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Historical Vs. Logical Order

One course I’m teaching this term is Modern Physics, which covers a diversity of topics in 20th century physics: relativity, basic quantum physics (atomic structure and spectra, fun stuff like the uncertainty principle), solid-state physics, and a bit of nuclear and particle physics. It’s a survey course, in other words—we don’t spend too long on any given subject, but just try to introduce the main areas of what most physicists do.

In fact, “modern physics” is arguably physics for most of us. I’ve often thought that we should fold these topics into our regular introductory courses. I’m not sure how best to do it, but the problem is that we often treat physics in a compartmentalized way—this box contains “normal physics” (the stuff Newton did, 19th century matters like optics and electromagnetism), that box contains “quantum physics” (particle physics, the science of materials, atomic and nuclear physics), a third box has “astrophysics” (which draws on the other two, but with a lot of unique terminology and history) and that weird box over there has the stuff that doesn’t fit (general relativity, cosmology).

The introductory courses typically don’t have any physics that wasn’t known in 1900. There are some good reasons for this, of course: it’s frankly easier to understand Newtonian physics than quantum physics. On the other hand, very few people can find inspiration in Newtonian physics for the simple reason that there’s very little cutting-edge research in it. Those who study it today use the tools of modern mathematics and connect it to research in quantum physics or general relativity–in other words, tools we don’t teach in introductory physics. I often worry that we treat introductory physics as drudge work or a rite of passage students have to endure before they get to the real stuff. (I’m as guilty of this as anyone, so please don’t think I’m pointing fingers.)

In modern physics, we continue the (more or less) chronological order, setting the historical stage with the state of physics as of the end of the 19th century. Again, I think there are some good reasons for it, and the excitement of discovery (even by proxy) is thrilling. But do we need to? If we rethink the introductory physics curriculum by way of redrafting the entire curriculum, maybe we can get some of that inspiration early on. After all, physics is an historical continuum: quantum physics—weird as it is—grew out of 19th century physics. Even our discussion of Newtonian physics can reflect our current knowledge of relativity and quantum mechanics. How this is to be done in its details, I don’t know. However, it seems to me that we continue to teach things the way we do because we have always taught them that way.

Another way forward is suggested by the quantum mechanics book I’m using. There is a lot of historical information in the book, but the topics are arranged in a logical order rather than an historical order. The Bohr atom (AKA the “old quantum physics”) is hardly mentioned, and the first topic introduced is spin—a property of particles that doesn’t have a direct analog in Newtonian physics. It’s the first topic because it’s one of the simplest quantum systems, and so it serves to illustrate how to think. (The Schrödinger equation doesn’t show up until chapter 4, and the “particle-in-a-box” is left to chapter 6.) I think a similar approach could inform the other physics courses.

What if we started introductory physics with energy, or with forces? What if we started sprinkling concepts from quantum mechanics into the class early? What if we introduced some astrophysics here and there? There are so many ways to rethink the curriculum. But I can’t do it now: I need to finish writing tomorrow’s modern physics lecture.

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