Sci: All good mixers

Herbicides, we are assured are applied selectively, only to their targets, and pollutants are only found in limited areas. When people say this, they are relying on your ignorance of entropy, the natural cussedness of matter. One of the most basic laws of nature is that everything spreads.

To see this happening, you need a deep glass container like a measuring cylinder or a long glass vase, a piece of fairly stiff glass or plastic tubing, taped to a small funnel, and some colour, either dye, or food colouring, or old-fashioned fountain-pen ink.

Fill the container almost to the top with water, put the tube in, and leave the container in a place where it will be safe for a week or two, to let random currents stop. Then pour your coloured material into the funnel and down the tube, so it gently reaches the bottom of the container.

Pour some fresh water down the tube to wash the last of the colour away and then take the tube out, again gently. You should now have a layer of colour at the bottom of the container, but over the next few weeks, random movement of the atoms and molecules will cause the coloured zone to expand and get lighter as the dye spreads in a process called diffusion.

Diffusion happens because of random events. Imagine you have an island, divided by a fence into a paddock full of rabbits (A), and another paddock (B) with no rabbits. The rabbits cannot get off the island, but there are gaps in the fence. This is my thought experiment, so I have blindfolded all of the rabbits, and they are wandering around at random, bumping into each other, tripping over carrots, banging into fence posts and whatever else blindfolded rabbits may do. I also go in regularly to blindfold any new rabbits.

Look, don’t argue. These are my imaginary rabbits, so I can do what I like in my thought experiment—even put them in 200 litre drums and blow Rugby whistles at them, so pipe down and pay attention! Now that I have your attention again, what is the probability of rabbits wandering into paddock A? The answer: none at all, because there are no rabbits in paddock B, yet.

The probability of rabbits wandering out into paddock B is higher. After a while, there will be about the same number of rabbits in each paddock, and now the flow of rabbits from A to B is about the same as from B to A. In real life, the rabbits would keep spreading wider and wider. We say that diffusion is always from areas of high concentration to areas of low concentration.

Play with this.

Most weeks, I use a herbicide, very selectively, in a sensitive ecosystem, and it doesn’t spread, because it breaks down quite rapidly. It is squirted onto the target plants, and we do no “carpet-bombing”.

Now about the oil drums and Rugby whistles: somebody did that once, while testing to see if stressed rabbits survive longer. The subject under study was the effects of stress on mortality in feral rabbits in Australia. There was reason to suspect that some fluctuating populations might be driven to vary by deaths that arose as a result of stress caused by living in a dense population. The experimenters needed stressed bunnies.

The rabbits were subjected to stress in the form of intense cold in a deep freeze running at -15° to -12°C, or in the form of noise made by a referee’s whistle attached to a powerful two-stage vacuum pump. The whistle and the rabbits were housed in a closed 44-gallon drum.
— Griffiths, M. E., J. H. Calaby and D. L. McIntosh, ‘The Stress Syndrome in the Rabbit’, C.S.I.R.O. Wildlife Research, 5(2), 1960, 134–149.

And that was why the rabbits were exposed to cold and whistles. Sadly, nothing came of it. Sometimes, you need a bit of luck on your side: it was not, so far as I can tell, meant to imply that Rugby players are bunnies.

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