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Why Old Physics Still Matters

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By Chad Orzel, Forbes, 7/30/18

(The following is an approximation of what I will say in my invited talk at the 2018 Summer Meeting of the American Association of Physics Teachers. They encourage sharing of slides from the talks, but my slides for this talk are done in what I think of as a TED style, with minimal text, meaning that they’re not too comprehensible by themselves. So, I thought I would turn the talk into a blog post, too, maximizing the ratio of birds to stones…

(The full title of the talk is Why “Old Physics” Still Matters: History as an Aid to Understanding, and the abstract I sent in is:

A common complaint about physics curricula is that too much emphasis is given to “old physics,” phenomena that have been understood for decades, and that curricula should spend less time on the history of physics in order to emphasize topics of more current interest. Drawing on experience both in the classroom and in writing books for a general audience, I will argue that discussing the historical development of the subject is an asset rather than an impediment. Historical presentation is particularly useful in the context of quantum mechanics and relativity, where it helps to ground the more exotic and counter-intuitive aspects of those theories in a concrete process of observation and discovery.


The title of this talk refers to a very common complaint made about the teaching of physics, namely that we spend way too much time on “old physics,” and never get to anything truly modern. This is perhaps best encapsulated by Henry Reich of MinutePhysics, who made a video open letter to Barack Obama after his re-election noting that the most modern topics on the AP Physics exam date from about 1905.

This is a reflection of the default physics curriculum, which generally starts college students off with a semester of introductory Newtonian physics, which was cutting-edge stuff in the 1600s. The next course in the usual sequence is introductory E&M, which was nailed down in the 1800’s, and shortly after that comes a course on “modern physics,” which describes work from the 1900s.

Within the usual “modern physics” course, the usual approach is also historical: we start out with the problem of blackbody radiation, solved by Max Planck in 1900, then move on to the photoelectric effect, explained by Albert Einstein in 1905, and then to Niels Bohr’s model of the hydrogen atom from 1913, and eventually matter waves and the Schrodinger equation, bringing us all the way up to the late 1920’s.

It’s almost become cliche to note that “modern physics” richly deserves to be in scare quotes. A typical historically-ordered curriculum never gets past 1950, and doesn’t deal with any of the stuff that is exciting about quantum physics today.

This is the root of the complaint about “old physics,” and it doesn’t necessarily have to be this way. There are approaches to the subject that are, well, more modern. John Townsend’s textbook for example, starts with the quantum physics of two-state systems, using electron spins as an example, and works things out from there. This is a textbook aimed at upper-level majors, but Leonard Susskind and Art Friedman’s Theoretical Minimum book uses essentially the same approach for a non-scientific audience. Looking at the table of contents of this, you can see that it deals with the currently hot topic of entanglement a few chapters before getting to particle-wave duality, flipping the historical order of stuff around, and getting to genuinely modern approaches earlier.

There’s a lot to like about these books that abandon the historical approach, but when I sat down and wrote my forthcoming general-audience book on quantum physics, I ended up taking the standard historical approach: if you look at the table of contents, you’ll see it starts with Planck’s blackbody model, then Einstein’s introduction of photons, then the Bohr model, and so on.

This is not a decision made from inertia or ignorance, but a deliberate choice, because I think the historical approach offers some big advantages not only in terms of making the specific physics content more understandable, but for boosting science more broadly. While there are good things to take away from the ahistorical approaches, they have to open with blatant assertions regarding the existence of spins. They’re presenting these as facts that simply have to be accepted as a starting point, and I think that not only loses some readers who will get hung up on that call, it goes a bit against the nature of science, as a process for generating knowledge, not a collection of facts.

This historical approach gets to the weird stuff, but grounds it in very concrete concerns. Planck didn’t start off by asserting the existence of quantized energy, he started with a very classical attack on a universal phenomenon, namely the spectrum of light emitted by a hot object. Only after he failed to explain the spectrum by classical means did he resort to the quantum, assigning a characteristic energy to light that depends on the frequency. At high frequencies, the heat energy available to produce light is less than one “quantum” of light, which cuts off the light emitted at those frequencies, rescuing the model from the “ultraviolet catastrophe” that afflicted classical approaches to the problem.

Planck used this quantum idea as a desperate trick, but Einstein picked it up and ran with us, arguing that the quantum hypothesis Planck resorted to from desperation could explain another phenomenon, the photoelectric effect. Einstein’s simple “heuristic” works brilliantly, and was what officially won him the Nobel Prize. Niels Bohr took these quantum ideas and applied them to atoms, making the first model that could begin to explain the absorption and emission of light by atoms, which used discrete energy states for electrons within atoms, and light with a characteristic energy proportional to the frequency. And quantum physics was off and running.

