Chien-Shiung Wu and the Experiment That Broke Mirror Symmetry

Chien-Shiung Wu (1912–1997) was a Chinese American experimental physicist at Columbia University (Atomic Heritage Foundation^[1]). In 1956–57 she led the experiment that showed nature can tell left from right, overturning a symmetry physicists had taken for granted. Our Chien-Shiung Wu sticker puts it in five words: “She Broke Symmetry. And Records.” Her story is one of the best-known cases of missing credit in physics, which we cover in Who Really Discovered It?

What is parity?

Parity is the mirror test. Imagine reflecting an experiment through a point, so every position arrow flips to point the opposite way. In symbols, the parity operation P does this:

P: r → −r, p → −p

Here r is a position and p is a momentum, both ordinary arrows (vectors) that reverse. Richard Feynman’s lectures explain that not every arrow behaves this way (Feynman Lectures, Vol. I, Ch. 52^[2]). Angular momentum, the arrow that describes spin, is built from a cross product of two ordinary arrows, so it does not flip:

L = r × p → (−r) × (−p) = L

Feynman calls ordinary arrows polar vectors and spin-like ones axial vectors. A magnetic field is also axial. This difference is the key to the experiment: in the mirror world, a spinning nucleus keeps its spin direction but anything flying out of it reverses.

The idea that physics is the same in the mirror is called parity conservation. Feynman notes that the laws of gravitation, of electricity and magnetism, and of nuclear forces all satisfy this reflection symmetry, and it was assumed to hold everywhere.

Lee and Yang ask the question

In 1956 the theorists Tsung-Dao Lee and Chen Ning Yang published “Question of Parity Conservation in Weak Interactions” (Lee & Yang, 1956^[3]). Feynman explains the motive: two particles, then called τ and θ, had the same mass and lifetime but decayed in ways that parity conservation said one particle could not. Lee and Yang suggested experiments to test whether parity held in weak decays such as beta decay. The Atomic Heritage Foundation records that they approached Wu because of her expertise in beta decay (Atomic Heritage Foundation^[1]).

How the cobalt-60 experiment worked

The Nobel committee’s 1957 presentation speech gives the plain-language version (NobelPrize.org, 1957 ceremony speech^[4]). Nuclei of radioactive cobalt were cooled to very low temperature and placed in a magnetic field. The nuclei are themselves small magnets, so they lined up like compass needles. The team then measured which way the beta-decay electrons flew out.

1. Line up the spins of cobalt-60 nuclei with a magnetic field, at a temperature low enough that heat does not scramble them.
2. Count the electrons emitted along the spin direction and against it.
3. Compare with the mirror version. In the mirror, the spins stay the same but the electron directions reverse.

If parity held, electrons would come out equally in both directions, because otherwise the real experiment and its mirror image would differ. They did not. With the nuclear north poles pointing up, the electrons were preferentially thrown downward, and the speech states that from this it follows unambiguously that the process lacks right-left symmetry. Feynman describes the same result: with the field pointing up, most electrons go down (Feynman Lectures, Ch. 52^[2]). He also notes that in this decay the cobalt nucleus’s spin changes from 5ħ to 4ħ.

The result was published in 1957 as “Experimental Test of Parity Conservation in Beta Decay” by C. S. Wu, E. Ambler, R. W. Hayward, D. D. Hoppes and R. P. Hudson (Wu et al., 1957^[5]). A second test, using the decay of pions and muons, appeared in the same journal volume (Garwin, Lederman & Weinrich, 1957^[6]). In his Nobel lecture, Lee described the cobalt experiment with polarised nuclei as establishing parity non-conservation without reference to any theory (T. D. Lee, Nobel Lecture^[7]).

Who got the credit?

Lee and Yang received the 1957 Nobel Prize in Physics “for their penetrating investigation of the so-called parity laws which has led to important discoveries regarding the elementary particles” (NobelPrize.org, Physics 1957^[8]). Wu was not included. The Atomic Heritage Foundation notes that her work was not acknowledged in that prize. Recognition came in other forms. She was the first woman to serve as president of the American Physical Society, and in 1978 she received the Wolf Prize in Physics “for her persistent and successful exploration of the weak interaction which helped establish the precise form and the non conservation of parity for this new natural force” (Wolf Foundation^[9]).

She also worked on the Manhattan Project at Columbia, helped confirm Fermi’s theory of beta decay, and wrote the 1965 book Beta Decay, which the Atomic Heritage Foundation calls still a standard reference.

Why it matters today

Parity violation is now a built-in feature of the weak force, the same force that drives the beta decay on our Decay Dancer tee. For how the weak force works, read Beta Decay Unveiled. Wu and other scientists are in our Physicist Stickers collection.

Quick FAQ

What did Chien-Shiung Wu discover?

That the weak force does not respect mirror symmetry: in the beta decay of aligned cobalt-60 nuclei, electrons come out preferentially in one direction relative to the spin.

Why did Wu not get the Nobel Prize?

The 1957 prize went to the theorists Lee and Yang. The committee’s reasons for leaving her out are not stated on the prize page, so we do not speculate.

Does parity violation happen in everyday physics?

Only through the weak force. Feynman notes that gravitation, electromagnetism and nuclear forces satisfy mirror symmetry; weak decays do not.

References

1. Atomic Heritage Foundation / National Museum of Nuclear Science & History, “Chien-Shiung Wu” profile. https://ahf.nuclearmuseum.org/ahf/profile/chien-shiung-wu/
2. R. P. Feynman, R. B. Leighton & M. Sands, The Feynman Lectures on Physics, Vol. I, Ch. 52 “Symmetry in Physical Laws” (Caltech online edition). https://www.feynmanlectures.caltech.edu/I_52.html
3. T. D. Lee & C. N. Yang (1956), “Question of Parity Conservation in Weak Interactions,” Phys. Rev. 104, 254–258 (DOI 10.1103/PhysRev.104.254; metadata verified via Crossref). https://doi.org/10.1103/PhysRev.104.254
4. NobelPrize.org, Award ceremony speech, Nobel Prize in Physics 1957. https://www.nobelprize.org/prizes/physics/1957/ceremony-speech/
5. C. S. Wu, E. Ambler, R. W. Hayward, D. D. Hoppes & R. P. Hudson (1957), “Experimental Test of Parity Conservation in Beta Decay,” Phys. Rev. 105, 1413–1415 (DOI 10.1103/PhysRev.105.1413; metadata verified via Crossref). https://doi.org/10.1103/PhysRev.105.1413
6. R. L. Garwin, L. M. Lederman & M. Weinrich (1957), “Observations of the Failure of Conservation of Parity and Charge Conjugation in Meson Decays: the Magnetic Moment of the Free Muon,” Phys. Rev. 105, 1415–1417 (DOI 10.1103/PhysRev.105.1415; metadata verified via Crossref). https://doi.org/10.1103/PhysRev.105.1415
7. T. D. Lee, “Weak Interactions and Nonconservation of Parity”, Nobel Lecture, 11 December 1957 (NobelPrize.org). https://www.nobelprize.org/uploads/2018/06/lee-lecture.pdf
8. NobelPrize.org, “The Nobel Prize in Physics 1957” (C. N. Yang & T. D. Lee). https://www.nobelprize.org/prizes/physics/1957/summary/
9. Wolf Foundation, “Chien-Shiung Wu” (Wolf Prize in Physics 1978). https://wolffund.org.il/chien-shiung-wu/

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.