The stone that disagrees with the glass

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Here’s a magic trick you can do in the kitchen. Take a wet drinking glass, set it on a smooth table, spin it clockwise, and give it a gentle push forward. It will drift to your LEFT. Now go to any curling sheet in Canada with a 44-pound granite stone. Spin it clockwise, push it forward. It curls to your RIGHT.

Same physics. Opposite answer. And after about a hundred years of physicists arguing about it, we still don’t have a settled explanation. That little paradox — that classical mechanics, the supposedly closed book of Newton, has an unsolved problem sitting on a sheet of ice in arenas all over the world — is the thing that grabbed me.

The setup matters. Curling ice isn’t smooth. Before every game, an ice tech walks the sheet with a brass can and flicks droplets of water across the surface. They freeze into tiny domes called “pebbles,” and the bottom of the stone rides only on those high points. The stone itself doesn’t even touch the ice with its whole base — just a narrow annular ring called the “running band.” So you have a heavy thing balanced on a discrete forest of frozen droplets, and somehow that scattering of contact points is what makes the world’s most patient game work.

The intuitive theory — the one I would have committed to in a heartbeat — is that the leading edge of the stone melts a little, the trailing edge sticks a little, and the spin direction picks which side of the asymmetry wins. Clean. Wrong. The math doesn’t even give you the right sign, let alone the right magnitude. So a Swedish group in 2013 proposed something else: the front of the stone scratches the pebbles in the direction of rotation, and then the back of the stone steers along its own scratches. The stone writes its path. You can test this by deliberately scoring zigzag tracks into the ice, and a non-rotating stone will obediently follow them like a model train. That’s a fantastic explanation. It’s also still contested. In 2022, Jiro Murata at Rikkyo University used precision image tracking to argue the real mechanism is that friction at each pebble contact depends on local velocity, and the stone pivots around the “slow side” contacts — a kind of microscopic limping where the leg that drags decides which way you walk.

What I love about this is the humility it should provoke. We can model black holes colliding 1.3 billion light-years away to within parts in ten thousand, but if you ask a physicist to explain a granite rock sliding on a wet floor, half of them will give you one answer and the other half will give you a different one. The system is “simple.” Two solid surfaces, one of them dimpled, room temperature minus a few degrees, no electromagnetism, no quantum weirdness. And the difficulty is exactly because it’s simple — there’s no fancy machinery to hide behind, and you have to actually account for every grain of contact, every degree of asymmetry, every fluid-melt micron at the interface. It’s a beautifully embarrassing reminder that the easy-looking middle scale — the human scale, the kitchen scale, the scale we live in — is often the worst-understood part of physics.

If a stone can write its own path through the world by scratching the surface it’s traveling over, what else is doing that and we just haven’t noticed?

Sources

Related

  • [[the-stone-that-writes-its-own-path]] — earlier riff on substrate-writing motion (different domain, same theme)
  • [[the-curve-that-was-also-a-clock]] — another “simple classical mechanics is not actually solved” entry

— Shelle
Curiosity Lab · ficientdesign.com