Evolving Science and Theories - TOI - 18/01/2008

The Truth About Models

Scientific explanations are never regarded as infallible

John Gribbin

What do scientists mean when they say that they “know” what goes on inside an atom or what happened in the first three minutes of the life of the universe? They mean that they have what they call a model of the atom, or the early universe, or whatever it is they are interested in, and that this model matches the results of their experiments, or their observations of the world. Such a scientific model is not a physical representation of the real thing, the way a model aircraft represents a full-scale aircraft, but is a mental image which is described by a set of mathematical equations.
The atoms and molecules that make up the air that we breathe, for example, can be described in terms of a model in which we imagine each particle to be a perfectly elastic little sphere (a tiny billiard ball), with all the little spheres bouncing off one another and the walls of their container. That is the mental image, but this is only half the model; what makes it a scientific model is that the way the spheres move and bounce off one another is described by a set of physical laws, written in terms of mathematical equations.
In this case, these are essentially the laws of motion discovered by Isaac Newton more than 300 years ago. Using those mathematical laws, it is possible to predict, for example, what will happen to the pressure exerted by a gas if it is squashed into half its initial volume. If you do the experiment, the result you get matches the prediction of the model (in this case, the pressure will double), which makes it a good model.
Of course, we should not be surprised that the standard model of a gas which describes it in terms of little balls bouncing off one another in accordance with Newton’s laws makes this particular correct prediction, because the experiments were done first, and the model was designed, or constructed, to match the results of the experiments. The next stage in the scientific process is to use the model you have developed from measurements carried out in one set of experiments to make predictions about what will happen to the same system when you do different experiments.
In fact, all scientific models have restricted applicability. None of them is “the truth”. The model of an atom as a perfectly elastic little sphere works fine for calculating changes in pressure of a gas under different circumstances, but if you want to describe the way an atom emits or absorbs light, you
need a model of the atom in which it has at least two components, a tiny central nucleus surrounded by a cloud of electrons. Scientific models are representations of reality, not the reality itself, and no matter how well they work or how accurate their predictions under the appropriate circumstances, they should always be regarded as approximations and aids to the imagination, rather than the ultimate truth.
When a scientist tells you that, say, the nucleus of an atom is made up of particles called protons and neutrons, what they should really say is that the nucleus of an atom behaves, under certain circumstances, as if it were made up of protons and neutrons. Lesser scientists, and many non-scientists, often think that the role of scientists today is to carry out experiments which will prove the accuracy of their models to better and better precision — to more and more decimal places. Not at all! The reason for carrying out experiments which probe previously untested predictions of the models is to find out where the models break down. It is the cherished hope of the best physicists to find flaws in their models, because those flaws — things that the models cannot predict accurately, or explain in detail — will highlight the places where we need a new understanding, with better models, to make progress.
The archetypal example of this is gravity. Newton’s law of gravity was regarded as the most profound piece of physics from the 1680s to the beginning of the 20th century. But there were a few, seemingly tiny, things that the Newtonian model could not explain (or predict), involving the orbit of the planet Mercury and the way light gets bent when it goes past the Sun. Albert Einstein’s model of gravity, based on his general theory of relativity, explains everything that Newton’s model explains, but it also explains these subtle details of planetary orbits and light bending. In that sense, it is a better model than the older model, and it makes correct predictions (in particular, about the Universe at large) that the older model does not. But Newton’s model is still all that you need if you are calculating the flight of a space probe from the Earth to the Moon.
The trick is to use the right tool for the job. Just as a carpenter has many tools in his toolbox, and would never dream of using a chisel instead of a screwdriver, so the scientist has many models in his kitbag, and needs to choose the right one to apply in different circumstances. By doing so, he has been able to tell the whole story of the universe, from its birth 13.7 billion years ago, right up to the present day. This is surely the greatest achievement of the human intellect.

This is not my article - The writer is a visiting fellow in astronomy, University of Sussex - as printed in TOI