The mysterious balancing stones on frozen lakes | Hacker News Reader

The mysterious balancing stones on frozen lakes

physicstoday.scitation.org

121 pointssohkamyung17 comments

The article mentions reproducing this under careful lab conditions, but I reproduced this accidentally in my freezer. A few years ago, there were a series of storms that were likely to knock out power in my area. This had happened before, and I worried about food spoiling in my freezer. So I froze water in a plastic container, and (once frozen solid) I placed a coin on top. The thinking was: if the freezer thaws enough for the coin to drop into the ice, I may have spoilage. This never happened, but the ice around the edges of the coin slowly receded leaving the coin “balancing” on a spire of ice. This happened over the course of several weeks. Give it a shot yourself!

I had the same exact phenomenon. Bought a freezer for my garage and immediately did the coin-on-ice trick for a week before using it and then checked again 2 months later and found the strange position of the coin.

Which suggests that the umbrella shade explanation isn't very good and pressure melting point of water is probably more significant. That would also explain why the rock is balanced - highest pressure is just under center of gravity.

Pressure depresses the melting point of ice, so (as pointed out in the article) it is not a candidate explanation for ice remaining underneath the coin. Furthermore, if the pressure is insufficient to depress the melting point below the ambient temperature, it has no effect.

The claim that the highest pressure is just under the center of gravity does not hold in general, either. More generally, the pressure distribution depends on the distribution of the contact points between the body and its support (consider a four-legged table, for example.)

The article does not say that the umbrella shade hypothesis explains the phenomenon, though it is a factor, as it affects heat flow. The article suggests (very plausibly) that it is due to sublimation. As this only occurs where the ice is in contact with the air[1] (and that air is free to move and carry away the sublimated vapor), the erosion proceeds inwardly from the boundary of contact.

For single pedestals, the boundary does have to shrink in such a way that it always contains the point below the body's center of gravity, so I would like to see the experiment conducted with irregularly-shaped bodies (if I had more free room in my freezer, I might try it myself.) Hypothesis: for bodies of uniform thickness and density, this is often the case.

[1] As demonstrated by the apparatus used in the experiment (where the sublimation occurred into a vacuum), what really matters is exposure of the ice to the void between the molecules of the air.

The umbrella shade explanation might still be valid. There is still infrared radiation emitted from the roof of the freezer to encourage sublimation. It's just not visible light.

This means that ANY OF US can be at the leading edge of science, as long as we have a freezer.

[on further consideration]

Seriously, this unexplored corner of physics is like being in the position of Henry Cavendish when he first weighed the earth.[1]

You could use an PC with a web cam, and some lights, fans, heaters, etc... to try all sorts of experiments to see what the causes of this phenomenon are. Its rare to find such unexplored corners of knowledge that are so easily reached by a single experimenter.

[1] https://en.wikipedia.org/wiki/Cavendish_experiment

Depending on the sharpness of the spire, it may even be the cutting edge!

Well, the knowledge gained from the balancing stones might be as impressive as learning the density of earth at the time, but I wonder how hard it would be to explain "ice can evaporate without melting first, and everything is always faintly glowing with a type of warm light that you can't see, and can also cast shadows you can't see..."

"The thinking was: if the freezer thaws enough for the coin to drop into the ice, I may have spoilage."

Very clever :)

I was unable to find any pictures of penguin bodies on pedestals of ice as mentioned at the end. Even though it happens the same way I'm curious to see what that looks like.

Ars technica wrote about an earlier version of this research a year ago, it seemed familiar. https://arstechnica.com/science/2021/09/physicists-may-have-...

Reminded me of this[1]: leaves (and a stick in the background) creating a "rain shadow" on a river beach, resulting in sand pedestals.

[1] https://secure.flickr.com/photos/l0b0/50830815242/

That's the "hoodoo mechanism" which is also mentioned in the article: on a bigger scale, when you have a layer of harder rock on top of softer rock, you get hoodoos (also called earth pyramids or tent rocks): https://en.wikipedia.org/wiki/Hoodoo_(geology)

Nice, TIL. I wonder if there's a name for these ephemeral sand formations, as hoodoos seem to be more permanent.

The pictures made me think of this: https://www.physics.utoronto.ca/~smorris/edl/icespikes/icesp...

Oh that is interesting. There also are ice ribbons, flowers and needle ice. The ice flowers seem to be created by a similar mechanism but the hole is in a plant rather than the top of a container of water. https://www.americanscientist.org/article/flowers-and-ribbon...

Cool! I guess this only works with stones where the center of gravity is more or less the same as the geometric center (or more likely, the point where the shadow is the strongest, which may not be the geometric center depending on the angle of the sun).

This phenomenon[1] which you can use to find the center of gravity of an object might greatly increase the number of stones that work.

[1] https://www.exploratorium.edu/snacks/center-gravity

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