In late 1956 and early 1957, the experimental physicist Chien-Shiung Wu of Columbia University led a small team in performing one of the most consequential experiments of the twentieth century. The experiment used a sample of radioactive cobalt-60 cooled to about 0.01 K and placed in a strong magnetic field, with detectors monitoring the directions of emitted electrons.
The cobalt-60 was chosen because its nuclear spin can be aligned by a magnetic field at low temperature, breaking the rotational symmetry of the system. When the nuclei decay by beta-electron emission, the emitted electrons can either come out preferentially in the direction of the nuclear spin, preferentially against it, or symmetrically. If parity (the symmetry under spatial reflection) was a real symmetry of the weak interaction, the emission had to be symmetric. If parity was violated, an asymmetry would appear.
Wu found a clear asymmetry. The electrons came out preferentially against the direction of the nuclear spin — by a factor of about 1.4 to 1 in the easily-measured polarization. The weak interaction violated parity.
The result, announced in January 1957, immediately overturned a foundational assumption of physics. Parity, which had been thought to be an exact symmetry of nature, was definitively shown to be broken by one of the four fundamental forces. Tsung-Dao Lee and Chen-Ning Yang — who had proposed the experimental test a few months earlier — received the 1957 Nobel Prize in Physics. Wu, who had actually performed the experiment, did not.
What “parity” actually means
In physics, parity (P) is the operation of mirror reflection — replacing every position vector r with −r. A parity-symmetric physical law looks the same in a mirror as it does in the original. Most physical laws known up to 1957 obeyed this symmetry. Newton’s laws, Maxwell’s equations of electromagnetism, the gravitational interaction, the strong nuclear force — all were parity-symmetric.
The question of whether the weak interaction was also parity-symmetric had simply never been carefully tested. By 1956, several anomalies in the decay patterns of certain “strange” particles — particularly the so-called “tau-theta puzzle” involving the decays of kaons — had suggested that something unusual was going on. Lee and Yang, both Chinese-American physicists at Columbia and Princeton respectively, undertook a careful theoretical survey and concluded that the weak interaction’s parity behavior had never been directly verified.
In a paper submitted in June 1956, Lee and Yang noted that the answer was uncertain and proposed several experiments that could resolve it. One specific suggestion was a measurement of the angular distribution of electrons emitted in beta decay from oriented nuclei. Wu — already an expert in beta-decay measurements — saw the proposal, recognized its importance, and immediately set about designing the relevant experiment.
The experimental challenge
The basic idea is simple in principle: align the spins of decaying nuclei in a known direction, then measure whether the emitted electrons come out preferentially in any particular direction relative to that spin.
The practical difficulty is alignment. At ordinary temperatures, the thermal motion of atoms randomizes nuclear spin orientation. To produce significant alignment, you need either extremely strong magnetic fields (impractical) or extremely low temperatures (technically demanding).
Wu’s experiment combined a strong magnetic field with cooling to about 0.003 K — barely above absolute zero. This required a complex cryogenic apparatus including a paramagnetic salt (cerium magnesium nitrate) used as a cooling stage via adiabatic demagnetization.
The cobalt-60 sample was placed in the cooled apparatus, the magnetic field was applied to align the nuclear spins, and detectors counted the electrons emitted both along and against the nuclear-spin direction. By comparing the count rates in the two directions, Wu could measure the parity asymmetry.
The experiment was performed at the National Bureau of Standards in Washington, D.C., where the necessary low-temperature equipment was available, with collaborators Ernest Ambler and others. Wu directed the experiment but the actual cryogenics required NBS expertise.
The result and its impact
By December 1956, Wu and her collaborators were seeing a clear asymmetry. The electron emission was anisotropic in exactly the way that parity violation would produce. The result was rushed to publication.
The asymmetry was about 40% — much larger than the precision of the measurement. The data were not ambiguous. The weak interaction violated parity by a substantial amount.
The result was published in Physical Review in February 1957, with Wu as first author. The acceptance in the physics community was almost immediate. By the end of 1957, the implications had been worked out in detail, and several follow-up experiments by other groups (Lederman’s at Columbia, Telegdi’s at Chicago) had confirmed the result in different physical systems.
