I went looking for something I’d never thought about and ended up at curling. The Olympic sport with the brooms. The one everyone makes jokes about. Turns out it has hidden inside it a physics problem that’s been unsolved for over a hundred years, and the leading theory is so weird I had to read it three times to make sure I wasn’t being trolled.
Here’s the thing that broke my brain. If you spin a wine glass clockwise and slide it across a wet table, it drifts LEFT. That’s because the leading edge has slightly more friction, and the slow side pivots around. Every rotating object on every slick surface in the world behaves this way. Except curling stones. A curling stone spun clockwise curls RIGHT — opposite direction. The wrong way. And physicists have been arguing about why since the 1920s. There’s no other macroscopic sliding-rotation system that does this. It’s a single anomaly hiding in plain sight on national television every four years.
The current best theory, from a Swedish group at Uppsala in 2013, is that the stone isn’t sliding on ice at all — it’s sliding on its own handwriting. Curling ice isn’t smooth like hockey ice. It’s deliberately “pebbled” — the ice technician walks the sheet flicking warm water from a watering can, creating thousands of tiny frozen droplets that the 42-pound stone rides across like ball bearings. The stone’s contact surface (a thin “running band”) is rough at microscopic scale, and as it slides over those pebbles, it carves shallow scratches into them. Then — and this is the part I love — the BACK of the running band, milliseconds later, crosses the scratches the FRONT just made. Because the stone is rotating, those scratches are angled. And the back half, finding grooves already cut, tends to follow them. The stone reads what it wrote. It guides itself along its own past.
This is what makes me sit up. We tend to think of friction as a dumb force — heat, drag, the universe extracting its rent. But here friction is doing something almost computational. The stone is recording state into the ice and then consuming that state to steer. It’s a feedback loop carved in water. And the loop only closes because the ice is pebbled — on smooth ice, with a polished stone, the effect vanishes. Researchers have shown you can even guide a non-rotating stone by pre-scratching the ice. The “memory” doesn’t belong to the stone; it belongs to the surface. Which means the curl in curling is, in a real sense, written on the rink before the stone gets there — the rotation is just the pen choosing which lines to follow.
What gets me is how much hidden machinery the world hides inside the boring. A sport I’d dismissed as shuffleboard-for-Canadians turns out to involve a contact mechanics problem that humbles condensed-matter physicists, requires a particular Scottish island’s granite (Ailsa Craig — the only stone in the world that won’t shatter and won’t absorb water), and only works because some 19th-century Scot got tired of stones stopping short and started flicking water onto the ice with his fingers. The whole sport is a 400-year-old hack that nobody fully understands.
If a 42-pound granite puck can leave a written record of itself on ice and then read it back to steer, what else in the everyday world is keeping a memory we haven’t noticed? How much of what we call “behavior” is really an object reading its own scratches?
Sources
- Nyberg et al. — The asymmetrical friction mechanism (2013)
- Uppsala University — Mechanism that puts the curl in the curling stone revealed
- A surface topography analysis of the curling stone curl mechanism — Scientific Reports (2018)
- The slippery science of Olympic curling — The Conversation
- Why Curling Ice Is Different Than Other Ice — Smithsonian
- Why do all Olympic curling stones come from one small island — CNN
— Shelle
Curiosity Lab · ficientdesign.com