Ada Lovelace Day: Chien-Shiung Wu

By Sean Carroll | March 25, 2009 9:12 am

240x240_wu.jpg March 24 was designated Ada Lovelace Day. To honor the world’s first computer programmer, bloggers posted something about a woman who made a significant contribution to science or technology. Serious bloggers wrote detailed and engaging pieces, but we overdue authors don’t have time for that. So instead, only one day late, here’s a short excerpt from my book draft, about Chien-Shiung Wu and the discovery of parity violation.


It came as quite a surprise in the 1950’s when parity was shown not to be a symmetry of nature, largely through the efforts of three Chinese-born American physicists: Tsung-Dao Lee, Chen-Ning Yang, and Chien-Shiung Wu. The idea of parity violation had been floating around for a while, suggested by various people but never really taken seriously. In physics, credit traditionally accrues not just to someone who makes an offhand suggestion, but to someone who takes that suggestion seriously enough to put in the work and turn it into a respectable theory or a decisive experiment. In the case of parity violation, it was Lee and Yang who sat down and performed a careful analysis of the problem. They discovered that there was ample experimental evidence that electromagnetism and the strong nuclear force both were invariant under P, but that the question was open as far as the weak nuclear force was concerned.

Lee and Yang also suggested a number of ways that one could search for parity violation in the weak interactions. They finally convinced Wu, who was an experimentalist specializing in the weak interactions and Lee’s colleague at Columbia, that this was a project worth tackling. She recruited physicists at the National Bureau of Standards to join her in performing an experiment on Cobalt-60 atoms in magnetic fields at very low temperatures.

As they designed the experiment, Wu became convinced of the project’s fundamental importance. In a later recollection, she explained vividly what it is like to be caught up in the excitement of a crucial moment in science:

Following Professor Lee’s visit, I began to think things through. This was a golden opportunity for a beta-decay physicist to perform a crucial test, and how could I let it pass? — That Spring, my husband, Chia-Liu Yuan, and I had planned to attend a conference in Geneva and then proceed to the Far East. Both of us had left China in 1936, exactly twenty years earlier. Our passages were booked on the Queen Elizabeth before I suddenly realized that I had to do the experiment immediately, before the rest of the Physics Community recognized the importance of this experiment and did it first. So I asked Chia-Liu to let me stay and go without me.

As soon as the Spring semester ended in the last part of May, I started work in earnest in preparing for the experiment. In the middle of September, I finally went to Washington, D. C. for my first meeting with Dr. Ambler. … Between experimental runs in Washington, I had to dash back to Columbia for teaching and other research activities. On Christmas eve, I returned to New York on the last train; the airport was closed because of heavy snow. There I told Professor Lee that the observed asymmetry was reproducible and huge. The asymmetry parameter was nearly -1. Professor Lee said that this was very good. This result is just what one should expect for a two- component theory of the neutrino.

Your spouse and a return to your childhood home will have to learn to wait – Science is calling! Lee and Yang were awarded the Nobel Prize in Physics in 1957; Wu should have been included among the winners, but she wasn’t.

  • joe

    I never knew there was parity violation in the “week” interactions. I know Saturday and Sunday are treated differently from the other days of the week, but that can hardly be considered a parity violation.

    Now that I’m done having fun at the expense of your typo, do I get a prize for proof reading a section of your book?

  • Sean

    We don’t give prizes for finding typos, we dole out punishments. But in this case we’ll let it pass.

  • Bryan

    Sorry for the nitpicking, but it’s Chen-Ning Yang.

  • John R Ramsden

    Has anyone come up with an explanation, speculative or otherwise, for parity violation? Or is that one of those “why”s that professionals frown on?

    Also, is it likely to have any bearing on the Universe’s apparent slight matter-antimatter asymmetry.

  • David B.

    Dear John:

    The Standard model of particle physics explains parity violation. Indeed, the weak interactions violate parity to the maximum amount allowed by nature. The reason for Parity violation lies in the representations of the Lorentz group for particles with zero rest mass. They are classified by helicity (the spin along the direction of motion). This allows a massless particle to set up a preferred orientation for a coordinate system that is universal for all observers: left handed or right handed. Basically, this is because you can not set up an experiment where you overtake a particle traveling at the speed of light. This is what you would need to do in order to change one spin orientation into the other one. In general theories left and right handed massless particles interact differently, as they are not related to each other by a symmetry. This property survives even after particles get a mass via the Higgs mechanism. This is called chirality, and general chiral theories break parity.

    Parity violation is not enough to describe the matter-antimatter asymmetry. You need a violation of CP (a much more involved symmetry that is a combination of charge conjugation and parity) to do that. Breaking CP is more difficult than breaking parity in general.

  • Sean

    Obviously that was not a good typing day. Typos fixed.