The Nobel Prize for the discovery was awarded to Lee and Yang for the theoretical proposal in October 1957 — less than a year after Wu’s experimental result. The speed of the recognition was unusual. The omission of Wu was widely noted at the time and has remained a point of historical discussion ever since.
What parity violation meant for neutrinos
Wu’s experiment did not directly measure neutrinos. The cobalt-60 nuclei emitted electrons (visible to the detectors) and antineutrinos (invisible). The asymmetry Wu measured was in the electron angular distribution.
But the implications for neutrinos were immediate. The same beta-decay process that produced the asymmetric electrons also produced antineutrinos. If parity was violated maximally — which the data suggested — then the antineutrinos themselves had to be produced in a specific helicity state. They had to be right-handed (their spin parallel to their direction of motion).
By extension, neutrinos (rather than antineutrinos) had to be produced in the opposite helicity — left-handed. The weak interaction couples only to left-handed neutrinos and right-handed antineutrinos. This conclusion was confirmed within a few years by direct measurements of neutrino helicity (notably the 1958 Goldhaber-Grodzins-Sunyar experiment).
The “left-handedness” of the neutrino became a defining feature of the Standard Model, with deep consequences for how neutrinos appear in the broader theory. As discussed in our chirality and handedness article, this feature is still one of the most striking and least-fully-understood aspects of the weak interaction.
Wu’s broader career
Chien-Shiung Wu’s career extended far beyond the parity-violation experiment. Some of her other major contributions:
The Manhattan Project. Wu joined Columbia in 1944 and worked on the development of gaseous diffusion for uranium-235 enrichment as part of the wartime project.
Beta-decay physics. Throughout the 1940s, 1950s, and 1960s, Wu was one of the world’s leading experts on the detailed properties of beta decay. Her textbook on the subject was a standard reference for decades.
The Goldhaber-Grodzins-Sunyar experiment measured the helicity of the neutrino emitted in europium-152m electron capture. Wu was not a direct author of the paper, but the experiment was inspired by and built on her parity-violation work.
Vector-current conservation tests. Wu’s group performed several precision tests of the conserved vector current hypothesis, which is a foundational element of the modern theory of weak interactions.
Academic leadership. Wu became a full professor at Columbia, was elected to the National Academy of Sciences, and received numerous awards (the National Medal of Science in 1975, the Wolf Prize in 1978, and many others). She was active in mentoring younger physicists, particularly women in physics, and was a strong advocate for women’s participation in science.
She died in 1997 at age 84, in New York City. Her ashes were eventually interred at her birthplace in Liuhe Town, in Jiangsu Province, China.
The Nobel question
The decision to award the 1957 Nobel to Lee and Yang but not to Wu has been the subject of substantial historical analysis. Several factors have been cited.
Nobel rules limit laureates to three. The Nobel Committee had to choose among multiple plausible candidates. Lee and Yang’s theoretical proposal was the trigger for the experiment; including Wu would have meant excluding one of them or finding a way to add a fourth person.
Theory-versus-experiment biases. The Nobel Committee has historically favored theoretical contributions over experimental ones in cases where both were involved. This bias has been criticized over time and has been corrected in some later awards.
Gender bias. Wu was one of the few prominent female physicists of her era. The Nobel Committee’s record on female laureates has been the subject of extensive criticism. Wu’s omission is often cited as one example of broader gender-related gaps in Nobel recognition.
Wu’s own assessment. In her later writings, Wu expressed some frustration with the omission but emphasized that the actual scientific contribution was what mattered to her, not the prize. Her career and influence continued for decades after 1957, including major contributions that did not depend on Nobel recognition.
The historical record now treats Wu as a co-equal contributor to the parity-violation discovery, even without the formal Nobel recognition. Her name is on the experimental paper, her experimental design is the basis for the discovery, and her broader career is widely recognized as Nobel-worthy in retrospect.
A simple result with enormous consequences
The Wu experiment is one of the cleanest examples in modern physics of an unambiguous experimental result that immediately restructured a foundational principle. Before January 1957, parity was assumed to be an exact symmetry of nature. After Wu’s announcement, the weak interaction was understood to violate parity maximally, and the entire structure of weak-interaction theory had to be rebuilt.
The reconstruction took only a few years. By the early 1960s, the modern V-A theory of weak interactions — incorporating maximal parity violation as a foundational feature — was in place. By the late 1960s, the broader electroweak theory was being developed. By the 1970s, the Standard Model itself was assembled.
