When Frederik Gareis was still in secondary school, he poured water into a container, shook it, and watched air bubbles dance—a simple experiment that would eventually reveal a blind spot in physics that had persisted for decades. That observation, made during a student research project at the TAO student research center at the University of Bayreuth, sparked a discovery that physicists at the institution have now confirmed: a mathematical force so difficult to calculate that scientists routinely ignored it, distorting their predictions of particle motion in fluids by up to 60 percent.
The force in question is the Basset–Boussinesq history force, an elegant but computationally demanding phenomenon that arises whenever particles move through liquids or air with changing velocities. When a particle accelerates or decelerates, vortices form around it in the surrounding fluid. The fluid, in a sense, "remembers" these previous motions and influences where the particle goes next. It is physics as interconnected memory—but few researchers had bothered to account for it in their models.
The reason is practical rather than theoretical. "The history force is often ignored because it is mathematically complex and makes calculations significantly more demanding," explains Gareis, now a doctoral researcher in the Theoretical Physics I group at Bayreuth. "It is frequently unclear whether neglecting it leads to larger errors in modeling particle motion in fluids." So for years, researchers built simulations and predictions without it, assuming the omission was negligible.
Gareis and his supervisor, Prof. Dr. Walter Zimmermann, decided to test that assumption systematically. They proposed a deceptively simple experimental framework: horizontally shaken fluids containing particles—microplastics, bacteria, droplets, or solid particles—that undergo constant acceleration and deceleration. In such conditions, they could measure with precision how the history force actually affects particle motion and compare those measurements against theories that ignored it altogether.
The results were striking. Theories that neglected the history force overestimated the motion of small particles relative to the fluid by as much as 60 percent. This is not a minor correction; it is the difference between understanding a system and systematically misunderstanding it. The Bayreuth team also identified a crucial practical tool: specific patterns in measurement curves that would allow researchers to detect when the history force is significant enough to matter in their work.
"Our results now make it possible to estimate when the history force should not be neglected in calculations of particle motion in flows in nature and technology," Gareis says. The implications ripple outward. Particle motion in fluids is fundamental to countless fields—from industrial filtration and chemical engineering to environmental science and medicine. Microplastics in water, aerosol behavior in air, bubble dynamics in fermentation—all of these involve particles moving through fluids, and all could be modeled more accurately with this correction.
What makes this story particularly hopeful is not just the science but its origin: a secondary school student, curious and persistent enough to follow a puzzle wherever it led, working alongside a mentor willing to listen. Gareis's observation became a research project alongside his university studies, and that project is now published in Physical Review Fluids. The history force, once forgotten, is now documented and practically testable. Sometimes the most overlooked forces—in physics and perhaps in life—are simply waiting for someone attentive enough to notice them.