    John– parity violation is pretty well understood within the Standard Model. The weak interactions couple to particles with left-handed helicity, but not to particles with right-handed helicity; they violate parity as much as possible. Parity violation is not directly tied to matter/antimatter asymmetry, but CP violation, combining parity with charge conjugation, almost certainly is. See Mark’s series of posts, beginning here.

  • Sean

    Oops, my comment crossed with David’s in the tubes.

  • JoAnne

    Let me just add that there is popular possible extension of the Standard Model, called the Left-Right-Symmetric Model, which restores parity at high energies. These theories have a new symmetry with interactions that couple to particles with right-handed helicity. There are new neutral and charged gauge bosons in this theory that have easy to detect signatures at the LHC. This theory is also part of a Grand Unified Theory which can naturally explain the smallness of neutrino masses in comparison to the other fermions.

  • uncle sam

    Here’s an interesting consequence of handness in the universe: the violation of parity is important in the “Ozma problem” as described by Martin Gardner (and hinted much earlier, like by Kant.) It is: there is no way to describe a right-hand or a left-hand version of a shape directly if the universe is inherently free of parity bias. One can only say “this is the mirror image of something else” but without a standard of handness already in place, we can’t inform distant aliens that we are “mostly right-handed” – all informational represenations of hands could be interpreted either way. (If skeptical, imagine sending a binary image file – and the alien’s reconstruct it right-to-left instead of LTR.) You’d have to send an actual sample – ick. But after the Co-60 experiment, we can use that nuclide as a reference to explain “right” or “left.”

  • SLC

    Madame Wu is not the only woman who deserved a Nobel Prize but was passed over for male colleagues. Rosalind Franklin and Lisa Meitner were also passed over.

  • chemicalscum

    Rosalind Franklin was not really passed over for the Nobel Prize as she was already dead from cancer at the time the Nobel Prizes were handed out. If she had lived she should not only have won the Nobel Prize for her work on DNA at Kings, she should have also won one for her work on Tobacco Mosaic Virus at Birkbeck.

    Here collaborator at Birkbeck, Aron Klug who won the Nobel Prize for their work there has often pointed out that if she had lived the prize should have been shared between them.

  • Dileep

    Ms. Franklin still could have been awarded it posthumously.

  • Dileep

    Oh wait. Nobels aren’t awarded posthumously.

  • Peter Coles

    A few years ago I had the pleasure of meeting Tsung-Dao Lee because he was awarded an honorary degree by the University of Nottingham. He was the second-youngest ever winner of the physics Nobel prize (the youngest being Lawrence Bragg, son of William Bragg). Having been teaching undergraduates about parity violation for years, it was nice to meet one of its architects in the flesh. I had to deliver an oration to accompany the presentation of his degree, but was told in no uncertain terms not to mention Dr Wu. I agree that it was wrong that she wasn’t included in the citation, but I also felt it would have been wrong to spoil the day for Professor Lee. It wasn’t his fault that the committee screwed up.

  • graviton383

    Actually the Standard Model does not explain parity violation but only describes it…the fact that SM fermions are in LH doublets and RH singlets is experimental INPUT and not required by the model, ab initio, in any sense. WHY the weak interactions violate parity cannot be addressed within this context but requires new physics to explain, e.g., grand unification or….

  • Sean

    Well, the fact that the Standard Model couples differently to right-handed and left-handed fields may have a deeper explanation, or it may not. But certainly the SM is a perfectly well-defined theory that completely accounts for the observed parity violation in experiments. Whether or not the theory itself demands an explanation depends on what one counts as an “explanation.”

  • Jeff

    Peter, why would it spoil TD Lee’s day to mention the experimentalist who validated his theoretical work? You can acknowledge her contribution without diminishing his. By omitting mention of Dr. Wu you are perpetuating the Nobel committee’s injustice.

  • Sean

    It might be worthwhile to click on the link above labeled “in a later recollection”!

  • Peter Martel

    Thought this excerpt from my book, “Memoirs of a Hayseed Physicist” might give yet another slant to the Madame Wu story:

    In his early days Professor Samuels’ research involved
    measuring the polarization of emitted gamma rays from
    radioactive Co60 (cobalt-60), whose nuclear spins had been
    aligned by external magnetic fields at very low temperatures
    near -273 degrees Celsius. The radioactivity of Co60 consists


    Peter Martel

    of gamma rays (high energy photons) and lower energy Beta
    rays (electrons). Measurements of gamma ray polarization can
    define the direction of transverse photon vibration; thus
    permitting deduction of the nature of transitions between
    different energy levels of the Co60 nucleus. There was a great
    deal of physics associated with these measurements, which
    involved low temperature, solid state, and nuclear physics. The
    results were very interesting to nuclear physicists but probably
    not extremely exciting for the Nobel Prize committee and the
    physics community at large.


