A Giant Passes: C. N. Yang (1922-2025)

This post is about C. N. Yang, one of the greatest physicists of the 20th century, who passed away last week at the age of 103. Yang received the Nobel prize in 1957 and made towering contributions to several areas in physics, as detailed below.

Quick academic bio: Yang was born and educated in China until the masters level; he obtained his thesis in theoretical physics from the University of Chicago under Edward Teller (sometimes known as the father of the Hydrogen bomb). Subsequently, he worked for a year as an assistant to Enrico Fermi. Then he was at the Institute of Advanced Study at Princeton for a decade and a half, after which he moved to Stony Brook. He retired from Stony Brook in 1999, and moved to a position at the Tsing Hua University in Beijing.

Contributions:

Yang had wide interests in theoretical physics. Some of his major contributions:

1. Parity violation: This is the work for which he (and T. D. Lee) were awarded the Nobel in '57. They predicted that the laws of nature were not mirror symmetric: some experiments and their mirror reflections gave different outcomes. This was experimentally confirmed by C. S. Wu by looking at the radioactive decay of Cobalt-60 nuclei.

   
   

   It turns out that the weak nuclear force (responsible for radioactivity) does not respect mirror symmetry (i.e. parity). A very accessible description of the physics can be found here. The violation of parity is directly related to the fact that in our universe neutrinos are always left-handed and anti-neutrinos are always right-handed.

   
   

2. Yang-Mills theory: Arguably his biggest contribution to physics. This theory generalizes Maxwell's theory of electromagnetism and provides our current understanding of fundamental particles and their interactions.

   
   

   In other words, it underlies the Standard Model of physics, providing a unified framework for understanding electromagnetism and the weak and strong nuclear forces. The only force it does not apply to is the gravitational force. Solutions to problems related to Yang-Mills theory are worth a million dollars.

3. Lee-Yang theory: This is his contribution to the study of phase transitions (e.g. when liquid turns to gas).

   
   

4. Yang-Baxter relations: These relations are very important in the systems where many particles scatter of off each other.

   
   

5. Lee-Huang-Yang correction: This formula applies to the study of Bose gases and has recently been used to create a new form of liquid matter, quantum droplets.

   
   

6. Byers-Yang theorem: This result describes the connection between the behavior of magnetic fields passing through loops of superconducting material (as in devices like SQUIDs) and the current in the superconductor.

   
   

7. The C. N. Yang Institute for Theoretical Physics: At Stony Brook university; Yang was the founding director.

One of the dominant themes in Yang's work was the role of symmetry in physics. His talk here reflects (pun intended) that interest.

Afterword

Anecdote: When C. N. Yang and T. D. Lee took a graduate course in astronomy at the University of Chicago in the late 1940s, they found they were the only two students enrolled. Their professor S. Chandrasekhar drove about 80 miles each way to come from Yerkes Observatory to teach them. In the end it was worth the effort, as the entire class won the Nobel prize (Chandrasekhar was awarded in 1983).

There are some biographical notes available on Yang's life; of course the original papers and books; and many videos on YouTube. Nonetheless, when the dust settles from the fall of this colossus, a full fledged biography, if not several, would definitely be in order.