Chien-Shiung Wu

Chien-Shiung Wu (31 May 1912 – 16 February 1997) was a Chinese-American experimental physicist. In 1956 she showed that the weak interaction — the force that governs beta decay and all neutrino processes — distinguishes between left and right.

Until then that had been regarded as out of the question.

Career

Wu was born near Shanghai. Her father had founded a school for girls — unusual in China at that time — and vigorously encouraged her education. In 1936 she went to the United States and in 1940 took her doctorate at Berkeley under Ernest Lawrence.

During the war she worked on the Manhattan Project on the enrichment of uranium. She then went to Columbia University in New York, where she remained until her retirement. Her field was beta decay, and in it she was soon regarded as the most precise experimentalist of her time.

The Wu experiment

In 1956 the theorists Tsung-Dao Lee and Chen Ning Yang put forward a disturbing line of thought: perhaps the conservation of parity — the assumption that the laws of nature look the same in a mirror world — did not hold at all for the weak interaction. Nobody had tested it.

Wu took the test on. The set-up was demanding: cobalt-60 nuclei had to be aligned in a magnetic field and at the same time cooled to within a few hundredths of a degree above absolute zero, so that thermal motion would not destroy the alignment. She carried out the work together with the National Bureau of Standards.

The result was unambiguous. The electrons from the decay flew preferentially in one direction — opposite to the nuclear spin. In a mirror world it would have been the other. Nature therefore distinguishes left from right.

The paper appeared in February 1957. In the same year Lee and Yang received the Nobel Prize. Wu did not receive it.

Why this matters for neutrinos

Parity violation is not an incidental finding but the key to understanding the neutrino. Shortly after Wu's experiment, Maurice Goldhaber showed that neutrinos possess a fixed helicity: their spin always points against the direction of flight.

That is precisely what distinguishes the neutrino from every other particle of the Standard Model — and precisely why it was long open whether it can have a mass at all. That question was answered only by neutrino oscillation.

Recognition

Wu received numerous honours. In 1975 she became the first woman president of the American Physical Society; in 1978 she received, as the first person ever, the Wolf Prize in Physics.

The Nobel Prize did not come. Lee and Yang have always emphasised Wu's contribution, and in the field the experiment bears her name to this day. An asteroid and a study centre at Columbia University are named after her.

Her example shows what an experiment can achieve: a conjecture that nobody took seriously became established knowledge within a few months — because someone had the patience and the technique to look closely.

Weak interaction — the field concerned
Helicity — the consequence for the neutrino
Beta decay — her field of work
Standard Model

Sources

C. S. Wu, E. Ambler, R. W. Hayward, D. D. Hoppes, R. P. Hudson: Experimental Test of Parity Conservation in Beta Decay, Physical Review 105, 1413 (1957).
T. D. Lee, C. N. Yang: Question of Parity Conservation in Weak Interactions, Physical Review 104, 254 (1956).