Symmetry Destroyed: The Failure of Parity

Copyright © 1984 Krishna Myneni

December 10, 1984


  1. Introduction
  2. Lee and Yang
  3. The Theta-Tau Puzzle
  4. The Beginnings of Doubt
  5. The Proposed Experiment
  6. Madame Wu
  7. Success and Reaction
  8. Conclusion
  9. References


In 1947 Cecil F. Powell photographed cloud chamber tracks of charged particles from space descending upon the Andes mountains. In those photographs Powell identified the pi meson --- the particle postulated twelve years earlier by the physicist Hideki Yukawa, as the intermediary for the nuclear force. Two years later Powell would discover new particles with his cloud chamber, and the pi meson would play a crucial role in the postulation of a new force, the weak interaction. Both of these discoveries were to lead physicists of the mid-twentieth century to a contradiction. Its resolution involved the overthrow of cherished conservation law, and, as relativity challenged common sense earlier, once again physicists would be shocked by the fallibility of their intuitions.

Symmetries have long played a crucial role in physics. The conservation laws of the past had more fundamental roots within the symmetry of the universe. Such laws as conservation of linear and angular momentum arise from an even more fundamental requirement: physical laws are invariant under translation and rotation[1]. The law of conservation of parity arose from the symmetry between the left and right hands.

In 1848 Louis Pasteur discovered, almost by sheer luck, the property of optical isomerism. Two forms of the same chemical compound, isomers, were found to rotate polarized light in two different directions --- one to the left, the other to the right. Isomers are essentially identical chemical compounds. They have the same number and type of atoms and the same structure, almost. The difference in the two isomers of a compound is that one is the mirror image of the other. This is the same symmetry that exists between the left and right hands. Pasteur observed as well that living organisms were able to synthesize and use only one isomer and never the other. But nature itself appeared to have no preference over which form it produced --- in reactions the isomers were produced in equal quantities. That is, nature appears to exhibit complete symmetry between the left and right. Until 1957 physicists believed this symmetry to hold for all physical processes. A mirror image of any reaction should be identical in every way to the actual reaction. This idea was intuitive to the physicist --- what could it mean if nature preferred left over right or vice-versa?

To describe more precisely the symmetry between left and right, physicists used the concept of parity. Parity originated with the development of quantum mechanics. In 1924 O. Laporte classified the wavefunctions of an atom as either even or odd, depending upon the symmetry of the wavefunction. Laporte discovered that when an atom transitions from one state to another and a photon is emitted, the wavefunction changes from even to odd or vice-versa, but never remains the same [1]. Even functions were defined to have parity of +1 and odd functions a parity of -1. In addition, the emitted photon was defined to have parity of -1. Laporte's rule could then be stated as a conservation law, the conservation of parity. For mathematical reasons, the parity of any system is the product of the parities of the individual components. Parity is conserved in atomic transitions. If the initial wavefunction was odd (-1 parity), Laporte's rule asserts the final wavefunction must be even (+1 parity). Since the initial system has -1 parity and the final system has as its parity the product of parities of the final wavefunction and the parity of the emitted photon, (+1)(-1) = -1, parity is conserved in the transition. The parities of the inital and final wavefunctions can be interchanged; conservation of parity will still hold.

The importance of parity conservation, its fundamental nature, was discovered in 1927 by the physicist Eugene Wigner, Wigner proved that Laporte's rule was a consequence of right-left symmetry (or mirror image symmetry) of the electromagnetic force[1]. Conservation of parity rested upon Maxwell's equations describing electromagnetism, but more important, the intuitive idea that nature should be left-right symmetric had been established on the quantum level. Thus, when in 1949, the weak force was postulated to explain disintegration of elementary particles, physicists could not conceive that parity conservation would not hold for reactions involving the weak force. It was a minor oversight however that there was no direct evidence for the extension of this law to the fourth force of nature. Seven years later physicists would come full circle to question their acceptance of parity conservation.

Lee and Yang

The same war that would later finance the particle accelerators in the United States would also bring together two young Chinese students. In 1943 Tsung Dao Lee was a student in the Kweichow province of China. It was the time of the Sino-Japanese War, and the Japanese invasion of the mainland forced Lee to move to Kunming. There he attended the National Southwest University where he met Chen Ning Yang [2]. Lee and Yang had only a nodding acquaintance then. In 1946 both students received fellowships to study in the United States. Yang had pursued Enrico Fermi from Columbia to the University of Chicago --- he was to have a close association with Fermi. Lee, on the other hand, had little choice. Only one school in the U.S. then allowed an undergraduate to work towards the PhD without the intermediate degrees, the University of Chicago. The two graduate students fast became friends.

