Physics: An Illustration of Discovery
Peter Hodgson FAITH Magazine November-December 2006
Physics is the activity by which we attain our knowledge of the structure and dynamic interactions of the inorganic natural world. But what is knowledge? It is vital to distinguish between speculation and knowledge. Speculation is an essential starting point, but until it is tested and verified it is not knowledge. Failure to make this distinction can easily confuse discussions of the relation between science and faith.
When a scientist is interested in a certain phenomenon he tries to imagine what interaction lies behind it, what is the hidden ‘mechanism’. Then perhaps he has an idea. He thinks about it and it seems to make some sense, in a qualitative way, of his observations. The next stage, where physics begins, is to make some measurements of the phenomenon and to see how the resulting numbers can be connected. He somewhat speculatively constructs a mathematical formalism that enables him to calculate those numbers, and then compares the calculated numbers with his measured numbers. If they agree they provide some support for his idea, but if they disagree they show that his idea is false.
This can be illustrated by some examples. Aristotle thought about the world and made some observations. It seemed to him that there is a sharp distinction between the celestial world where everything is unchanging and the terrestrial world of change. He thought that all material particles seek their natural places, so a stone falls towards the centre of the earth. The heavier the stone, the more strongly it seeks its place, so an object of twice the weight falls twice as fast.
All this is speculation. It never occurred to him to test this idea by seeing if it is true. All he had to do was to drop a pea and a potato at the same time and notice that they hit the ground simultaneously. This experiment, taking just a few seconds, is enough to demolish his idea. He did not do so; he thought that it was beneath the dignity of a philosopher to make experiments and he had no knowledge of mathematics. He was not a physicist.
Much the same can be said of two other speculative philosophers, Descartes and Kant. Descartes suggested, with persuasive rhetoric, a theory of the solar system whereby the planets are carried round the sun by vortices, but he made no calculations to see if it is true. He based his theory of motion on false premises, and so the results are absurdly wrong. He would easily have found this out if he had made a few experiments, but he did not do so. Kant put forward a theory of the origin of the solar system, but again made no calculations. These ideas were no more than unsupported speculations.
In sharp contrast, Newton had the idea that the same force that pulls a stone to the ground also keeps the moon in its orbit. This was also just a speculation, but Newton went on to propose his theory of universal gravitation that enabled him to calculate the strengths of the forces on the stone and the moon. He found that they differed by 20\%, so he concluded that his idea was false, and put his calculations aside. Some time later he heard about a new determination of the radius of the earth, which comes into his calculations. With the new value, the results agreed, and so supported his idea. This is physics. He went on to show that the theory describes the motions of the planets to high accuracy and implies Kepler’s empirical laws. It also shows that the speculations of Aristotle andDescartes are false.
In his book on optics, Newton distances himself from speculative philosophers like Descartes by declaring: ‘My design in this book is not to explain the properties of light by Hypotheses, but to propose and prove them by Reason and Experiment’. He knew that what is essential is a combination of reason and experiment, and without one of them it is just respectively speculation or observation, and without both of them it is not physics.
This distinction between speculation and physics is not always understood. The French physicist Pierre Duhem, a devout Catholic, once attended a conference of Catholic philosophers and theologians who discussed the philosophy of science. After listening for some time he could contain himself no longer and told them straight out that unless they had studied the pure sciences for their own sake for at least ten or fifteen years, they should stay silent on such questions.
This distinction between speculation and reality is found in other fields. Many people had the idea of a jet engine, but it was Frank Whittle that actually built one and made it work. Many people say we can obtain all the power we need by building windmills. A good idea, but please build one and carefully measure the power output, the reliability, the cost, the safety and the effects on the environment and when you have the numbers compare them with similar numbers obtained for other sources of energy.
Only then will you be able to say whether the idea is worth implementing. ‘Science is measurement’, thundered Lord Kelvin, ‘unless you can measure what you are talking about and express it in numbers, your knowledge is of a most meagre and unsatisfactory kind’. He could well have added that unless you have a theory that can be expressed mathematically and which enables you to calculate those numbers and compare them with your measurements they are worthless observations.