Wu Chien-hsiung (31 May 191 2-), known as C. S. Wu, outstanding physicist and professor at Columbia University who was best known for her experimental investigations of nuclear beta decay.

A native of Liuho, Kiangsu, C. S. Wu was brought up in a scholarly atmosphere. Her father. Wu Zong Yee, was the principal of a school in Liuho. He instilled in his children a love of reading and a deep respect for Chinese tradition, along with an awareness of the Western values and ideas which then were reaching China.

After receiving her early education at a local elementary school, C. S. Wu entered the Kiangsu Second Girls School in Soochow (later the Soochow Girls Normal School), which was famous for its emphasis on discipline and for its experiments with modern subjects and teaching techniques. It was while attending this school that C. S. Wu became strongly interested in mathematics and the sciences, especially physics. She also began to study English and German. In 1930, having completed her secondary education, she enrolled at National Central University in Nanking. She took all the available mathematics and physics courses and utilized her knowledge of German and English to keep up with the research in physics then being done in Europe and the United States. After being graduated in 1936, she worked for a short time as an assistant in the university's physics department.

Because C. S. Wu wished to pursue the study of physics and because opportunities for graduate work in that field were lacking in China, she went to the United States in the autumn of 1936 to enroll in the University of California at Berkeley. Her choice of Berkeley attests to her awareness of developments in physics at the time and of the institutions which were in the vanguard of research. Under the leadership of E. O. Lawrence, the winner of the 1939 Nobel Prize in physics for his development of the cyclotron, the Radiation Laboratory at Berkeley was one of the foremost centers of experimental nuclear physics research in the world. Beginning with the second semester, C. S. Wu held a teaching assistantship until she completed the Ph.D. degree in 1940. She then was given a position as research assistant to Dr. Lawrence. When the War in the Pacific began, pure research was pushed aside at Berkeley, and defense research became the order of the day. In 1942 C. S. Wu left Berkeley to become an assistant professor at Smith College. She spent the following year at Princeton University as an instructor. In March 1944 she joined the scientific staff of the Division of War Research (Manhattan Project) at Columbia University. After the war, she stayed on at Columbia as a research associate, becoming an associate professor in 1952 and a full professor in 1958. C. S. Wu was best known for the accurate and extensive experimental investigations of nuclear beta decay which she carried out. Nuclear beta decay is the transformation of the nucleus of an atom of some particular element to that of another element by the spontaneous emission of positive or negative particles known as beta particles. A positive beta particle is called a positron; a negative beta particle, an electron. This transformation of a nucleus into another nucleus is characterized by the probability that it will occur in a certain interval of time and by the energy spectrum of the emitted beta particles. A successful theory of nuclear beta decay must predict these two characteristics correctly in any particular case. C. S. Wu's investigations in this field began with a study of the X rays which are excited by beta particles. This study, as well as a study of the decay schemes of some radioactive noble gases from uranium fission, was performed at Berkeley as her doctoral thesis. She later demonstrated the existence of a dependence of the beta spectrum upon the thickness of the source, a problem which before identification had caused some misinterpretation of the theory of beta decay. She also made a systematic study of various types of nuclei for which the beta spectra are quite different, providing the necessary evidence of the correctness of the theory of beta decay. In 1956 two theoretical physicists, T. D. Lee of Columbia and C. N. Yang of the Institute for Advanced Study at Princeton, carried out a thorough survey of the existing experimental evidence concerning the conservation of parity in processes involving the weak interaction. This survey was prompted by the existence of a peculiar phenomenon: the existence in nature of two sub-nuclear particles which were identical in all respects except in the way they decayed. The assumption that these were but one particle with two different modes of decay required, however, that parity not be conserved. Parity conservation in a physical process is equivalent to saying that a distinction between left and right is not possible in such a process. For example, when a charged particle is accelerated, it emits electromagnetic radiation. If it emits more radiation on the left side of its path than on the right side, one would say that parity is not conserved. The survey of Li and Yang showed that evidence for parity conservation existed for all processes except those involving the weak interaction. They therefore postulated that in such processes parity is not conserved, and suggested some experiments which could be attempted in order to test their hypothesis.

One of the processes which involves the weak interaction is the beta decay of atomic nuclei. Lee and Yang, aware of C. S. Wu's experimental work in this field, asked her to carry out an experiment which would test their theory. The experiment, which G. S. Wu carried out in collaboration with scientists of the National Bureau of Standards, was essentially the following: The nuclei of radioactive cobalt-60 were aligned in the direction of a magnetic field while at a temperature close to 273°C below zero. The reason for using such a low temperature was to reduce the amount of thermal agitation which at higher temperatures causes a group of nuclei to be aligned randomly. The electrons from the decay of cobalt-60 which came out forward and backward along the direction of alignment then were counted. The results, announced early in 1957, showed that the electrons were emitted preferentially in one direction, differentiating right from left, and, hence, proving that parity was not conserved in nuclear beta decay. Within a few weeks, other results, obtained at Columbia and the University of Chicago, showed that parity was not conserved in other physical processes involving the weak interaction.

In 1963, in collaboration with Columbia research physicists Y. K. Lee and L. W. Mo, C. S. Wu reported the experimental confirmation of a new fundamental theory of beta decay. This theory, proposed in 1958 by Richard Feynman and Murray Gell-Man of the California Institute of Technology, is known as the theory of conservation of vector current. For her part in the overthrow of the principle of parity conservation, as well as for her work in nuclear beta decay and other fields, C. S. Wu was awarded many honors. Among these were the 1958 Research Corporation Award, the 1962 John Price Wetherill Medal of the Franklin Institute, the 1964 Cyrus B. Comstock Award of the National Academy of Sciences, and the 1965 Award for Achievement in Science from the Chi-Tsin Cultural Foundation in Taiwan. In 1958 she received the first honorary doctorate in science given to a woman by Princeton University. She also received honorary Sc.D. degrees from Smith College (1959), Rutgers University (1963), and Yale University (1967). She was the author of many research papers and coauthor with S. A. Mozkowski of Beta Decay, published in 1966.

C. S. Wu married the physicist Luke Chia-liu Yuan, a grandson of Yuan Shih-k'ai, in 1942. They had one son, Vincent.