For a while Yang had tried experimental physics, but it was not to be. Other graduate students had teased him, "Where there was a bang, there was Yang" [2]. Yang eventually did his doctoral thesis under the supervision of Edward Teller. Lee on the other hand knew he was a theorist from the start. He did his doctoral thesis under Fermi. Yang recalls Fermi's advice on his career: As a young man, work on practical problems; do not worry about things of fundamental importance [3]. For all of his admiration of Fermi, Yang chose to ignore this bit of advice. Both Lee and Yang graduated and for awhile worked as staff members at the Institute for Advanced Study in Princeton. Lee had become a reputable theoretical physicist, invoking praise from J. Robert Oppenheimer as "one of the most brilliant theoretical physicists then known" [4]. Thus the individual physicists T. D. Lee and C. N. Yang had established their reputations by 1956, when their work together would help clear a mystery known as the theta-tau puzzle and topple of the most fundamental conservation laws.

The Theta-Tau Puzzle

Within the cosmic rays in which C. F. Powell had discovered the pi meson (pion) were other new particles. In 1949 Powell identified a cosmic ray particle which disintegrated into three pions. He dubbed this new particle the tau meson. Another particle called the theta meson was also discovered. It disintegrated into two pions. Both particles disintegrated via the weak force. Now, a problem arose when the masses and the lifetimes of the tau and theta particles were considered. The two particles turned out to be indistinguishable other than their mode of decay. Their masses and lifetimes were identical, within the experimental uncertainties. Were they in fact the same particle? The problem itself was not that the tau and theta, if indeed they were the same particle, decayed in two different modes, one by two pions, the other by three pions. The problem dealt with the more fundamental parity conservation law. In 1953 the physicist R. H. Dalitz argued that since the pion has parity of -1, two pions would combine to produce a net parity of (-1)(-1) = +1, and three pions would combine to have total parity of (-1)(-1)(-1) = -1. Hence, if conservation of parity holds, the theta should have parity of +1, and the tau of -1. Hence, they could not be the same particle [5]. Thus was born the theta-tau puzzle. It's resolution would involve an almost unacceptable proposition to the physicists of the time.

The Beginnings of Doubt

The events which led to the publication of Lee and Yang's historic paper, Question of Parity Conservation in Weak Interactions, began at the International Conference on High Energy Physics at the University of Rochester in April 1956. Lee and Yang attended the conference with a proposal for ending the theta-tau puzzle. Their idea was that certain kinds of elementary particles occur in two forms with different parities. The idea was called parity doubling [5]. Also attending the conference was the theoretical physicist Richard Feynman, who is renowned for his development of the field of physics called quantum electrodynamics. Feynman's roommate at the conference was the experimentalist Martin Block. Block suggested to Feynman on the first night of the conference that parity just may not be conserved in certain interactions. The next day, following Yang's presentation of the parity doubling idea, Feynman brought up the question of non-conservation of parity. Feynman himself later said, "I thought the idea (of parity violation) unlikely, but possible, and a very exciting possibility." Indeed Feynman later made a fifty dollar bet with a friend that parity would not be violated [6]. Yang's reply was that he and Lee had considered the idea but had arrived at no conclusions. During the discussion, Wigner, who had formulated the law of conservation of parity in the first place, also suggested that perhaps it did not hold in weak interactions [5].

Lee and Yang pursued the question further after the conference. "Early in May, when they were sitting in the White Rose Cafe near the corner of Broadway and 125th Street, in the vicinity of Columbia University, it suddenly struck them that it might be profitable to make a careful study of all known experiments involving weak interactions" [6]. After several weeks of reviewing past experiments, they had come to two conclusions:

  1. "Past experiments on the weak interactions had actually no bearing on the question of parity conservation."

  2. "In strong interactions, ... there were indeed many experiments that established parity conservation to a high degree of accuracy..." [1].
As Yang commented in his Nobel lecture, "The fact that parity conservation in the weak interactions was believed for so long without experimental support was very startling. But what was more startling was the prospect that a space-time symmetry law which the physicists have learned so well may be violated. This prospect did not appeal to use." [1].