This history is useful because it grounds an exceptionally weird subject in concrete solutions to concrete problems. Nobody woke up one morning and asserted the existence of particles that behave like waves and vice versa. Instead, physicists were led to the idea, somewhat reluctantly but inevitably, by rigorously working out the implications of specific experiments. Going through the history makes the weird end result more plausible, and gives future physicists something to hold on to as they start on the journey for themselves.

This historical approach also has educational benefits when applied to the other great pillar of “modern physics” classes, namely Einstein’s theory of special relativity. This is another subject that is often introduced in very abstract ways– envisioning a universe filled with clocks and meter sticks and pondering the meaning of simultaneity, or considering the geometry of spacetime. Again, there are good things to take away from this– I learned some great stuff from Takeuchi’s Illustrated Guide to Relativity and Cox and Forshaw’s Why Does E=mc2?. But for a lot of students, the abstraction of this approach leads to them thinking “Why in hell are we talking about this nonsense?”

Some of those concerns can be addressed by a historical approach. The most standard way of doing this is to go back to the Michelson-Morley experiment, started while Einstein was in diapers, that proved that the speed of light was constant. But more than that, I think it’s useful to bring in some actual history– I’ve found it helpful to draw on Peer Galison’s argument in Einstein’s Clocks, Poincare’s Maps.

Galison notes that the abstract concerns about simultaneity that connect to relativity arise very directly from considering very concrete problems of timekeeping and telegraphy, used in surveying the planet to determine longitude, and establishing the modern system of time zones to straighten out the chaos that multiple incompatible local times created for railroads.

Poincare was deeply involved in work on longitude and timekeeping, and these practical issues led him to think very philosophically about the nature of time and simultaneity, several years before Einstein’s relativity. Einstein, too, was in an environment where practical timekeeping issues would’ve come up with some regularity, which naturally leads to similar thoughts. And it wasn’t only those two– Hendrik Lorentz and George FitzGerald worked out much of the necessary mathematics for relativity on their own.

So, adding some history to discussions of relativity helps both ground what is otherwise a very abstract process and also helps reinforce a broader understanding of science as a process. Relativity, seen through a historical perspective, is not merely the work of a lone genius who was bored by his job in the patent office, but the culmination of a process involving many people thinking about issues of practical importance.

Bringing in some history can also have benefits when discussing topics that are modern enough to be newsworthy. There’s a big argument going on at the moment about dark matter, with tempers running a little high. On the one hand, some physicists question whether it’s time to consider alternative explanations, while other observations bolster the theory.

Dark matter is a topic that might very well find its way into classroom discussions, and it’s worth introducing a bit of the history to explore this. Specifically, it’s good to go back to the initial observations of galaxy rotation curves. The spectral lines emitted by stars and hot gas are redshifted by the overall motion of the galaxy, but also bent into a sort of S-shape by the fact that stars on one side tend to be moving toward us due to the galaxy’s rotation, and stars on the other side tend to be moving away. The difference between these lets you find the velocity of rotation as a function of distance from the center of the galaxy, and this turns out to be higher than can be explained by the mass we can see and the normal behavior of gravity.

This work is worth introducing not only because these galaxy rotations are the crux of the matter for the current argument, but because they help make an important point about science in context. The initial evidence for something funny about these rotation curves came largely from work by Vera Rubin, who was a remarkable person. As a woman in a male-dominated field, she had to overcome many barriers along the course of her career.

Bringing up the history of dark matter observations is a natural means to discuss science in a broader social context, and the issues that Rubin faced and overcame, and how those resonate today. Talking about her work and history allows both a better grounding for the current dark matter fights, and also a chance to make clear that science takes place within and is affected by a larger societal context. That’s probably at least as important an issue to drive home as any particular aspect of the dark matter debate.

So, those are some examples of areas in which a historical approach to physics is actively helpful to students, not just a way to delay the teaching of more modern topics. By grounding abstract issues in concrete problems, making the collaborative and cumulative nature of science clear, and placing scientific discoveries in a broader social context, adding a bit of history to the classroom helps students get a better grasp on specific physics topics, and also on science as a whole.

About the author: Chad Orzel is Associate Professor in the Department of Physics and Astronomy at Union College

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