I often lecture on famous female scientists and if I do not mention Chien-Shiung Wu, someone almost invariably asks why. This shows how well known she is among scientists. Not only is Wu highly respected – she is known to some as the “First Lady of Physics” or the “Chinese Marie Curie” – but there is a general opinion that it was an injustice that she did not receive the Nobel Prize for Physics together with Tsung-Dao Lee and Chen Ning Yang in 1957 for her part in the experiment that proved that parity is violated in the weak force. But was this really an example of gender discrimination? To find out, I decided to look into this question and weigh up the evidence.

Born in China in 1912, Wu’s father was himself an advocate of gender equality, founding one of the first schools in China that admitted girls, and he instilled the value of education in his daughter. In 1934 Wu received her bachelor’s degree in physics, graduating at the top of her class from the National Central University in Nanjing. She then did a few years of research but, unsatisfied with the opportunities for physicists in China at that time, moved to the US where she completed a PhD at the University of California at Berkeley in 1940, and then took up a brief research position. In 1942 she married Luke Chia-Liu Yuan, who was the grandson of the first president of the Republic of China.

In 1943, with many physicists in the US working on military projects as the Second World War reached its peak, Wu was offered a teaching position at Princeton University in New Jersey – one of several “firsts” in her career (see “Fair treatment” below). Her appointment was remarkable given that at that time women were not even allowed to study at Princeton. As a young immigrant Chinese woman teaching one of the most difficult subjects – physics – to the male students of Princeton, her presence was unprecedented. But Wu’s teaching time at Princeton did not last long because the following year she was asked to do defence work, joining the Manhattan Project to work on radiation detectors at Columbia University in New York.

In 1945, with the turbulent war years over, Wu started working at Columbia’s physics department where she could continue research in the field that she felt very close to, nuclear physics, and within that, beta-decay – one of the weak interactions associated with radioactive decay. Wu was to stay at Columbia for the rest of her career and took an active interest in physics well into her retirement. Wu died in 1997 at the age of 84 following a stroke.

Concept genesis

The work for which Lee and Yang were awarded the Nobel Prize for Physics in 1957 had its roots in the so-called “tau-theta puzzle”, which perplexed particle physicists in the early 1950s. Tau and theta were two subatomic particles – types of K-meson – the behaviour of which was hard to explain. They were identical in every aspect but one: they had the same mass, the same spin and the same lifetime, but they decayed to products with different net “parity”.

Parity is an intrinsic symmetry property of particles that is characterized by the behaviour of their wave functions under reflection through the origin of their spatial coordinates. In everyday terms, it refers to the relationship between a particle or a process and its mirror image. The mirror image of, say, a right-handed screw is a left-handed screw. Similarly, a particle spinning clockwise produces a mirror image that spins anticlockwise. Based on its behaviour, the parity of a particle is defined as either +1 or –1, and the net parity of a group of particles is the product of the parities of all particles in the group.

The tau-theta puzzle was that the tau decayed to three pions, with a net parity defined as (–1)(–1)(–1) = –1, while the theta decayed to two pions with a net parity of (–1)(–1) = +1. If tau and theta were indeed the same particle – as their other properties indicated – they should have the same parity as well; according to the parity conservation law the parity of a system cannot change under particle decay or production. The implications were that either tau and theta were different particles and we had not learned how to distinguish them yet, or they were the same particle and parity was not conserved. This latter idea was highly controversial.

At a conference in 1956 Lee and Yang opted for the former explanation and suggested that certain elementary particles might occur in two forms with different parities. But during the conference there followed some discussion about the possibility that parity is violated in weak interactions. Lee and Yang later searched the literature and found that there were many cases confirming parity conservation in strong interactions, but in experiments on weak interactions this conservation law had not been tested, so it was impossible to tell whether it was valid for them. Lee and Yang then published their famous paper “Question of parity conservation in weak interactions” (1956 Phys. Rev. 104 254), in which they briefly discussed the possibility that parity might be violated in weak interactions. They also suggested ideas for experiments that might test this possibility, each involving two sets of experiments that were mirror images of each other. If the two gave identical results then parity conservation was valid, while if the two results were different, it showed that parity was violated.

Months before publication of their famous paper, Lee, who also worked at Columbia University, consulted Wu on the subject. As Wu later recounted (1973 Adventures in Experimental Physics: Gamma Volume ed. B Maglich), one day in the spring of 1956 Lee went to Wu’s office and asked her about the status of experimental knowledge on parity conservation in beta decay. According to Wu, “People not only took it for granted that parity was conserved in all interactions, but this untested notion was also used to discourage others from doing any experiments to test, much less challenge, the validity of this concept.”

Wu asked Lee whether anyone had thought of experiments that could show that parity is conserved in weak interactions. Lee mentioned ideas such as using polarized nuclei resulting from nuclear reactions, or a polarized slow neutron beam from a reactor. “Somehow I had great misgivings about using either of these two approaches,” wrote Wu. “I suggested that the best bet would be to use a cobalt-60 beta-source polarized by the demagnetization method.” After Lee’s visit, Wu realized “This was a golden opportunity for a beta-decay physicist to perform a crucial test, and how could I let it pass?”