The Proposed Experiment

When Lee and Yang's paper appeared in the October 1, 1956 issue of The Physical Review, physicists were not immediately prompted into action. The proposition of parity nonconservation was not unequivocally denied; rather, the possibility appeared so unlikely that experimental proof did not warrant immediate attention. The physicist Freeman Dyson wrote of his reaction to the paper: "A copy of it was sent to me and I read it. I read it twice. I said, `This is very interesting,' or words to that effect. But I had not the imagination to say, `By golly, if this is true it opens up a whole new branch of physics.' And I think other physicists, with very few exceptions, at that time were as unimaginative as I." [6]. Hence, the initial reaction among most physicists to verifying parity conservation was not enthusiastic.

In their paper, Lee and Yang stated, "To decide unequivocally whether parity is conserved in weak interactions, one must perform an experiment to determine whether weak interactions differentiate the right from the left." [7]. And they proposed several experiments. One of the simplest experiments (conceptually) invovled measurements on the beta decay of cobalt-60. The idea involved orienting cobalt nuclei with a strong magnetic field so that their spins are aligned in the same direction. Beta rays (electrons) are emitted at the poles of the nuclei. A mirror image of the system would also show beta rays being emitted from the poles of the mirror cobalt nuclei, the only difference being that the north and south poles of the mirror nuclei would be reversed since they spin in opposite direction of their real counterparts. Hence parity conservation demands that the emitted beta rays be equally distributed between the two poles. If more beta particles emerged from one pole than the other, it would be possible to distinguish the mirror image nuclei from their counterparts. Thus an anisotropy in the emitted beta rays would be tantamount to parity violation.

Madame Wu

Another immigrant was now to play the next major role, Madame Chien-Shiung Wu. Arriving at Berkely in 1936 from Shanghai, Wu was one of the most ardently pursued coeds on campus. But she was also a hard worker who abhorred the marked absence of women from the American scientific establishment. She says, " ... it is shameful that there are so few women in science... In China there are many, many women in physics. There is a misconception in America that women scientists are all dowdy spinsters. This is the fault of men. In Chinese society, a woman is valued for waht she is, and men encourage her to accomplishments --- yet she retains eternally feminine." [8]. In this view, there is a clear distinction between American and Chinese cultures. Yang, too, had to come to terms with the differences between the two cultures. In his Nobel address, he says, "I am heavy with awareness of the fact that I am in more than one sense a product of both the Chinese and Western cultures, in harmony and in conflict... I am as proud of my Chinese heritage and background as I am devoted to modern science, a part of human civilization of Western origin..." [2]. Returning to Madame Wu, the physicist Emile Segre', one of her teachers, said of her, "She is a slave driver. She is the image of the militant woman so well known in Chinese literature as either empress or mother." But by 1956 she had a world-wide reputation for her work on beta decay. Beta decay involves the weak interaction. Wu's experiments were highly regarded for their simplicity and elegance [8]. At the time Lee and Yang considered the question of parity, Wu was a professor at Columbia and a long time friend of both men. She was the first to act on the proposed experiment involving beta decay in cobalt 60.

Even before Lee and Yang's paper had been submitted to The Physical Review, Lee had discussed the experiment with Wu. At the time, Wu and her husband had planned a trip to Europe and the Far East. But she chose instead to remain and perform the experiment rather than lose the opportunity to other physicists who might recognize its importance. However, the experiment could not be performed with only her expertise. Reaching the low temperatures necessary to be able to orient the cobalt nuclei spins required equipment few laboratories possessed. Nevertheless, one such laboratory existed in the United States --- the Cryogenics Physics Laboratory at the National Bureau of Standards in Washington. Early in June of 1956, Wu sought the help of Ernest Ambler at NBS. Ambler accepted enthusiastically. Indeed his doctoral thesis dealt with the orientation of cobalt-60 nuclei. In addition, Ralph Hudson, with expertise in cryogenics, and Raymond Hayward and Dale Hoppes, with experience in radiation detection, joined the team. By early October they began to assemble and test their equipment. The same month saw the publication of Lee and Yang's paper.

Success and Reaction

The experimental problems were enormous. Temperatures as low as one hundredth of a Kelvin were necessary to attain a high degree of spin orientations for the cobalt nuclei. While such temperatures could be reached through a process called adiabatic demagnetization, maintaing the super coldness posed quite a problem for the group. Another problem was leaks in the apparatus --- the experiment required the detectors and cobalt sample to be placed in a vacuum. Nevertheless, after reconstructing their equipment, several trials, and the use of cotton thread, the experiment finally succeeded. The day was December 27, 1956 [9].

