Atomic nuclei—the dense cores of atoms that make up matter and fuel the stars—hold answers to some of the most fundamental questions in physics.
In the Department of Physics and Astronomy in UT’s College of Arts and Sciences, Thomas Papenbrock is working to discover some of those answers using advanced theory and computation to better understand how matter behaves at its most basic level.
“Exploring the properties of atomic nuclei helps us understand where the elements in the universe are made and answers basic questions about the nature of the strong nuclear force—one of the four forces that govern interactions between matter in the universe,” he said.
An unexpected path to nuclear theory
For Papenbrock, the journey into nuclear theory wasn’t planned. Trained alongside nuclear physicists during his doctoral studies, he initially found himself working adjacent to the field. But during a postdoctoral appointment at the Institute for Nuclear Theory in Seattle, his perspective shifted.
“That’s where I realized how exciting and broad nuclear theory is,” Papenbrock said. “Those three years converted me.”
From there, he became a Eugene P. Wigner Fellow at Oak Ridge National Laboratory and joined UT in 2004, remaining active at ORNL through a joint faculty appointment.
Now Papenbrock has built a career focused on developing more accurate models of atomic nuclei and the computational tools needed to solve them. His work increasingly draws on emerging techniques, including machine learning and quantum computing, to push the boundaries of what these models can achieve.
Modeling the building blocks of the universe
At the center of Papenbrock’s research are atomic nuclei—complex systems of protons and neutrons that contain nearly all of an atom’s mass. Understanding how those composite particles interact requires both deep theoretical insight and immense computational power.
Papenbrock and his collaborators develop sophisticated models that describe nuclear structure from first principles, meaning they aim to explain nuclear behavior starting from the most fundamental theories of matter. Their models help answer sweeping scientific questions—from how elements are formed in stars to how matter behaves under extreme conditions.
To run the models, his team relies on some of the world’s most powerful supercomputers, including systems at ORNL. By improving both the accuracy of the models and the efficiency of the algorithms used to solve them, they are expanding what scientists can predict about atomic nuclei.
From fringe ideas to the forefront of emerging technologies
Over the course of his career, Papenbrock has helped move once-uncommon ideas into the mainstream of nuclear theory.
“What we do was on the fringes two decades ago but has become more mainstream now,” he said.
His work has pushed nuclear theory toward more unified predictive models that can describe a wide range of nuclear behavior without relying on separate approximations for each scenario. Recently he and his collaborators developed models that connect phenomena across multiple energy scales within the nucleus, offering a more complete picture of how those systems behave.
Papenbrock has been an early adopter of emerging technologies. His group was among the first to explore how quantum computing could be used to solve nuclear structure problems, a direction that could ultimately transform how scientists simulate complex quantum systems.
“Computing plays a big role in my research,” he said. “I’ve also been part of national collaborations where computer scientists and physicists work together to tackle these challenges.”
A collaborative research environment
Papenbrock’s success is closely tied to UT’s unique research environment. Through his joint faculty appointment with ORNL, he bridges the strengths of a flagship university and one of the nation’s leading national laboratories, powering collaborations that bring together theorists, experimentalists, and computational scientists at a scale rarely found in academia.
This collaborative environment allows him to tackle problems that would be more difficult to solve in isolation—bridging scales from the smallest particles to the largest astrophysical phenomena.
“UT has been a great place for me,” he said. “As a joint faculty member with UT and ORNL, I have access to the best of what the worlds of academia and national laboratories offer.”
Research in the classroom—and beyond
For Papenbrock, research and teaching are closely intertwined.
“I only fully understand a problem that I’ve solved in research or had to teach,” he said.
That philosophy shapes his approach in the classroom, where students are introduced to the same computational tools and concepts used in cutting-edge nuclear physics. By working with real-world models and data, students gain hands-on experience that prepares them for careers in research and other fields.
He recently collaborated with UT students and colleagues at ORNL to publish an open-source software package that allows users to solve simplified models of atomic nuclei, making advanced concepts accessible to a broader audience.
His work also contributes to training the next generation of scientists. Some of his former students have gone on to careers at national laboratories and research institutions.
Looking ahead: Tackling dynamic nuclear
processes
While significant progress has been made in understanding nuclear structure, Papenbrock sees new challenges ahead.
“We want to understand how to model dynamical processes such as fission and fusion with the same level of accuracy,” he said.
Those processes—in which nuclei split or combine to create energy—are central to energy production and to understanding how elements are created.
Advancing this area of research will require new theoretical approaches, improved computational tools, and continued collaboration across disciplines.
For Papenbrock, that challenge is exactly what makes the field so compelling. By refining the models and methods that describe atomic nuclei, he and his collaborators are revealing how the smallest building blocks of the universe shape everything from the elements around us to the stars above.