicists have received this recognition, placing her alongside greats such as Albert Einstein, Enrico Fermi, Richard Feynman, Robert Millikan, John Bardeen, and Maria Goeppert Mayer.

References:

1. Physics Today, Volume 77, Issue 12, December 2024.
2. Ruth Howes, Their Day in the Sun: Women of the Manhattan Project, Temple University Press, Philadelphia, 1999.
3. Sai-Chi Yen Chiang, Madame Wu Chien-Shiung: The First Lady of Physics, World Scientific, 2013.
4. Biographical Encyclopedia of Scientists, World Book, Chicago, 2003.
5. Anna Reser and Leila McNeill, Forces of Nature: The Women Who Changed Science, Frances Lincoln, London, 2021.