What Is It That A Physicist Aims To Do?

To answer this we must consider how a physicist works. Suppose that he is faced by some new "animal." It may be a living thing or it may be some inanimate system such as a thunderstorm. He first watches what it does. He compares it in his mind with other things he has met. He adopts those useful inventions the integers and makes numerical comparisons. After a while he may venture some theories, predicting what it will do next, perhaps, or guessing what it may look like from the other side, or from the inside. Certain studies, notably astronomy, restrict one to this approach. But every physicist tries to advance to the next stage of enquiry. This is when he, so to speak, points a finger and gently pokes the animal. It reacts, and by repeating the poking in a number of ways he rapidly extends his knowledge. This is called the "controlled experiment," for he can control just how he pokes it or when or where. I would like to illustrate this by something told to me by Mr Benseman of the Physics and Engineering Laboratory. He was directing his attention to geysers in the New Zealand thermal area. Most people study geysers by watching them, but Mr Benseman wished to poke his animal. The geyser was erupting at regular intervals through a hole in a shallow pool of water at the foot of a sloping bank of ground. One might, of course, plug the hole of the geyser, and what would it do then? But rather than risk destroying this scenic attraction he dealt with it more ingeniously. He argued that the water in the pool must in some degree impede the eruptions, so what could be simpler than to pile sandbags round the rim of the pool. After the next eruption the water in the pool was about a foot deeper than before. The eruptions now became more feeble in the sense that less water was being delivered. The volume of overflowing water was being measured at a weir so this change in behaviour expressed itself numerically. Soon, however, the activity returned; each eruption was producing the customary volume of water. What should be done now? Between two eruptions Mr Benseman removed the sandbags so that the pool fell to its usual level. The next two eruptions were big ones. When the measurements were compared it was found that they had delivered, in addition to the usual amount, all the volume of water that had failed to be delivered at the time when the eruptions were weak. You see the implication. The increased pressure had caused water to be stored somewhere underground. This immediately invites one to propose some system of underground channels and chambers that would store water in this way.

This look at the actions of a physicist will, I hope, suggest to you, as it does to me, that a physicist is trying to find recipes for action. He describes a body by the ways in which it would affect other bodies and ultimately ourselves. Consider the statement that this wooden bench before me occupies a certain position in space. What does the statement mean? Among other things, it means that if I am advancing my finger downwards I would do well to arrest it before it penetrates the wood, for this would cause me discomfort. So I suggest that we adopt the following view:—

All the laws of physics are recipes for action.

If we adopt this view it disposes of most of our previous difficulties. We asked whether the Earth is really flat or really round. When we ask if a thing is "really there" we are asking whether the recipes for action that we deduce from the statement "it is there" will in all conceivable circumstances be correct. But the real world does not "is"; it acts. We are concerned with events; events are the real world. We develop concepts of position in space and a flow of time but these are only maps to help us guide ourselves among the events. If the maps provided by Newton and by the advocate of the flat Earth are equally successful in guiding us (as they might well be) then both are right.

But the advocate of the flat Earth has, so to speak, taken Newton's map, stretched it like an India rubber skin, and tied it in knots, with the result that it is more difficult to read. So we prefer Newton's map. If the modern physicist chooses to picture hypothetical waves in empty space this need not discompose us. Events are real. His curious picture is only a map.

Occam's rule becomes commonsense; one adopts the theory whose recipes are most simple to work out. Yet some may say that a difficulty remains. How does the world come to be such a place that it is possible to find simple theories or indeed any theories at all? Why are not events quite unrelated? The only explanation I have ever been offered is that God is one and not many. Some people do have a strong conviction that the world is orderly. Look at Kepler struggling for fifteen years with the numbers so rigorously collected by his master Tycho Brahe. Kepler did not know what he was looking for; he only felt he would recognise it when he saw it. One good theory leads in a very striking way to another and Newton follows Kepler, It can seem very surprising that de Broglie could declare that electrons were waves before the matter was tested, or that Yukawa could use the new quantum theory ideas to assert the nature of a kind of primary particle, the meson, that no one had ever thought of; yet these were found when they were looked for. A physicist does not have the emotional energy to go about in perpetual astonishment at the orderliness of the world. Some physicist may say that there is no point in being astonished at so obvious fact, yet I imagine that, if one of these sudden theoretical clarifications took place in him, the emotions he might record would have that flavour.

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Attitude Of A Physicist

Continued from page 7.

I will mention here two attitudes that are important. I think, in the practice of physics and, presumably, in the teaching of it. The first is:—

Every law in physics has been invented by a man.

One hopes that, if one can take students to the situation in which that man found himself, they will declare that he was a sensible fellow and that they would have wished to invent the same theory. Later they may be able to say, "I know why he wrote the theory as he did, but had he been aware of the results of my recent experiments he would have written it differently." And they proceed to write it differently. The second is:—

Always idealise.

A theoretical physicist never attempts to deal with the entire complexities of the world. He makes up a fictitous world in which, for example, all solids may be perfectly rigid, all fluids quite frictionless, and so on. This is called "idealising" the situation. It gives him a problem with which his logic can cope. A physical oceanographer may contemplate the sea. There are many different ways of idealising that. But you see what he is searching for. He needs an idealisation of the sea about which he can ask himself questions and which is sufficiently simple for him to answer the questions, yet the answers must be unexpected and new. When he has seen through all this and has found an intriguing answer, he turns himself into an experimentalist and goes about to find whether the sea actually behaves in this way. This mixing of fantasy and fact is the art, or artfulness, of a physicist.