# History and a Q&A Approach

I recently came upon the Review of Modern Physics series from the end of the 20th century where prominent figures wrote pieces about the history of subjects ranging from gravity and dark matter to biophysics and neural networks. I read Kohn’s article about the history of condensed matter physics (pdf!) from this series. Since the piece was written in 1999, it does not include a discussion of two dimensional materials, nor of topological insulators.

Nonetheless, I found the article to be quite illuminating because of the author’s historical question-and-answer perspective. Before embarking on the discussion of a topic, Kohn explained historically the importance of both the question and the answer. For example, an outstanding question at the start of the 1900s was: why do electrons in metals, which are assumed to be “free”, not contribute to the specific heat of solids at room temperature? Classically, each free electron should contribute $\frac{3}{2}k_BT$ to the total energy by the equipartition theorem. The answer, of course, has to do with quantum statistics, which was only developed a decade later.

I have noticed that textbooks often don’t take this approach to presenting material and the important questions from a historical perspective are not explicitly stated. In my opinion, this omission leaves students often staring despairingly at the whiteboard wondering: what is the point of all of this again…? Most of us take a question-and-answer approach to our work on a day-to-day basis. It is therefore imperative that we not only relay basic material to undergraduate and graduate students, but also communicate the question-and-answer method we use to solve problems.

Can you imagine learning the photoelectric effect without understanding its  quantum mechanical consequences? Or Young’s two-slit experiment without knowing about the particle/wave debate concerning light?

Then we should not take such an approach in condensed matter physics either. For instance, Kittel’s textbook on solid state physics starts the chapter on the Debye $T^3$ relation with the following sentence: We discuss the heat capacity of a phonon gas and then the effects of anharmonic lattice interactions on the phonons and on the crystal. I think that this is an approach we should try to avoid. Now that I understand the historical context, I find this book to be quite valuable, but I remember struggling with it as an undergraduate.

In this sense, Kohn’s article is excellent in that it provides a historical context to some of the most important advances in condensed matter physics. It starts from classical physics, and goes through the Born-Oppenheimer theorem, the Sommerfeld model, the Bloch band paradigm, Landau Fermi liquid theory to Mott and Wigner insulators, among many other stops. I recommend it as some bedtime reading, which is exactly what it was for me.