Chemistry or Physics? A Nobel Riddle
By: Dimitry Zakharov
How did a physicist end up winning the Nobel Prize for Chemistry?
Throughout his life, Gerhard Herzberg thought of himself as a physicist. He always worked in physics departments, first at Darmstadt Technical University, and later in Gottingen and Bristol. At Gottingen, in fact, he learned from towering figures in physics — Nobel laureates Max Born and James Franck. After leaving Europe, Herzberg held the position of professor of physics at the University of Saskatchewan and director of the NRC’s physics division.
Given all that professional background, why did he win the Nobel Prize in Chemistry?
The answer begins with the fact that the fields of chemistry and physics are inter-related. Indeed, a physicist had won the Nobel for chemistry previously. Ernest Rutherford, often called “the father of nuclear physics,” was awarded the Nobel Prize in Chemistry in 1908 for “for his investigations into the disintegration of the elements, and the chemistry of radioactive substances.”[i]
One way to understand this overlap is to consider spectroscopy’s position within the scientific world. Spectroscopy and various spectroscopic methods were at the center of physics research in the late 19th century and considered fundamental for confirming theories about temperature radiation and quantum mechanics. Spectroscopy had become a dominant experimental method when Herzberg was at university. When he moved to the University of Saskatchewan, he built a six-meter “grating spectrograph” similar to the one he had built at his previous laboratory in Darmstadt. Later, at the NRC, he and fellow physicist Alexander Edgar Douglas constructed a seven-meter concave grating spectrograph.
The field of spectroscopy expanded in the following decades with different types of spectrographs and other equipment like lasers and masers, which investigated molecules using spectra from all the different regions of light including the infrared, ultraviolet, and x-ray region. There are even spectrographs for studying spectra in the highly radioactive alpha-particle regions.
However, back in the 1930s, a concurrent invention emerged as a powerful tool in the arsenal of physicists. In 1930, UC Berkley professor Ernest O. Lawrence built the first small cyclotron for artificially creating radioactive isotopes like phosphorous-32, which was used in medical applications at the time. From there, cyclotron research eventually led to the development of particle accelerators. Consequently, some physicists changed their focus to what became known as high energy physics (HEP).
Science historian Andrew Pickering argues that HEP began to dominate physics by the 1950s,[ii] not least because of the display of the nuclear power over Nagasaki and Hiroshima in 1945. By 1967, there were 14 high energy particle accelerators in existence, primarily in the U.S., the Soviet Union, and Europe. Many physicists were fully engrossed in studying sub-atomic particles like hadrons and bosons. Eventually, this research led to developments like the discovery of quarks, which were even smaller elementary particles, as well as the formulation of different theories, and symmetries, like the Yang-Mills gauge theory. The 1954 Yang-Mills gauge theory mathematically united understandings of how the weak nuclear force and electromagnetic force functioned at high energy levels.[iii] By the 1980s, these insights led to the Standard Model of physics for understanding the interaction of sub-atomic particles like quarks, and how these elementary particles plus the four fundamental forces ‘hold together’ to form atomic nuclei, and then molecules.
Spectroscopy had not fallen behind. The field continued to expand, with new applications in medicine and astronomy. The discipline focused on molecules and the many unresolved problems associated with this level of analysis. Scientists like Herzberg, who combined spectroscopy with their knowledge of quantum physics, were welcome and needed within the world of chemistry. This overlap helps explain why, beginning in the 1960s, prominent spectroscopists, many of whom began as physicists, started winning the Nobel for Chemistry.
The first was Robert Mulliken, in 1966. Herzberg and Mulliken worked briefly as colleagues at the University of Chicago’s Yerkes Observatory. Mulliken, a New York-born physicist, had spent several years in the 1920s at Gottingen as a post-doctoral fellow. Like Herzberg, he also worked with the who’s who of physics, including Schrodinger, Paul Dirac, Heisenberg, and others. Mulliken spent his career working with spectroscopy and on problems of molecular orbits and structure. In the mid-1920s, he and Gottingen physicist Friedrich Hund developed the “molecular orbital theory” for studying the electronic structure of atoms and molecules using quantum physics. When Herzberg was at Gottingen several years later, in the theoretical division of the institute headed by Max Born, he worked on (and made several corrections to) Mulliken and Hund’s molecular orbital theory.
In 1967, George Porter, another physicist, won the Nobel Prize in Chemistry. Like Herzberg, Porter worked on the problem of detecting free radicals and won because his team at Cambridge developed the flash photolysis technique for the spectroscopic study of free radicals. The process was similar to the flash discharge technique developed by Herzberg and his team at the NRC, completely independently, during 1949 and 1950.
There were others after Herzberg, who likewise contributed to the development of a field that intersected both quantum physics and chemistry, and which gradually developed into what is presently known as quantum chemistry. This discipline still relies heavily on spectroscopy as an experimental methodology, but also draws on a multitude of new techniques, like nuclear magnetic resonance spectroscopy and laser induced breakdown spectroscopy, to name a few.
In 1984, an applied quantum chemistry symposium was held in Hawaii in honor of Herzberg, Mulliken, William Lipscomb, Kenichi Fukui, and Roald Hoffmann, all Nobel laureates. Along with a few other great scientists, like Erwin Schrodinger and Linus Pauling, these men had established quantum chemistry. For their part, Herzberg and Mulliken represented a direct connection to an earlier period, and when spectroscopy and the study of molecules proved to be valuable tools in the development of quantum physics.
Quite apart from the discoveries of individual scientists, these overlaps remind us that the laws of nature don’t easily classify. The distinctions between chemistry and physics is a human invention. While the two may be taught as separate subjects in schools and university classrooms, the differences disappear in the realm of high level of research, where scientists like Herzberg operated. The fact that Nobel prizes for chemistry sometimes go to physicists is merely another reminder that these boundaries don’t matter when it comes to our understanding of the ways in which matter works.
[i] Ernest Rutherford, “The 1908 Nobel Prize in Chemistry,” NobelPrize.org, Nobel Organization, January 28, 2022, https://www.nobelprize.org/prizes/chemistry/1908/summary/.
[ii] Andrew Pickering, Inventing Quarks: A Sociological History of Particle Physics, (Chicago: University Press, 1984): 21-73.
[iii] Pickering, Inventing Quarks, 159-160.