News of the success reached Lee and Yang. At Columbia, in those days, many of the physicists would gather on Fridays for "Chinese lunch" under the supervision of T. D. Lee. When Lee, during such an occasion, announced that positive results to parity violation were being given by Wu's group, the physicist Leon Lederman was among those present [5]. Lederman, who worked with Columbia's cyclotron, realized that he could perform an independent test of parity with the cyclotron. His experiment, which involved the decay of pi and mu mesons, had also been proposed by Lee and Yang in their paper. Soon, Lederman, along with his graduate students, Marcel Weinrich, and Richard Garwin began their experiments. At the same time, the group under Wu was running into problems. Wanting to verify their results from December 27, they repeated the experiment. Their original finding of a large asymmetry in the beta ray distribution was not consistently reproducible. However, after a week of solving problems with the apparatus, consistent results were obtained. And the results pointed to parity violation. Much consideration was given to the question of the origin of the beta ray asymmetry --- was it really an indication of the failure of parity or some result intrinsic to the experiment? "The group worked around the clock, assembling the apparatus many times, and took their breaks for a few hours sleep when the superfluid helium spoiled their vacuum by finding its way around the stopper at the bottom of the cryostat. Hoppes then slept beside the apparatus, telephoning to the others as soon as its temperature was low enough to begin their experiments again. Finally, on Januray 9th, at 2 o'clock in the morning, Hudson brought out a bottle of Chateau Lafite-Rothschild, 1949, and they drank to the overthrow of the law of parity" [9]. As the closing door to the question of parity violation in weak interactions, results from Lederman's group at the cyclotron came quickly. They too had obtained distinct evidence for parity violation. Both groups submitted their papers together to The Physical Review on January 15, 1957. On that day, Columbia called for a press conference.

As newspaper headlines told of a physics principle demolished, startled reactions emerged from the physicists. Feynman had lost his bet (and fifty dollars). From Zurich, Wolfgang Pauli wrote to Victor Weisskopf at MIT, "Now after the first shock is over, I begin to collect myself. Yes, it was very dramatic." At Columbia's press conference, Isador Rabi said, "A rather complete theoretical structure has been shattered at the base and we are not sure how the pieces will be put together" [6]. Credulity of parity nonconservation had taken hold among physicists.


The failure of the physicists' intuition had been enormous. Nature, as it had done with relativity, did not oblige itself to follow the rules of "common sense". To physicists, results of the Columbia experiments left them in a profound quagmire: Why did nature distinguish between the left and the right? Though the question remains unanswered, the physicists had set themselves up for the shock. They had ignored Ernst Mach's warning: "Even instinctive knowledge of so great a logical force as the principle of symmetry ... may lead us astray ... The instinctive is just as fallible as the distinctly conscious. Its only value is in provinces with which we are very familiar" [10]. The new province of weak interactions had not been tested before Lee and Yang made the suggestion. Nevertheless, the physicists' assumption that nature will present a simple understanding is unflailing. Chen Ning Yang has stated, "In the study of nature, one believes in something simple underlying all". Madame Wu agrees, "One hopes that nature possesses an order that one may aspire to comprehend. When we arrive at an understanding, we shall marvel how neatly all the elementary particles fit into the great scheme." Violation of the law of conservation of parity, then, should lead one to search for an even more fundamental symmetry to the universe.


  1. Yang, C. N., The law of parity conservation and other symmetry laws of physics, Nobel Lectures Physics: 1942--1962, 1964.

  2. Bernstein, J., Profiles: A Question of Parity, The New Yorker Magazine, 38: May 12, 1962.

  3. Segre', E., Enrico Fermi, Physicist, 1970.

  4. Biography, Nobel Lectures Physics: 1942--1962, 1964.

  5. Trigg, G. L., Disproof of a Conservation Law, in Landmark Experiments in Twentieth Century Physics, 1975.

  6. Gardner, M., The Fall of Parity, in The Ambidextrous Universe, 1964.

  7. Lee, T. D., and C. N. Yang, Question of Parity Conservation in Weak Interactions, The Physical Review, 104, Oct 1, 1956.

  8. Queen of Physics, Newsweek, 61, May 20, 1963.

  9. Forman, P., The Fall of Parity.

  10. Mach, E., Science of Mechanics, 1942.

  11. Yang on Yang, Newsweek 59, Jan 22, 1962.

Additional References