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Olympic sport's physics remain unsolved after 500 years
Curling, the Winter Olympics' centuries-old sliding game, still puzzles physicists despite decades of research. While players glide granite stones across pebbled ice and sweep brooms to steer them, the exact mechanics behind the stone's signature curl remain unexplained.
From frozen lochs to Olympic rinks
Originating in 16th-century Scotland, curling involves hurling 20-kilogram granite discs toward a circular target called the house. Players sweep the ice ahead of the stone to adjust its speed and trajectory. Despite its simplicity, even elite coaches debate the most effective techniques due to gaps in scientific understanding.
The central puzzle: why stones curl
When a stone is launched with a clockwise spin, it curves to the right as it slows-counterintuitive to basic physics. A spinning object on a smooth surface, like a glass on carpet, typically curls in the opposite direction of its rotation. Curling stones defy this expectation, leaving researchers baffled.
"The scientific community hasn't reached consensus on curling's physics, despite over a century of study," says Jennifer Vail, a tribology expert and author of Friction: A Biography. "The mechanisms behind the stone's curl are still unsolved."
Engineered for unpredictability
Curling's complexity stems from its materials. The stones, sourced from only two quarries in Wales and Scotland, are uniquely durable and water-resistant. Their concave underside features a narrow running band that contacts the ice. Meanwhile, the playing surface is "pebbled"-sprinkled with water droplets to create a textured, low-friction path. Without this roughness, stones wouldn't glide far enough to reach the target.
As the stone slides, friction melts the ice, forming a microscopic water layer that reduces drag. Sweepers amplify this effect by brushing the ice ahead of the stone, generating more meltwater to prolong its travel. A 2024 study identified three phases of movement: initial hydroplaning at high speed, gradual curling as meltwater diminishes, and final dry friction as the stone slows to a stop.
Theories, controversies, and rule changes
Physicists have proposed numerous explanations for the curl, none definitive. The earliest theory, from 1924, suggested uneven friction between the stone's left and right sides. Later models included the "snowplow" effect, "scratch-guiding," and "slip-stick" mechanisms, but all fell short of consensus.
In 2022, physicist Jiro Murata of Rikkyo University in Tokyo approached the problem experimentally. By filming stones in motion, he observed they rotated around a pivot point, akin to a pendulum. "The spin creates a friction difference that acts as a pivot," Murata explains. "If the stone catches the ice on its left, it curls left-like grabbing a pole while running."
Murata's 2024 research also addressed sweeping techniques. While players debate whether to sweep inside or outside the stone's curve, his experiments confirmed that sweeping the outside enhances the curl by reducing friction there, forcing the stone to pivot more sharply around the higher-friction inside edge.
Technological advances have further complicated the sport. In 2015, brushes capable of scratching the ice sparked a "broomgate" scandal, prompting the World Curling Federation to ban certain materials in 2016. By January 2026, sweeping techniques that decelerate the stone-such as single-push motions without a follow-through-were prohibited.
Unanswered questions and future research
Despite progress, the curl's physics remain contested. Variables like ice temperature, humidity, pebble condition, and even microfractures in the stone introduce layers of complexity. Murata's pendulum theory, while compelling, is one of many competing explanations.
As Vail notes, "Curling's mystery persists because it's a perfect storm of tribology, fluid dynamics, and material science." Until a unifying theory emerges, players and scientists alike will continue to strategize-on and off the ice.