    What subsequently interested the Nobel Prize committee
    were Beta (electron) measurements on Co60 initiated by a
    Madame Wu of Columbia University. For much of the first
    half of the 1900’s it was thought electrons coming from Co60
    had a property called “parity”—a concept very dear to nuclear
    physicists until the fifties when two theoreticians (T.D. Lee and

    C.N. Yang) began uttering heretical questions about its validity
    as applied to the “weak” force in nuclei. What appeals to
    physicists is symmetry—they like to believe that nature is
    symmetrical and makes no distinction between opposite sides
    of a subatomic particle. In its simplest terms, the notion of
    parity holds that events arising from subatomic decay have
    mirror symmetry.
    As an aside, it should be noted that there is also a strong
    force binding neutrons and protons together in the nucleus.
    Physicists are also happy with the existence of two other forces
    known to high school students as electromagnetic and
    gravitational forces. Besides these four forces, astrophysicists
    sometimes speculate about the possibility of other forces in the
    outer reaches of the cosmos.

    Anyway, there were a lot of similarities between the
    Samuels and Wu measurements. They both required very low
    temperatures and magnetic fields, the latter serving to align the
    nuclear spins in cobalt. To the amazement of Wu and her
    collaborators when the magnetic field on the nuclear spins was


    Memoirs of a Hayseed Physicist

    reversed the electrons ejected from the nucleus did not reverse
    direction. Parity had not been conserved.

    In spite of Professor Wu’s earth-shattering results she was
    passed over for the Nobel, much to the surprise of the physics
    community. The award went to Lee and Yang. The reason was
    probably because she did little if any of the measurements
    herself; instead she headed a large team at Columbia and the
    US National Bureau of Standards in Maryland. And then, of
    course, gender discrimination can never be ruled out in such

  • graviton383


    Accounting for something is not the same thing as explaining it. The SM accounts for the fermion mass hierarchy but certainly doesn’t explain it. Ditto parity-violation…sorry.

  • Tszap

    The C. S. Wu Nobel issue is pretty complicated. There was a strong competing claim from Valentine Telegdi at Chicago, who did a very different type of experiment. Lots of bad blood, and accusations of Phys. Rev. having a bias toward Columbia, came out of it. Telegdi and Wu despised each other to their deaths (even years after Wu died, Telegdi would insult her gratuitously in public).

  • Sili

    Franklin was dead by the time the Nobel was awarded, but Jocelyn Bell-Burnell is still alive and well. And she was passed over.

    All in all, M. Curie comes across as the exception. If I were more bitchy I’d say that the prize comittee didn’t realise Marie was a woman (it’s not unusual for French men to have Marie as a middle name). Her daughter, Irene, does rather screw up that idea, though.

  • Spear Mark

    Someone saw parity violation in the 1930’s in beta emitters…. see `Adventures in Experimental Physics’ by Maglic for a description. Think it was a condensed-matter guy who actually published it, found the particle physicists of the time uninterested, and moved on. Don’t think he complained about a lost Nobel Prize.

    Then there was the collective effort of experimentalists who unraveled the tau-theta puzzle which motivated Lee and Yang… That was a huge and fascinating effort, involving cosmic-ray emulsion experiments at a level of organization that rivals large collaborations today, as well as the emerging accelerator groups in the 1950’s. History as recited just neglects that whole chapter.

    I think there were 3 experiments that published nearly simultaneously on parity violation.. Wu and a bunch of other collaborators (why are her collaborators Ambler, Hayward, Hoppes, and Hudson forgotten about?)… also Lederman, Garwin, and Weinrich, and then Telegdi and Friedman. The latter two groups used pi-mu-e decay, and really did devilishly clever experiments.

    Not to run down Madame Wu at all, she was great! But there are solid reasons why she did not end up with a Nobel Prize. It is not so easy to isolate discrimination in her case. Meitner is a much, much stronger case to blame on discrimination.

    I think the Standard Model merely `describes’ parity violation. It remains a terrific question… where are the other handedness of neutrinos? Maybe they’re the dark matter. Or maybe they will pop up at the LHC. But excluding one whole handedness from the Standard Model feels more unnatural then any fine-tuning argument.

    BTW, I’ve been really sickened by all the priority arguments over the dark energy. Yuk, a pox on them all.

  • david Hwang

    Very randomly I came to read this article in China and I am quite interested in Mr Sean’s unpublished book on C S Wu since I am going to choose a research topic in the cluster of history of science and technology.Can I have a further contact with Mr Sean?

  • Mike G.

    Yes, echoing the comments by Spear Mark, I’ve often wondered why the work of Garwin, Lederman, and Weinrich, received on the same day and published in the same issue of the Physical Review (Letters) as Wu’s work, is rarely mentioned in the discussion of the confirmation of parity violation.


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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