The experiment that went wrong—from Wu's perspective

The experiment that went wrong—from Wu's perspective

• Jan 24, 2022
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In the second half of Chapter 8 of his Wu biography, Chiang Tsai-Chien summarizes the events of the Rustad-Ruby affair. Only you will notice that he always puts Ruby first. That may be because Wu herself remembered it that way.

Before Wu started her earthshaking experiment on parity violation in 1956, she continued to focus on research in beta decay. Beta decay was very complicated, with many sometimes conflicting experimental results, which made it difficult to be consolidated into a general theory. She made a regrettable mistake in the process of solving this puzzle. A particular experiment was done by her students, and her name was not on the paper. But people never forgot the mistake as it had come out from Wu’s laboratory, which was well known for its standards of precision and accuracy.

As the saying goes among physicists, “People may not remember all your other experiments with correct results, but will surely remember your single mistake.”

Chiang then undertakes a summary of the physics and the significance of the lettered "transition matrices"—A, T, S, V, and P. There are better places to come up to speed on the physics so I'll skip these pages and get right to the good stuff. 

In the early 1950s, two of Wu’s students, S. Ruby and B. Rustad, performed an experiment to investigate the beta decay in the transition from radioactive helium (He-6) to lithium (Li-6).

Wu held discussions with the students during the experimental process. The students published a short article Physical Review Letters in 1952, followed by a long article in the Physical Review in 1955. They determined that the Fermi theory had a scalar (S) transition matrix, and the Gamow-Teller theory had a tensor (T) transition matrix.

As their experiment had Wu’s endorsement, and she had a long record of precision, the Ruby-Rusted papers initially carried a lot of credibility. Later experiments, however, showed conflicting results.

Richard Feynman, M. Gell-Mann (who won the Nobel Prize for a proposal of “quarks” and their interactions), R. Marshak and his student E. Sudarshan, and another physicist, J. Sakurai, all argued that the transition matrices in beta decay were vector (V) and axial vector (A). Before this was settled, some said that Marshak must be mad. How could the He-6 experiment be wrong?

Not long afterward, Maurice Goldhaber and two collaborators did an elegant experiment and proved that the V-A theory was correct. That settled the dispute.

There are errors and simplifications in the above, but the basic narrative is correct. Now for the ramifications.

Wu was very unhappy about the mistake made in the experiment of Ruby and Rustad. Ruby discussed the experiment in the Plaza Hotel...in January 1990, and regretted that he was so careless. He did not finish his Ph.D. degree, worked for IBM for some time, and resumed research work at Stanford University. Rustad died in the early 1960s.

The incident bothered Wu. She later built a larger experimental setup at Columbia, and did a similar experiment with He-6. She and her collaborator Arthur Schwarzchild wrote a paper in 1958 pointing out the factors causing the mistake in the earlier experiment.

This bad mark did not change very much the position of authority in the field of beta decay that Wu enjoyed. Her reputation as the most precise experimentalist was intact. The saying in the physics circle was: “If the experiment was done by Wu, it must be correct.” 

There are a few things to say here. His summary of Stan's career post the experiment is bullshit. He mentions the Ph.D. but does not give Stan's explanation about it. IBM is mentioned in Stan's resume but not the far more important Westinghouse and Argonne years, nor any of Stan's contributions to nuclear resonance spectroscopy. It feels dismissive.

He mentions twice how the incident affected Wu, and he puts it in the context of her protecting her reputation. Though she is not directly footnoted, I think Chiang gets this interpretation from Wu. It amounts to an acknowledgement that the erroneous experiment posed a serious threat to her carefully crafted reputation both for beta decay expertise and for experimental accuracy.

Her response to this threat was to conduct the rather extraordinary Critical Examination review process together with Schwarzchild, resulting in the unpublished CU-173 report that we have written about. 

The happy result, for Wu if not for her former students, was that her position of authority was unchanged and her reputation remained intact.