The physicist who finds fundamental truths in spilled coffee

Physicists have a reputation for caring about the grandest questions: the formation of the universe, the collisions of black holes, the nature of time and space.

But Sidney Nagel was also curious about the more modest things that make up the texture of our world. The physics as a raindrop stretches and falls from an overhang. Why a spill from a coffee cup forms a ring instead of a spot. The shifting of grains of sand.

“No one thought you’d find basic truths by looking at these sorts of things,” he said. “But the fact that we don’t know how to think about answering these questions means there’s some fundamental physics we don’t know.”

Nagel spent nearly 50 years doing exactly that. This year, Nagel, the Stein-Freiler Distinguished Service Professor of Physics at the University of Chicago, is accepting the 2023 American Physical Society Medal for Exceptional Achievement in Research. The award, sometimes referred to as the ‘lifetime achievement Oscar of physics,’ recognizes “contributions of the highest level that advance our knowledge and understanding of the physical universe in all its facets.”

“The thing that makes you a candidate for a prize like this is that you change a field,” said Thomas Witten, Prof. Emeritus of Physics and a longtime colleague of Nagel’s. “Sidney changed many things.”

In particular, Nagel is one of the pioneers of the field of physics known as soft matter. Today it is a primary focus in physics, but decades ago it was not only little known, but disdained.

Order and disorder

As a young physicist at the University in 1976, Nagel was drawn to questions that the field itself dismissed. “There was a certain prejudice that you were supposed to look at clean systems, clean questions,” he said. “‘This is just dirt—is it really even physics?’”

But Nagel was struck that there weren’t answers to some of the simplest questions one could ask when looking at the world around them.

“We knew nothing about some of these areas, but we learned that if we jumped in, we could swim to shore,” Nagel said.

And as they did so, he and his collaborators found fundamental truths woven into the questions.

They unraveled the physics behind how grains of rice or coffee in a container pack tighter as it is shaken. They ran endless experiments to figure out why spilled coffee forms a ring. They took high-speed photographs of droplets separating, discovered a hitherto unseen world of physics, and developed the mathematics to explain how the breakup of ordinary liquids mimics features not only found in other materials, but at the largest scale in black holes. They introduced the importance of “jamming,” the phenomenon that turns loose grains into a rigid solid when squeezed together—opening up a whole new line of intensive research across several fields.

Through it all, themes emerged. “Considered as a whole, they open up the idea that disorder and inequilibrium is crucial,” Nagel said.

The classic approach to physics is to envision the simplest system in order to find fundamental truths about how it behaves. For example, physicists often cool down their experiments to near absolute zero to remove confounding variables like temperature and the disorder that comes with it.

But Nagel found he was interested in the opposite question: “Is there an epitome of disorder, and if so, what are the principles that govern it?”

Most of the equations we learn in high school physics are written about a perfect, ordered system at equilibrium with its surroundings—that is, when all the forces are balanced.

“But the thing is, most systems are not actually in equilibrium,” Nagel said. “The atmosphere is not. People are not. In fact, if you think about it, biology is set up to keep us from reaching equilibrium. This results in complex behaviors, but are there simple things you can say about how they work?”

In fact, there are—and they lay the foundations for a deeper understanding of our world as well as for applications in biology, materials science, and robotics.

“The impact of Sid’s work goes far beyond any one particular phenomenon or any particular subfield of science, because the basic principles he uncovered are so widely applicable,” said Heinrich Jaeger, Nagel’s friend, colleague and collaborator for many years.

“Sid is amazingly creative and profound, and yet he has such clarity that things look simple—and it’s only later that you (meaning me, of course) realize how much he has changed your worldview,” said Prof. Susan Coppersmith, a collaborator and former colleague of Nagel’s.

Beauty in the beaker

Nagel’s approach is known for another characteristic: an appreciation of the beauty in the world.

His office is lined with images taken of experiments, often in black and white, that showcase the perfect curve of a droplet about to fall or the sly lines of water and oil sliding past each other. Several of these are part of the Smart Museum of Art’s permanent collection.

“The phenomena themselves have an aesthetic quality that I don’t think of separate from physics,” he said.

Sharing the beauty and intrigue of physics has always been an interest of Nagel’s; he, Jaeger, and others worked with Chicago’s Museum of Science and Industry to create a permanent exhibit called “Science Storms,” which includes an activity demonstrating the physics behind sandcastles. He is also a fixture in an annual outreach event held at the University called “Physics with a Bang!” in which he and Jaeger explode balloons, launch bottles across the room with gas, and blow smoke rings with garbage cans.

“What I find most inspiring in Sid’s experiments is that they have a conceptual and poetic appeal that transcend the specific phenomena he studies,” said colleague Prof. Vincenzo Vitelli.

But it’s been a journey to get here; Nagel said he grappled with imposter syndrome as a young physicist, especially one interested in unorthodox areas of study. “I still have it,” he said. “But I don’t care about it anymore. Now I feel that if I can ask a question, I don’t have to defend the fact I’m asking it.”

Nagel is the second recipient of the award from the University of Chicago; his late, longtime colleague Eugene Parker was recognized in 2018 for seminal contributions to space physics.

APS President-Elect Robert Rosner and CEO Jonathan Bagger will formally present the Medal at a ceremony during the Society’s Annual Leadership Meeting in Washington, DC on January 26, 2023.