The Standard Model that emerged was a theory that fundamentally distinguished left from right. The weak interaction operates on left-handed components only. Neutrinos are produced left-handed. The universe, at the most fundamental level, is not symmetric under mirror reflection.
That single fact about nature was experimentally established in a Washington, D.C., low-temperature laboratory in late 1956, with cobalt-60 as the source and Chien-Shiung Wu as the lead experimenter. It is one of the foundational moments of twentieth-century physics.
For the consequences for neutrinos specifically, see Why neutrinos are left-handed. For the broader theoretical context, see How neutrino oscillation works and The Higgs and neutrino mass. For the 2002 Nobel Prize that recognized later neutrino-astronomy work, see The 2002 Nobel Prize.
Frequently asked
Who was Chien-Shiung Wu?
Chien-Shiung Wu (1912–1997) was a Chinese-American experimental physicist at Columbia University. She earned her PhD at Berkeley in 1940 and joined Columbia in 1944, working on the Manhattan Project during the Second World War. Wu was a leading expert in beta decay and weak-interaction physics by the 1950s, with a reputation for extraordinary experimental precision.
What did the Wu experiment show?
The experiment demonstrated that parity is violated in the weak nuclear interaction. By aligning the nuclear spins of cobalt-60 atoms in a strong magnetic field at very low temperatures, Wu was able to measure the angular distribution of the electrons emitted in beta decay. She found a clear asymmetry: electrons were preferentially emitted in the direction opposite to the nuclear spin. A parity-conserving universe would have produced symmetric emission. The asymmetry was unambiguous.
Why did this matter?
Before Wu's experiment, physicists generally assumed that parity — the symmetry under spatial mirror reflection — was an exact symmetry of nature, like energy conservation. Lee and Yang had proposed in 1956 that parity might be violated specifically by the weak interaction, but the proposal needed experimental verification. Wu's result confirmed the proposal definitively and triggered immediate reconsideration of the structure of the weak nuclear force.
Why didn't Wu share the 1957 Nobel Prize?
The 1957 Nobel Prize in Physics went to Lee and Yang for the theoretical proposal of parity violation. Wu — who performed the decisive experiment — was widely considered a natural co-laureate but was not included. The omission has been the subject of long-running discussion in physics history, and is often cited as one of the more conspicuous gender-related gaps in Nobel-recognition history.
How does this connect to neutrinos?
Parity violation in the weak interaction is essentially equivalent to the statement that neutrinos are 'left-handed' — their spin points opposite to their direction of motion. The Wu experiment did not directly measure neutrinos, but its discovery of parity violation immediately implied that the weak interaction treats left-handed and right-handed particles asymmetrically. Subsequent experiments specifically established that neutrinos are produced only in left-handed states, a key feature of the Standard Model.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, December 30). The Wu experiment: how a Columbia physicist showed nature distinguishes left from right. Neutrino Times. https://neutrino-times.com/articles/wu-experiment-1957-cobalt-60-parity-violation/
Chicago
Neutrino Times Editorial Team. "The Wu experiment: how a Columbia physicist showed nature distinguishes left from right." Neutrino Times, December 30, 2025. https://neutrino-times.com/articles/wu-experiment-1957-cobalt-60-parity-violation/.
MLA
Neutrino Times Editorial Team. "The Wu experiment: how a Columbia physicist showed nature distinguishes left from right." Neutrino Times, 30 Dec. 2025, https://neutrino-times.com/articles/wu-experiment-1957-cobalt-60-parity-violation/.
BibTeX
@misc{neutrino-times-wu-experiment-1957-cobalt-60-parity-violation,
author = {Neutrino Times Editorial Team},
title = {The Wu experiment: how a Columbia physicist showed nature distinguishes left from right},
howpublished = {Neutrino Times},
year = {2025},
month = {dec},
url = {https://neutrino-times.com/articles/wu-experiment-1957-cobalt-60-parity-violation/},
note = {Accessed: 2025-12-30}
} RIS
TY - GEN TI - The Wu experiment: how a Columbia physicist showed nature distinguishes left from right AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-12-30 PB - Neutrino Times UR - https://neutrino-times.com/articles/wu-experiment-1957-cobalt-60-parity-violation/ ER -