Competing experiments

And she did not. That spring Wu started planning the experiment and was so keen to do it that she even gave up a trip to China, which she had not visited for some 20 years. The experiment was a complex task because she had to combine two techniques that had never before been used together (see “The parity violation experiments” figure). Even though she was an expert in beta-decay experiments, she lacked the expertise and the equipment to perform them at the required temperatures of near absolute zero. Wu therefore contacted Ernest Ambler at the National Bureau of Standards (NBS, later renamed the National Institute of Standards and Technology) in Washington DC, who was happy to collaborate. In September Wu met Ambler in Washington, and they also invited three NBS associates to work with them: Ralph Hudson, an expert in cryogenics, and radiation-detection experts Raymond Hayward and Dale Hoppes.

The group immediately began to work out the details of the joint experiment and started the measurements in October, working through some serious difficulties. Wu could not be at NBS all the time since she had teaching duties at Columbia. This is why she was not present when, on 27 December 1956, her NBS colleagues saw the first signs of the asymmetry that showed that parity conservation can be violated in weak interactions. As part of my research for my upcoming book on women scientists, I was intrigued by Wu’s story and tried to collect information about it from the scientists who are still around; Hoppes told me in an e-mail that Wu probably regretted her absence to the last day of her life. As soon as she heard the news, naturally, she hurried to Washington. A few days later, back in New York at Columbia University, she told Lee and Yang about the promising preliminary results.

On 4 January 1957 Lee mentioned the great news to the physicists who had gathered for the regular Chinese lunch that took place at Columbia every Friday. That parity violation may be real triggered the imagination of many experimental physicists. One of those was Leon Lederman, who that night at around 8 p.m. called Richard Garwin at his home with an idea for an alternative experiment to demonstrate parity violation. Lederman had realized that the muons produced at Columbia University’s cyclotron might already be polarized and hence also suitable for proving parity violation – Lee and Yang had already suggested trying muon experiments. Garwin, an experienced experimental particle physicist, met Lederman at the cyclotron that very night. Their experiment, for which they used the apparatus built for another project by Lederman’s graduate student, Marcel Weinrich, not only worked but did so very convincingly. Within four days they had compelling results and even had a manuscript ready. However, Lee dissuaded them from submitting, saying that this would not be fair to the NBS team, which had by then been working hard on its experiment for months.

The NBS team must have been glad to see confirmation of the effect but the researchers may have felt disappointed by the competition. Having heard their competitors’ news, they literally worked around the clock until finally on 9 January at 2 a.m. they were absolutely sure that what they had measured was a real effect. Wu later recalled that “Dr Hudson smilingly opened his drawer and pulled out a bottle of wine and put it on the table with a few small paper cups. We finally drank to the overthrow of the law of parity.”

The Department of Physics at Columbia University – with two success stories to boast of – held a press conference on 15 January to announce to the world that a basic law of physics – parity conservation in the weak interactions – had been overthrown. The NBS and the Garwin–Lederman–Weinrich reports were submitted to Physical Review the same day and were published in the February 1957 issue back to back (Phys. Rev. 105 1413; Phys. Rev. 105 1415).

A third paper describing experimental verification of parity violation was submitted by Valentine Telegdi and Jerome Friedman from the University of Chicago and received by Physical Review on 17 January 1957. They had begun their own experiment the previous summer with no knowledge of the NBS attempt (Phys. Rev. 105 1681).

Prize question

Within a year of these historic experiments, Lee and Yang were awarded the 1957 Nobel Prize for Physics – one of the fastest ever Nobel prizes considering that their paper had appeared only in October the previous year. But as their paper suggested but did not prove parity violation, one may wonder whether the 15 January press conference announcing experimental verification of parity violation helped them get the prize – after all, the deadline for submitting Nobel prize nominations is the end of January. Unfortunately, as all nomination records in physics and chemistry have to be kept secret for at least 50 years or for as long as the nominee is still alive, we cannot yet access the official records to see if this was the case.

However, I corresponded with Anders Bárány, the former long-time secretary of the Physics Nobel Committee, who told me that back in 1956 it could not suggest a strong candidate, so when Lee and Yang emerged as truly strong candidates for the 1957 prize, following the experimental verification of parity violation, the committee must have felt pleased to have a compelling recommendation. Bárány’s comments are consistent with the citation of Lee and Yang’s Nobel prize, as it hints at the importance of the discoveries stemming from the theoretical predictions: “for their penetrating investigation of the so-called parity laws which has led to important discoveries regarding the elementary particles”.

And so we arrive at the question at the heart of this story: was it a fair decision not to award Wu a share of the prize? After all, there was an “empty slot” – according to the statutes of the Nobel Foundation a maximum of three people can share a prize in a given category. The NBS experiment was the first to verify parity violation on 27 December and Wu had suggested and was actively involved with the experiment. But Friedman and Telegdi, who also started their experiments in the late summer of 1956 and were already doing measurements in October, may have had some preliminary results by December too. However, what really counts is publications, and in that the NBS group and the Garwin–Lederman–Weinrich group were side by side, with Garwin and Lederman actually finishing their report days before the NBS group did. Whichever way success is measured, it would have been hard to pick one particular experimentalist for the prize.

