Opinion: That’s Not Physics

Where do the boundaries of physics begin and end? The debate has persisted for more than a century.

By Andrew Zangwill | April 12, 2024

Credit: Menno Schaefer/Adobe

Starlings flock in a so-called murmuration, a collective behavior of interest in biological physics — one of many subfields that did not always “belong” in physics.

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If you’ve been in physics long enough, you’ve probably left a colloquium or seminar and thought to yourself, “That talk was interesting, but it wasn’t physics.”

If so, you’re one of many physicists who muse about the boundaries of their field, perhaps with colleagues over lunch. Usually, it’s all in good fun.

But what if the issue comes up when a physics faculty makes decisions about hiring or promoting individuals to build, expand, or even dismantle a research effort? The boundaries of a discipline bear directly on the opportunities departments can offer students. They also influence those students’ evolving identities as physicists, and on how they think about their own professional futures and the future of physics.

So, these debates — over physics and “not physics” — are important. But they are also not new. For more than a century, physicists have been drawing and redrawing the borders around the field, embracing and rejecting subfields along the way.

A key moment for “not physics” occurred in 1899 at the second-ever meeting of the American Physical Society. In his keynote address, the APS president Henry Rowland exhorted his colleagues to “cultivate the idea of the dignity” of physics.

“Much of the intellect of the country is still wasted in the pursuit of so-called practical science which ministers to our physical needs,” he scolded, “[and] not to investigations in the pure ethereal physics which our Society is formed to cultivate.”

Rowland’s elitism was not unique — a fact that first-rate physicists working at industrial laboratories discovered at APS meetings, when no one showed interest in the results of their research on optics, acoustics, and polymer science. It should come as no surprise that, between 1915 and 1930, physicists were among the leading organizers of the Optical Society of America (now Optica), the Acoustical Society of America, and the Society of Rheology.

That acousticians were given a cold shoulder at early APS meetings is particularly odd. At the time, acoustics research was not uncommon in American physics departments. Harvard University, for example, employed five professors who worked extensively in acoustics between 1919 and 1950. World War II motivated the U.S. Navy to sponsor a great deal of acoustics research, and many physics departments responded quickly. In 1948, the University of Texas hired three acousticians as assistant professors of physics. Brown University hired six physicists between 1942 and 1952, creating an acoustics powerhouse that ultimately trained 62 physics doctoral students.

The acoustics landscape at Harvard changed abruptly in 1946, when all teaching and research in the subject moved from the physics department to the newly created department of engineering sciences and applied physics. In the years after, almost all Ph.D. acoustics programs in the country migrated from physics departments to “not physics” departments.

The reason for this was explained by Cornell University professor Robert Fehr at a 1964 conference on acoustics education. Fehr pointed out that engineers like himself exploited the fundamental knowledge of acoustics learned from physicists to alter the environment for specific applications. Consequently, it made sense that research and teaching in acoustics passed from physics to engineering.

It took less than two decades for acoustics to go from being physics to “not physics.” But other fields have gone the opposite direction — a prime example being cosmology.

Albert Einstein applied his theory of general relativity to the cosmos in 1917. However, his work generated little interest because there was no empirical data to which it applied. Edwin Hubble’s work on extragalactic nebulae appeared in 1929, but for decades, there was little else to constrain mathematical speculations about the physical nature of the universe. The theoretical physicists Freeman Dyson and Steven Weinberg have both used the phrase “not respectable” to describe how cosmology was seen by physicists around 1960. The subject was simply “not physics.”

This began to change in 1965 with the discovery of thermal microwave radiation throughout the cosmos — empirical evidence of the nearly 20-year-old Big Bang model. Physicists began to engage with cosmology, and the percentage of U.S. physics departments with at least one professor who published in the field rose from 4% in 1964 to 15% in 1980. In the 1980s, physicists led the satellite mission to study the cosmic microwave radiation, and particle physicists — realizing that the hot early universe was an ideal laboratory to test their theories — became part-time cosmologists. Today, it’s hard to find a medium-to-large sized physics department that does not list cosmology as a research specialty.

Not all disciplines have had such clean paths in or out of physics. Consider the case of biophysics, or biological physics — where physicists apply their methods to the study of living systems. Whether or not this field is physics or “not physics” has vexed the community for a century.

In 1920, the physics department at Harvard boldly hired an assistant professor named William Bovie to conduct research and teach courses in biophysics. Bovie did not get tenure, and the Harvard physics department did not hire another assistant professor in this area until 2013.

In 1970, the National Academy of Sciences charged the American physics community with analyzing itself to help the government establish funding priorities. The resulting 2,900-page report devoted only 20 pages to what it called “physics in biology.” Tenured appointments for younger people working in biophysics “have been very scarce,” it noted. “Among the reasons for this scarcity are the difficulty evaluating their work in a physics department and questions about the appropriateness of such work in a physics department.”

This may explain why, in 2000, only 20% of Ph.D.-granting institutions employed at least one physics professor engaged in biophysical research. But also in 2000, six physics departments had biophysics groups with more than three faculty members each, and three of those groups boasted a record of continuous funding and publication beginning before 1965.

Since 2000, the advent of conferences and federal funding devoted to biological physics convinced a growing number of physics departments to accept it as a legitimate and exciting subfield of physics. On the other hand, several prominent U.S. physics departments remain biophysics-free in 2024. It appears that a juror or two must be convinced before a unanimous verdict can be announced.

Perhaps the path of each field — acoustics, cosmology, and biological physics — toward physics or “not physics” seems unique. But all three are archetypes of stories that could be told about virtually every other subfield of physics. To hear the tale of another discipline, simply question its suitability as a part of physics the next time you share a meal with one of its practitioners.

Andrew Zangwill is a professor of physics at the Georgia Institute of Technology.

This article was adapted from a talk the author gave at the 2024 March Meeting in Minneapolis, during a session on APS’s 125-year history. The topic is also the subject of a forthcoming book by the author.

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