Since the beginning of the 20th century, nuclear physics has strived to understand the intimate structure of the atomic nucleus, this tiny region which concentrates most of the mass of an atom. For a long time, its internal organization resisted any attempt at coherent modeling, despite the accumulation of experimental data. It is in this context that the physicist Maria Goeppert Mayer proposed, at the end of the 1940s, a theory making it possible to explain the stability of certain nuclei thanks to a logic of successive layers, analogous to those of the electrons around the nucleus.
His model, based on a bold hypothesis concerning the interaction between the spin and the orbit of nucleons, made it possible to resolve an enigma that had remained open: the existence of “magic numbers”. Rarely has a theoretical contribution had such an impact on the understanding of the fundamental forces that structure matter.
A physicist trained at the heart of the quantum revolution
Maria Goeppert Mayer was born in 1906 in Katowice, then Germany. Very early on, she benefited from a stimulating scientific environment. The daughter of a professor of pediatrics and from a line of six generations of academics, she grew up in Göttingen, a city which became a global epicenter of theoretical physics between the wars. From 1924, she studied at the University of Göttingen, initially in mathematics. Then she found herself fascinated by new developments in quantum mechanics.
His training is exceptional. She attended the seminars of Max Born, who guided her towards physics. She is also influenced by Werner Heisenberg, Pascual Jordan, and James Franck. These names would later become major figures in 20th century physics. In 1930, she supported an innovative thesis on two-photon absorption. A process predicted by quantum theory but undetectable at the time. She introduces the notion of two-photon transition. This anticipates the development of two-photon microscopy and nonlinear optics.
His thesis jury includes three future Nobel Prize winners: Born, Franck and Adolf Windaus. Eugene Wigner, future co-winner of the Nobel Prize with her, would later say that her thesis was a “masterpiece of clarity and rigor”. At a time when women scientists remained rare, she managed to find a place for herself in an extremely competitive environment.
This intellectual base, built in Göttingen in a climate of academic freedom, will prove decisive for the future. But this theoretical excellence will not be enough to open the doors to a stable career for him. Upon her departure for the United States, she will have to face a long series of institutional obstacles.
A scientific career hampered by academic sexism
In 1930, Maria married Joseph Edward Mayer, an American chemist living in Göttingen. She followed him to the United States, where he obtained a position at the prestigious Johns Hopkins University in Baltimore. She, on the other hand, comes up against the harsh reality of the nepotism rules in force. In a context of economic crisis, American universities systematically refuse to employ professors' wives. Maria can only access unpaid statuses, or as a simple assistant.
However, she continues her research with tenacity. At Johns Hopkins, she collaborated with Karl Herzfeld on the electronic structure of organic molecules. She develops quantum chemistry models applied to benzene. In the 1930s, she co-wrote a reference manual with her husband: Statistical Mechanicsstill cited today.
But obstacles persist. In 1937, Joseph Mayer lost his position, partly because of the presence of his wife in the laboratory, considered annoying by the hierarchy. The couple moved to New York, where Maria got an office in Columbia, without salary. She taught for a time at Sarah Lawrence College, a second-tier women's establishment. During the war, she participated in the work of the Manhattan Project, but on peripheral research. In particular, she studies the isotopic separation of uranium by photochemical reactions, with few resources.
Its role remains marginalized, despite its growing expertise. It was not until 1946, in Chicago, that she joined a laboratory with recognized scientific status for the first time. This institutional invisibility does not slow down his intellectual production. It demonstrates that scientific merit is not enough to compensate for systemic discrimination. Her case powerfully illustrates the structural limits imposed on women in 20th century academic science.
A theoretical model to explain nuclear magic numbers
When Maria Goeppert Mayer joined the Argonne National Laboratory in 1946, nuclear physics was booming. Since the 1930s, we have observed that certain atomic nuclei with a precise number of protons or neutrons are particularly stable. These numbers, called “magic numbers”, are: 2, 8, 20, 28, 50, 82 and 126. But no theory explains their appearance or their regularity.
Goeppert Mayer formulates the hypothesis according to which nucleons (protons and neutrons) are organized in successive quantum layers, analogous to the electronic layers of the atom. It offers a detailed description of how these particles arrange themselves according to well-defined energy levels, inside the nucleus. However, this model alone does not explain the observed numbers.
The introduction of a key element that will change everything: the spin-orbit interaction. In the nucleus, each nucleon has a spin (a kind of rotation on itself) and follows a path, called an orbit, around the center. Goeppert Mayer proposes that these two movements are not independent: when they are aligned or opposed, this strongly influences the energy level that the nucleon can occupy. This interaction therefore modifies the structure of the internal layers of the nucleus. Contrary to previous hypotheses, she considers that this interaction is strong in the nucleus. This hypothesis inversion makes it possible to precisely reproduce the magic numbers observed experimentally.
Her model, formulated between 1948 and 1949, was independently validated in Germany by Hans D. Jensen, with whom she published the book in 1955 Elementary Theory of Nuclear Shell Structure. This publication establishes the theory of the nuclear layer model, today taught in all nuclear physics courses.
Late recognition and lasting scientific legacy
In 1960, Maria Goeppert Mayer finally obtained her first paid position as a full professor at the University of California, San Diego, at age 54. Three years later, she received the Nobel Prize in Physics alongside Hans Jensen and Eugene Wigner. This global recognition comes after more than three decades of work often carried out without official status or remuneration.
When his award was announced, a local daily headlined:
“San Diego mother receives Nobel Prize”illustrating the persistent sexism even at the height of scientific recognition. Maria became the first woman to receive a Nobel in theoretical physics. And to this day she remains one of the rare women winners in this discipline.
Its contribution goes far beyond the theory of magic numbers. In molecular physics, she is one of the first to formalize multiphoton interactions. These will give rise to cutting-edge technologies such as multiphoton laser microscopy. The unit of measurement of the two-photon absorption rate today bears his name: the GM (Goeppert-Mayer).
She has also influenced several generations of researchers. Her journey is often cited to illustrate the need for stronger institutional support for women's careers. As Suropriya Saha, physicist at the Max Planck Institute, pointed out, Maria embodies rare scientific rigor, combined with a capacity for adaptation and valuable interdisciplinarity.
Until her death in 1972, Maria Goeppert Mayer continued to teach, supervise and publish. His model remains an essential tool for understanding nuclear stability. Through her work, she left a lasting imprint on science. She opened a way for those to whom doors remained closed.
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