As it turns out, this discussion is superfluous for legalistic reasons. As Bárány pointed out, “The awarded work must have been published before the year of the prize, in this case before 1 January 1957.” Since all three experimental studies were published in early 1957, none of the experimentalists could have been considered for the prize that year. The Nobel Committee could have decided to wait another year to award the prize for parity violation, but with the three experiments and the large number of physicists involved, the decision would always have been a hard one. Also, the committee needed strong candidates in 1957 and it is by no means certain that they had any as strong as Lee and Yang to put forward.

Of course, irrespective of the Nobel prize, the question of which experiment first observed parity violation is important. Telegdi and Friedman, who began their experiment in the late summer of 1956 and started taking measurements in October without knowing of the other attempt, had their progress hampered when Telegdi had to go to Europe for two months that autumn on personal matters. “During this period,” says Friedman, “I was starting to see a hint of an effect and I wanted to get more scanning help. But [they] would not give it to me, because the only scanners available were involved in what was thought to be a more promising measurement.”

It appears most probable that the NBS team had the first genuine signs of asymmetry, on 27 December, but needed time to verify this under very difficult experimental circumstances. After hearing about these promising preliminary results, Garwin and Lederman began their experiment in early January and it was ready in a flash: they started to measure in the early hours of Saturday 5 January, and – with the machine shut down from Saturday morning until Monday evening – they finished the measurement at dawn on Tuesday 8 January. Theirs was the first clear and conclusive measurement and their article was written on the same day. The Wu et al. paper was completed on 10 January. The two groups submitted their papers on the same day and the papers were received at the journal on 15 January. The Telegdi–Friedman paper was received two days later. Certainly, all the participants of the three papers deserve credit for their hard work, their insight and for embarking on a project that most physicists assumed was a waste of time.

Final thoughts

There is one more question that deserves mention and that is the role of Wu compared with her NBS colleagues in what became known as “the Wu experiment”. I wondered about the propriety of this label because of two statements (see “Who deserves the credit?” below) – one by Telegdi and the other by Nicholas Kurti, then at the University of Oxford, and Christine Sutton, current editor of CERN Courier – that questioned it many years ago. Both statements emphasized the importance of the cryogenic measurements and that without the expertise of the respective specialists the experiments could not have been done. Incidentally, Ambler and Hudson of the NBS team had both been Kurti’s students at Oxford.

I contacted the surviving participants of the experiment carried out at NBS 56 years ago, and from these interactions I formed the impression that the role of Wu and of Columbia University may have been overemphasized during the first euphoric days following the discovery. On 15 January 1957 a press conference was held at Columbia University. Even though the members of the NBS team were present, the fact that the announcement was made at Columbia added emphasis to Wu’s participation. It was also she who had suggested the experiment, which, for brevity, was convenient to call the Wu experiment.

The notion that it was “the Wu experiment” was further strengthened by the fact that on the report about it Wu was listed as first author, followed by her affiliation, and then came the names of the NBS authors in alphabetical order, followed by their affiliation. This way of presenting the authors was suggested by the NBS team. In Ambler’s correspondence with me he said “I invited her to go first in the list of names out of courtesy for having brought the preprint of Lee and Yang’s paper prior to actual publication.” Other NBS authors think that it was their courtesy towards a woman that made them suggest that her name be listed first, despite this being contrary to the usual NBS custom of following alphabetical order. Wu could have declined this honour had she felt it improper, but apparently she did not.

Furthermore, at no point in the report was it mentioned that the experiment was conducted at NBS. Even its terseness – with the paper comprising a mere two pages – does not justify this omission. This was a misleading oversight; having Wu as first author with her Columbia University affiliation, only the initiated could have known that the experiment might not have been carried out at Columbia. When soon after the event one of the NBS authors was giving a talk at Yale University, during the discussion of the experiment someone in the audience interrupted him to ask if that was the Columbia experiment. The speaker had to respond that yes, it was, but it was done at NBS!

The verdict

My view is that Wu made an outstanding contribution to bringing down the axiom of parity conservation in weak interactions. But to say it was an injustice that she did not win a Nobel prize is an oversimplification of a complex story. In spite of the widespread suggestion that it was discrimination against women that prevented her from sharing the Nobel prize with Lee and Yang, there is no indication in her life that suggests such discrimination. Quite the contrary: from very early on, she was highly respected and by the end of her career she had received an extraordinary number of prizes and other distinctions.

There are plenty of cases in the history of science when talented women were truly denied the opportunity to do research, to participate in university life or to receive proper recognition for their achievements. But Wu was certainly not one of these. She was a remarkable scientist and with her perseverance, her thirst for knowledge, her experimental skills and rigour, and her dedication to her students, she was – and will always remain – a wonderful role model for all young people aspiring to start a career in physics.