Why physics uses models it knows are wrong
Why physics uses models it knows are wrong
Point masses, ideal gases, surfaces with no friction. A-level Physics is built on things that do not exist, and understanding why is the beginning of thinking like a physicist rather than a problem-solver.
A-level Physics is populated by objects that cannot be found anywhere in the world. Strings have no mass. Surfaces have no friction. Particles occupy a single point and take up no space. Gases are ideal, collisions perfectly elastic, pulleys perfectly smooth. Students sometimes treat these as unfortunate simplifications, the sort of thing a real physicist would apologise for. They have it exactly backwards. Idealisation of this kind is not a weakness of the method. It is the method.
Strip away to see the mechanism
Physics makes progress by removing detail on purpose. A real dropped ball meets air resistance, spin, buoyancy and a dozen other influences at once, and if you tried to account for all of them from the start you would learn nothing about any of them. So you begin with the frictionless, airless idealisation, which isolates the single mechanism you want to understand, gravity acting alone. Then you add the complications back one at a time, in order of importance, when the problem requires them. The statistician George Box put the principle in a sentence that physicists have adopted as their own: all models are wrong, but some are useful. A model is a deliberate simplification, and its worth is measured not by how faithfully it copies reality but by what it lets you calculate and predict.
The ideal gas, and its corrections
The ideal gas law is a clear example of how this works. It assumes that gas molecules have no volume and exert no forces on one another, both of which are false. Real gases deviate from it, especially at high pressure and low temperature, where the molecules are crowded and their mutual forces begin to matter. But the law is not therefore a failed description. It is a limiting case that real gases approach as those complications become negligible, and when they do not, physicists reach for a refined model, the van der Waals equation, which adds terms back to account for molecular size and attraction. The ideal law tells you the baseline behaviour, and the deviations from it are themselves informative, because they reveal the forces the ideal model left out.
The map and the territory
Borges wrote a one-paragraph fable about an empire whose cartographers made a map so detailed that it was the same size as the territory, coincided with it point for point, and was therefore completely useless. A map earns its usefulness by leaving things out. A model does the same. The skill in physics is not to build the most detailed model but to know which details can be dropped for the question at hand, and, just as importantly, which cannot. That judgement is the reading-the-question skill we describe in our guide to reading a Physics question properly, taken one level deeper.
Even the fundamental theories are models
The point reaches all the way up. It is tempting to think that Newtonian mechanics was simply wrong and Einstein corrected it, but that is not how physicists see it. Newton's mechanics is the correct description of the world at everyday speeds and scales, and it emerges as the limiting case of relativity when speeds are small, and of quantum mechanics when objects are large. It was not overturned so much as bounded: its domain of validity was mapped. Modern physics is a layered set of theories, each an excellent model within its range and each aware, in a way earlier physics was not, of where that range ends. Knowing the domain in which a model holds is not a footnote to the physics. It is part of the physics.
For applicants to Physics, Engineering or Natural Sciences, this is the terrain interviews love, because a good interviewer will take a model you know and push it until it breaks, then ask what to do next. Understanding idealisation is understanding what that question is really testing. Richard Feynman's lectures on the character of physical law are the ideal place to start, and they sit on our reading lists alongside others that open this out. Learning to think this way is at the heart of our Physics and admissions tutoring.
