After nearly two weeks of continuous curling coverage, a viewer of the Winter Olympics could be forgiven for concluding that curling, while it may technically qualify as a sport, does not seem terribly Olympian. Archery, too, is fringe and unexciting, but at least the Greek goddess Artemis did it. Hockey—Ares would have loved it. Hermes probably kept a luge uniform in his closet. But curling exists because (and with all due respect to the many fine athletes involved) . . . why?
Scientists have long wondered the same thing, and lately the urge to explain it has taken on a competitive edge. By now, you know the basic rules: two teams take turns sliding, or “throwing,” a fortyish-pound puck of granite down a lane of ice toward the center of a horizontal target. When the stone is set in motion, it is also made to rotate slightly, and this rotation causes it to curve, or curl, to one side or another. If you’re the thrower, you can aim your stone to block your opponents’ stones, knock them out of the way, or even slide around them; own the curl, you own the game. To help the stone reach its intended target, your teammates can, using special brooms, furiously sweep the ice directly in front of it, warming the ice, reducing the friction, and making the stone travel slightly farther. It’s shuffleboard meets Swiffer ad.
“It’s way harder than you think,” Mark Shegelski, a physicist at the University of Northern British Columbia, and a recreational curler, told me recently. “It’s like golf: it’s easy to watch a guy hit a golf ball, and you think, ‘This isn’t very athletic.’ And then you get out there yourself and find that it’s incredibly difficult.” Also incredibly difficult: understanding why curling stones curl the way they do, a problem that Shegelski has been chipping away at for two decades.
Unlike skating ice, which is made to be as smooth as possible— “burnt,” in industry parlance—curling ice is pebbled. Between games, it is sprayed with droplets of water, which freeze to form microscopic bumps. As all curlers know, pebbling is essential to the sport; without it, a curling stone wouldn’t curl. This, however, is where the certainty ends. In most other respects, Shegelski told me, curling defies traditional logic.
The bottom of a curling stone resembles the bottom of a beer bottle. It’s concave, not flat, so as it slides only a narrow ring of stone—the running band—actually interacts with the ice. Take a beer bottle or an upturned glass and send it spinning down a table: if it rotates to the right, clockwise, it will curl to the left; if it rotates to the left, it will curl to the right. That’s because the bottle, as it moves forward, also tips forward slightly, adding weight to the leading edge. More weight means more friction. As the leading edge turns to the right, it meets with greater resistance than the back edge, turning to the left, does. So the clockwise-spinning bottle follows the path of least resistance, curling to the left.
Weirdly, a curling stone on ice does exactly the opposite: if it rotates to the right, it curls right, and vice versa. Shegelski said that, at the bar after a game (“Drinking beer after curling is absolutely required; it’s a must”), he sometimes blows the minds of fellow-curlers by sending an upturned glass spinning across the table. “To their horror, the drinking glass curls the wrong way,” he said. “All the curlers would be, like, ‘Whoa, how’d you do that?!’ ”
Early attempts to explain a curling stone’s behavior essentially worked the beer-bottle mechanism in reverse. If a clockwise-spinning bottle curls left because there’s more friction in the front than in the back, a stone spinning the same way must curl right because there’s more friction in the back than in the front. But why? Several theories emerged under the umbrella of “asymmetrical friction,” including, in the late nineteen-nineties, one by Shegelski. He proposed that, like the beer bottle, the curling stone tips forward slightly as it slides; the added pressure warms the ice, creating a thin film of water that acts as a lubricant, which reduces the friction in the front and, by comparison, increases it in the back.
This became known as the thin-liquid-film model, and it reigned for a few years, largely for want of challengers. But there’s more to the mystery of the curl than just “Why?”; there’s also “How much?” A curling stone can curl by as much as a metre and a half to either side. It’s clear that the curl is caused by rotation, since a stone that’s thrown without rotation doesn’t curl. But, in a game, the typical stone rotates only a couple of times during its long slide, and asymmetrical friction doesn’t generate enough force to produce that much curl. Even stranger, the curl stays pretty much the same whether the stone rotates twice or twenty times. “These models will not work, because the effect will never be strong enough to explain what we see,” Harald Nyberg, a materials scientist and friction expert at Uppsala University, in Sweden, told me.
