Introductory physics textbooks can offer a few different perspectives on their subject matter. They can present physics as an established and known set of theories that are useful for solving problems of a practical nature. This perspective emphasizes what one can DO with physics. Alternatively, textbooks can present physics as an established and known set of theories that help us to understand the functioning of the natural world in a deep way. Here, the emphasis is on how physics makes sense of peculiar features of our world that might otherwise seem surprising or counterintuitive. Most textbooks in calculus-based introductory physics combine the practical and explanatory perspectives, perhaps leaning one way or the other depending on whether the anticipated audience consists of future engineers or future physicists. In The Forest of Physics, Travis Norsen uses a novel approach that melds the explanatory perspective with a historical approach. By incorporating historical details, Norsen presents physics as a dynamic subject that grows and changes in order to adapt to new observations, incorporate new theoretical insights, and adjust to the ever-changing goals of its practitioners.
Norsen incorporates a historical perspective by discussing episodes in the history of physics that led to important new insights in a roughly chronological order. He presents physics as a puzzle that, when pieced together, provides us with an understanding of the natural world, and he shows us how some key parts of that puzzle were assembled. Although the focus of the book is physics, not history, Norsen does not neglect discussing, respectfully, alternative approaches that were historically important but that are now viewed as incorrect, nor does he ignore situations in which there seemed to be no consistent way to put the puzzle pieces together. This acknowledgment of the difficulty of solving the puzzle serves to highlight the impressive achievement of those physicists who eventually found ways to put the pieces together into a seemingly coherent picture.
Although the amount of historical content varies by chapter, throughout the book Norsen emphasizes the way physics helps us to make sense of the world. Even at the level of deriving a single formula, Norsen engages readers in sense-making by analyzing whether the result has the proper dimensions and gives the expected behavior in certain limiting cases. One hopes that students reading the text will follow Norsen's example in their own work.
The book opens with a preliminary chapter on measurement and uncertainty that is one of the best discussions of this topic I have seen at an introductory level. The first major unit of the book examines the development of Newtonian mechanics, focusing on the development of astronomy from Ptolemy to Newton and showing how Newtonian mechanics can explain astronomical systems (including systems of many bodies).
The next section examines the development of the electromagnetic theory, beginning with early qualitative observations of electrical and magnetic phenomena and working up to the modern theory of electric and magnetic fields as described by Maxwell's equations. Only then, in Chapter 8, does Norsen introduce the concept of energy with a focus on how energy is stored in the electric and magnetic fields inside capacitors and inductors, which then leads into a chapter on electromagnetic radiation. The final section of the book covers the development of atomic theory from early observations about the properties of gases (Boyle's law, etc.) and mass ratios in chemical reactions to the development of kinetic theory and classical thermodynamics, followed by a chapter on statistical physics that is exceptionally insightful. The book closes with a glimpse at how the idea of energy quantization helped make sense of the nuclear atom and explained unexpected results for the specific heats of gases.
Each chapter includes several Questions, Exercises, and Projects which, like most of the book, feel like what you would find in an upper-level physics text even though they cover (advanced) introductory material. The Questions are conceptual in nature and often surprisingly deep. Exercises are more mathematical, but they usually involve deriving formulas rather than calculating numbers. Few plug-and-chug problems will be found, and those that are present deal with situations of practical, historical, or conceptual significance. Projects are more work-intensive and involve data analysis, computer simulation, extended exercises, or laboratory experiments. The Projects are supplemented by online material in the form of Jupyter notebooks that help to guide students (and instructors!) through the necessary steps.
The Forest of Physics is very well written, particularly for a self-published book. The style is conversational, and one can imagine that one is listening to Norsen deliver a clear and engaging lecture. Indeed, the book may be a bit too much like a classroom lecture, since it frequently includes references to what will be done “next week” or what will be “shown in class.” While these references are mostly a minor annoyance, some students may be put off when they are told they will see some interesting result derived in class only to find out that their instructor has chosen a different path. Historians of science may view Norsen's historical content as fairly shallow and would certainly bemoan the lack of citations (or even a bibliography!), but, as noted above, Norsen's goal is to teach historically conscious physics, not the history of physics.
The most pressing question about The Forest of Physics is: Who will use it? Norsen wrote the book for an audience of first-year physics majors who have previously completed two years of calculus-based AP physics. The book should work well for that audience, but less experienced students might struggle with the material and be left with significant gaps in their knowledge of introductory physics. Students need some familiarity with Newtonian mechanics and electromagnetism before tackling this book. Norsen assumes that his readers are fluent with the calculus of a single variable, as well as vector operations like the dot and cross products. Several of the worked examples involve multiple integrals, but Norsen makes clever use of symmetry arguments and sequences the examples such that only single variable integration is needed in each case. More advanced mathematical topics, such as vector calculus and differential equations, are mentioned in the text but not really used.
The Forest of Physics provides a way to offer an engaging one-semester introductory course to a select group of first-year students with strong physics backgrounds that will extend and, most importantly, deepen their understanding of introductory physics content. Such a course would not only motivate students to take more advanced courses, but would also serve as a nice bridge to those courses. I would not have been ready for The Forest of Physics in my first semester of college, but I do wish I could have used the book immediately after completing my introductory physics sequence. The historical perspective really helps to show how scientists took a bunch of scattered pieces and put them together into the powerful and coherent construction we call physics. Or, to adopt Norsen's own metaphor, the book can help students see how individual trees combine to form the forest that is physics.
Todd Timberlake is a Professor of Physics and Astronomy at Berry College. He conducts research in computational quantum mechanics and is a contributor to the Open Source Physics collection. He is a co-author of two books: Classical Mechanics with Maxima and Finding Our Place in the Solar System: The Scientific Story of the Copernican Revolution.
