[Creative science] Birth by perturbation: strangely stable sand waves


Science has a problem. As a field, it’s suffering from a critical shortage — not of resources, funding, or direction, but of people who care. On the street, you hear the plaintive cry go up: “I was never very good at maths!” The time has come for change.

If you follow Australian politics at all (and my sympathies if you do) you might have heard our Most Illustrious Grand Nabob Malcolm Turnbull and his science minion — sorry, minister — Christopher Pyne ranting on about innovation in science and how Australia just doesn’t have enough. They’re proposing measures like extra funding for CSIRO (in all seriousness, yes, please fund science, thank you) and investment in quantum computing.

I’m here to tell you these are unnecessary. You want to really get people interested in science? Plonk them down in front of a clear stretch of ocean shore for a few hours and have them watch the sand in shallow water.

This brilliant idea stems from an excursion I took with some friends down to the farflung shores of St. Kilda in Melbourne. Every day around dusk, hordes of fairy penguins come zooming in back home to their nests on a length of the beach. This trip is memorable not only for time spent watching these adorable critters waddle around and the mysterious “penguin flu” our party subsequently contracted, but for the time spent prior to the penguins’ arrival.

For the three hours before our flippered friends’ return, my friend and I crouched by a piece of shore lapped by very gentle waves. While slowly moving water offers myriad fascinations — the shifting, shimmering refraction of dying sunlight off of crests or the motion of random debris caught in the currents — what we were watching was a tiny patch of sand engraved with an ornate pattern. What was enthralling about this pattern was not only the squiggles and whorls that composed it but the fact that it appeared to gradually inch towards us across the seabed intact, its shape undisturbed by the motion of the ten centimeters of water above it.

The ocean is an overwhelmingly complex place, only very slightly less complicated than the atmosphere around us. It is dominated by gargantuan oceanic currents that stretch over vast distances and is capable of transmitting forces a great way, so that an earthquake in Japan can generate a tidal wave which crashes down on Hawaii or California. The sheer power of the ocean is such that it’s difficult for even second-rate disaster movies to exaggerate its capability for turmoil and destruction (though I admit Sharknado came close). Given the chaotic mix of forces that shape the ocean, it’s astonishing that formations as stable as sand waves can exist.

A sand wave is a rhythmic pattern of peaks and troughs on shallow ocean floor like that of the North Sea shelf by the Netherlands, rising and falling in underwater dunes that can reach a quarter of the way to the surface. These are regular structures with potentially hundreds of meters between peaks, and like the little pattern my friend and I stared at, they don’t sit still, travelling up to tens of metres every year. True, this is much slower than the obligatory snail’s pace — your average garden-variety snail can chew up twenty-five metres a day, if it doesn’t stop for lunch — but when you consider the incredible volume of sand being shifted, it’s amazing that they move at all. They’re tugged along by tides, and can cause all sorts of problems, either by exposing buried pipelines to the ravages of the sea or causing a nasty surprise for sailors who could swear the ocean was a lot deeper just a few months ago.

While the mechanisms which shape and transport sand waves are increasingly well-understood, it is almost as hard to predict their motion as it is to flawlessly forecast the weather. This is once again due to the vast complexity of the oceanic system they inhabit. Some models can successfully mimic the formation and gradual movement of a sand wave by incorporating a slew of factors like varying water density and wind-driven currents, but their ability to say what any particular sand wave will actually do is frustratingly limited. It’s a bit like how I can model economic growth and fluctuation based on the last twenty years but still be unable to predict the Global Financial Crisis. Moving your numerical or computational model from the general to the specific — especially in systems with chaotic potential — is a terribly tricky business that perplexes thousands of scientists each year.


The snails of chaos theory

Yet sand waves are interesting for another reason. They wouldn’t form at all if it weren’t for the regular oscillatory motion of the tides gradually scraping sand from irregular sections of the seabed and carrying them towards emergent peaks which then become self-sustaining. An artificially flattened shallow-sea shelf will eventually form sand waves because the sand grains are slightly looser here or more tightly-packed over there. This is an example of birth by perturbation — the necessity for some imperfection or non-uniformity to set a process in motion.

There are a lot of examples of birth by perturbation, and the perturbations themselves can be nearly anything. Generally, bubbles need a centre of nucleation to form on a surface, which is usually a microscopic imperfection. Some surfaces like Teflon, however, can be smooth down to the micro-scale — bubbles won’t grow for the same reason that geckoes’ feet won’t stick. In your brain, tiny perturbations lead to neural cascades of thought and recollection; in nebulae, small irregularities in matter density can birth stars. Other, stranger cases where perturbations matter exist in chaos theory, where even minute and seemingly irrelevant changes to your starting conditions can cause incredible deviations in a system’s later behavior. These are loosely gathered under the umbrella term butterfly effect, though my favorite (if hypothetical) examples are found in the pages of absurd sci-fi books on time travel.

We may have wandered somewhat far afield from the hypnotic pattern in the sand that so transfixed my friend and me, but I still think my notion holds a lot of promise. If I could just get Malcolm Turnbull and Christopher Pyne to have a wander on the beach with me, we could sort out this thorny issue of innovation in no time at all. And if it starts raining and gets too dark to see the sand? Well, we can always leave climate policy for another time.

Image credits Wild Junket (top) and Daniel Ranalli (bottom)


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