The Physics of Shopping

Physicists love to analyze complex systems. Face it, in order to try to figure out a world where everything affects everything else, they have to. And to analyze complex systems you have to Do The Math. Then you can apply the same mathematical tools to other complex systems, such as book sales.

A report in today's issue of Physical Review Letters shows that the sales of popular books on Amazon.com can be partly explained by complex-systems models. Or, as they so quaintly put it,

"These results are rationalized quantitatively by a simple model of epidemic propagation of interactions with long memory within a network of acquaintances. The observed relaxation of sales implies that the sales dynamics is dominated by cascades rather than by the direct effects of news or advertisements, indicating that the social network is close to critical."

Here is a "Physics News Update" from the American Institute of Physics that tries to use more English. (However, most English speakers will note the egregious usage error in the first paragraph. Physicists don't seem to feel they need editors.)

Apparently physicists speak three languages:

• Mathematics, when analyzing things,
• Physics Jargon, when formally communicating with other physicists, and
• Natural human language when interfacing with non-physicists or on non-physics subjects. (I have no direct evidence for successful physicist-nonphysicist communication by this third modality.)

"What physicists bring to economics is not new concepts but rather the determination to put these concepts to quantitative tests." Professor H. Eugene Stanley, Boston University.

The underlying concept is the idea of models. Models are mathematical constructions (equations, algorithms, calculations) that try to mimic and predict the behavior of real-world systems. For example, models using Newton's Laws of Motion and Kepler's Laws of Planetary Motion do a pretty good job of predicting the behavior of the real-world solar system. We could hardly expect space probes like Cassini/Huygens to get to Saturn without accurate predictions of orbital motions and trajectories.


If we can model the motions of planets, moons and probes, maybe we can model the behavior of groups of book-buyers, if we can find the patterns that predict their actions.