In June, 2013, Nyberg and his colleagues made that argument in an equation-laden paper in the journal Tribology Letters (“tribology” being the fancy word for “friction science”). Not long before, in the journal Wear, they had proposed a model of their own, which became known as scratch-guiding theory. Using images from an electron microscope, the researchers showed that, as a curling stone slides along, it leaves fine scratches on the ice in the direction of rotation. The scratches are laid down by the front edge of the running band, but when the back edge encounters them it has a tendency to follow them, causing the stone to curl in the direction of rotation. In follow-up experiments, Nyberg’s group found that, by scratching the ice themselves in various ways, they could alter the trajectory of sliding stones, even ones that weren’t rotating. In one setup, they created a lane in which the ice was scratched in one direction and then, farther on, in the other direction. Then they slid a stone down the lane and watched as it curled, first one way and then the other.
“All of this added up to a mechanism that we felt was reasonable,” Nyberg said. He feels further vindicated by a controversy that broke out a couple of years ago: curlers were using new brooms that scratched the ice rather than merely warming it, enabling them to control the curl like never before. The “Frankenbrooms” have since been banned by the World Curling Federation. “I don’t think they’d read our papers, though,” Nyberg said.
One person who did read Nyberg’s papers was Shegelski, and in January, 2016, he wrote a comment piece in Tribology Letters that called the dismissal of asymmetrical-friction models “inappropriate.” Nyberg and his colleagues soon replied, reiterating their earlier points. When I spoke with Shegelski, he noted that, while scratch-guiding theory is intriguing, the theorists haven’t attempted to demonstrate that the mechanism can generate a metre-wide curl, much less explain why the curl is the same regardless of the rotation rate. “The fact is, in a theory, you need to have quantitative results, and there aren’t any,” he said.
Coverage by New Yorker writers of the 2018 Winter Olympics.
Recently, Shegelski teamed up with Edward Lozowski, a physicist and atmospheric scientist at the University of Alberta, to reconsider the curling conundrum. Some years earlier, Lozowski had published papers on the science of bobsledding and speed skating. “Ed is just a wizard at the physics of ice,” Shegelski told me. Together, the two men developed an improved explanation, which they unveiled in the latest issue of Cold Regions Science and Technology. They call it the pivot-slide model. Lozowski, whom I spoke to over Skype, explained it to me by holding up a hair comb and running his finger slowly across the teeth. Notice, he said, that his fingertip adheres to each tooth long enough to bend it, until the elastic force becomes large enough that the tooth breaks free and snaps back into place—that’s stick-slip friction. The same force will cause a circular saw, when it binds, to jump up and try to pivot around the obstacle. That’s what’s happening with a curling stone, he said.
“Every time the running band encounters a pebble, it catches on it,” Lozowski explained. “And, because ice is elastic, the pebble is deflected, then snaps back, and the rock moves on, pivoting during the deflection.” The pebbles and the pivots are small but they are many, and Lozowski and Shegelski calculated that the net result is enough to curl the stone by a metre. They emphasized that there is still a lot of research to be done—making models of the curling ice, quantifying the number of pebbles per unit area. “It could be all wrong,” Lozowski said. “But whether it’s right or not, at least it’s testable.” Nyberg, for one, is unpersuaded. “To be honest, I don’t really understand what they’re getting at,” he said.
“We aren’t by any means saying we’ve figured it all out,” Shegelski said. “What curling rocks do is so complicated that there’s got to be more than one thing going on.” He said that he wouldn’t be at all surprised if the stone’s path was shaped by a series of mechanisms—one at the beginning of the slide, another in the middle, maybe another toward the end. “It will take more than two people working on this to solve it,” he said. “We all need to pitch in.”
