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Chien-Shiung Wu Sticker

She Broke Symmetry. And Records. | Physics

\[{}^{60}\mathrm{Co} \to {}^{60}\mathrm{Ni}^{*} + e^{-} + \bar\nu_e\]
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For decades physicists assumed nature could not tell left from right. She checked. Chien-Shiung Wu’s cobalt-60 experiment (1956–57) showed the weak interaction violates parity, or mirror symmetry. A pixel-art portrait on a 3" × 3" sticker; the physics panel below has the equation, every symbol and the sources.

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The physics

In Wu's experiment, cobalt-60 nuclei were lined up by a magnetic field, and the electrons from their beta decay came out preferentially opposite to the nuclear spin. A mirror image of that experiment would show the reverse, so the weak force distinguishes left from right.

The sticker says She Broke Symmetry. Strictly, she showed that nature had never kept it.

Until 1956 physicists assumed parity symmetry: that the mirror image of any physical process is also a possible process, happening at the same rate. Tsung-Dao Lee and Chen Ning Yang pointed out that nobody had actually tested this for the weak interaction, the force behind beta decay, and proposed ways to do it. Chien-Shiung Wu of Columbia University carried out the first, with a group at the National Bureau of Standards in Washington who could reach the necessary cold.

The logic is in the second formula on this page. Line up the spins of cobalt-60 nuclei with a magnetic field and record which way the beta-decay electrons fly. Under a mirror reflection of the whole experiment, an electron's direction of flight reverses, but a spin, like any rotation, does not. So the product of spin and electron direction changes sign. If parity held, the experiment and its mirror image would have to agree, which means that average would have to be zero: no preferred direction at all.

There was one. Keeping the cobalt within a small fraction of a degree of absolute zero, so heat would not scramble the spins, Wu's team found that the electrons came out preferentially opposite to the nuclear spin. The weak interaction tells left from right. The result was published in early 1957. That year's Nobel Prize in Physics went to Lee and Yang. Wu was not included.

Feynman's Lectures describe her experiment in Chapter 52 of the first volume, as the way to tell a distant civilisation, by radio alone, which hand you mean by left.

The sticker broke symmetry. The cobalt-60 did it first, and she was the one who caught it.

Equations

\[{}^{60}\mathrm{Co} \to {}^{60}\mathrm{Ni}^{*} + e^{-} + \bar\nu_e\]
\[\begin{gathered} \hat P:\ \vec J \to \vec J,\ \ \vec p_e \to -\vec p_e \\ \Rightarrow\; \langle \vec J\cdot\vec p_e\rangle \ne 0 \\ \text{violates parity} \end{gathered}\]
SymbolMeaningUnit
\(\vec J\) nuclear spin (axial vector) \(\mathrm{J·s}\)
\(\vec p_e\) electron momentum (polar vector) \(\mathrm{kg·m/s}\)
\(\hat P\) parity (mirror) operation —

Sources

  1. Wu, Ambler, Hayward, Hoppes, Hudson (1957) Experimental Test of Parity Conservation in Beta Decay, Phys. Rev. 105, 1413
  2. Lee & Yang (1956) Question of Parity Conservation in Weak Interactions, Phys. Rev. 104, 254
  3. NobelPrize.org — The Nobel Prize in Physics 1957 (Yang, Lee)
  4. Encyclopaedia Britannica — Chien-Shiung Wu
  5. Feynman Lectures on Physics Vol. I Ch. 52: Symmetry in Physical Laws

Source Wu, C. S. et al. (1957). Experimental Test of Parity Conservation in Beta Decay. Physical Review 105, 1413–1415.doi.org/10.1103/PhysRev.105.1413

Field notes

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