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Salient.Victoria University of Wellington Students' Newspaper. Volume 32, No. 17 July 23, 1969

Professor Barber – Physics Dept

Professor Barber – Physics Dept.

I have been brought up to think that theories are invented so that one can foretell what will happen. One invents them and reinvents them until they work successfully.

For example I find that when I stir a teacup or a bucket of water, and leave it, the leaves in the tea, or sand grains in the water, are left in a little heap in the middle on the bottom of the vessel.

My first theory says this is natural; it will always happen when the water rotates. So I make the situation a little different and hang the bucket on a string and make it spin. The sand grains move to the outside edge now. The same thing happens with tea leaves. This is not what my first theory predicted. The theory must be wrong.

My second theory is that the sand moves to the outside edge because it is denser than water; the bucket acts like a "centrifuge". But theories are of little use unless they predict successfully in new situations. I therefore reverse the argument and predict that if the particles were less dense than water they would move to the middle. So I sprinkle cork fragments on the water (they float of course, being less dense) and I spin the bucket. Nothing happens; the floating particles begin to go round as the water picks up the speed of the bucket but they show no inclination to move to the middle in preference to the outside.

Photo of Professor Barber. Physics Department

Things are getting mysterious and I seem to be losing sight of my first question 'why did the sand gather in the middle?' But in research it does not pay, I think, to insist on answering the question one first thought of. The point of research, to my mind, is to succeed in making sense of something. It doesn't matter quite where one begins. The point is to succeed in making sense of something, then with luck the area of enlightenment will spread.

So back to the problem. The sand gathers in the middle only when the water is spinning but the bucket is not. This is a third theory. Let's invert it and predict that if the bucket is spinning and the water is not, then the sand should move to the outside. This is a good prediction because we have already seen it happen. The sand moves to the outside as soon as one rotates the bucket, long before the water has had time to pick up the spin.

So let's push the theory a little further. One should never leave a theory alone. One should always keep on pushing a theory until it breaks down (or until one gets tired of pushing); because a theory that finally breaks down points the way to an even better theory.

So think of some further twist. Perhaps if the sand were lighter than water it would move in the reverse way. But we have tried cork dust and nothing happened. Perhaps this was because the water surface became curved. When the water spins it climbs up the side of the bucket. Incidentally this might page 5 explain the sand motion at the bottom, for if the water moved outward as it began to pick up the spin it might carry the sand with it, and then as it lost its spin and the surface became flat again it could carry the sand back.

So let's stop the water surface from becoming curved by filling the bucket to the top and putting a glass lid on it. I prefer to do this with a closed jam jar on a gramophone turntable. Now predict what will happen to the sand in the jam jar. I think one should always predict what will happen before one does an experiment. This helps one to realise whether the theory is working or not. It keeps one's mind on the job.

The last theory suggests that if the water surface is kept flat, the sand should not move. It's wrong. The sand moves to the outside when the jar starts turning, and then if one stops the jar and lets the water spin inside it, the sand gathers in the middle. The curve on the surface is not the cause of the motion.

So let's go back to the theory about the relative motion of the jar and the water. This says nothing about the density of the sand. Try cork fragments. There is a glass top to the jar now and the cork fragments float against it. Toss a coin and say perhaps that they will behave like the heavy sand. This proves true when you make the experiment. They move out at first, then gather in the middle after you have stopped the jar turning.

It seems that the relative density of the fragments and the water is unimportant. I won't go further, but merely say that I think that the relative rotation of the jar and the water inside it, sets up circulating currents that waft the fragments about in the way we see.

My point in saying all this is that I think that theories are made to allow us to predict what will happen. We alter them until they do. But if one cannot use a theory to predict a behaviour, there seems little use in the theory, and until it can be shown to give successful predictions in new situations it is not to be taken seriously.

One last point is that, in physics at least, one can usually invent a variety of theories and that these must be assessed by seeing whether or not their predictions are true. Occasionally one has different theories that are equally successful in prediction and so cannot be distinguished by experiment. In that case one always choses the one that works out most simply. I will give an illustration.

My friend and I live three miles apart and I set out one day at 9.00 by my clock at home and reach his house when his clock says 9 30. I leave when his clock says 10.30 and reach home at 12.00 by my clock. This seems wrong, since the first journey takes half an hour and the other takes one and a half hours. It might happen day after day till it occurred to me that things would make more sense if my friend put his clock forward half an hour. If he did so I should arrive at 10.00 and leave at 11.00, having taken an hour each way. This would seem better.

But if my friend were obstinate he could maintain that both clocks were perfectly good clocks and showed the right time. If I carried my own clock with me and showed him that it read 10.00 on arrival he might, if sufficiently obstinate, point out to me a new law of nature, that clocks always gain half an hour when carried from my house to his, and conversely lose half an hour when carried the other way. This might strike me as a wrong headed view, but I could live with it if he insisted, and could make all the necessary additions or subtractions to keep appointments with him. But sensible people would arrange to alter one of the clocks by half an hour. If my clock is right, then the right way to adjust his clock is the way that makes for plain simple sense. It doesn't mean (to me) that it is the Right way with a capital R but it is right in the sense that it makes for the simplest arguments. So in physics we choose the simplest way to look at things and call it "right".

We say that the earth moves round the sun because this theory allows us to invent simple universal rules in physics. A person might maintain otherwise if he did not mind his physics becoming extremely complicated. But it would be "wrong" because it was more complicated than it need be.