Case Study 1: Physics Envy — How Economics Imported and Misapplied Equilibrium
The Import
The concept of equilibrium in economics was borrowed directly from classical mechanics in the late 19th century. Léon Walras, one of the founders of mathematical economics, was explicit about the borrowing: he modeled his general equilibrium theory on the physics of his era, treating the economy as a system of forces that tend toward a balanced state.
The structural mapping seemed compelling. In mechanics, a system is in equilibrium when all forces cancel out. In economics, a market is in equilibrium when supply equals demand. The mathematics was identical: find the point where opposing pressures balance.
Where the Mapping Holds
The equilibrium concept genuinely illuminated certain economic phenomena: - Price determination in competitive markets, where the interaction of supply and demand does produce stable prices under certain conditions - Comparative statics: analyzing how equilibrium changes when external conditions change (tax changes, supply shocks) - Welfare theorems: demonstrating conditions under which market equilibria are efficient
Where the Mapping Breaks
Physical systems have known forces. Economic "forces" are human decisions — influenced by culture, psychology, information, power, and institutions. The analogy assumes that human decision-making can be modeled like gravitational attraction. The behavioral economics revolution (Kahneman, Tversky, Thaler) has demonstrated that this assumption is substantially wrong.
Physical equilibria are stable or unstable. The stability of a physical equilibrium can be analyzed rigorously. The stability of economic "equilibria" is often assumed rather than demonstrated. Financial markets, in particular, exhibit the opposite of equilibrium behavior: positive feedback loops, cascading failures, and self-reinforcing crises.
Physical systems don't anticipate. A ball rolling toward the bottom of a bowl doesn't anticipate its trajectory. Economic agents do — they form expectations about the future that affect their current behavior, creating feedback loops that have no counterpart in classical mechanics.
Physical systems don't innovate. The laws of physics don't change. Economic "laws" change constantly as technology, institutions, and culture evolve. An equilibrium model assumes a stable structure; economies are structurally dynamic.
The Consequences
The equilibrium import contributed to several consequential errors: - The efficient market hypothesis assumed that financial markets are always in (or near) equilibrium, making crashes "impossible" by the model's logic - DSGE models (Chapter 3 case study) assumed that economies return to equilibrium after shocks, making persistent unemployment and depression theoretically anomalous - Policy recommendations based on equilibrium models treated market outcomes as naturally optimal, providing intellectual justification for deregulation that contributed to the 2008 crisis
Discussion Questions
- Apply the Five-Question Mapping Test to the equilibrium import. Where does the mapping hold, break, and become invisible?
- If economics had borrowed from biology rather than physics — using concepts like evolution, ecology, and adaptation — how might the field look different today?
- Is the current movement toward "complexity economics" (which abandons equilibrium assumptions) a genuine metaphor replacement or just another set of epicycles?
- The chapter identifies cases where borrowing was productive (Shannon, epidemiology). Is the economics-physics borrowing entirely unproductive, or has it also been partly useful?
References
- Mirowski, P. (1989). More Heat Than Light: Economics as Social Physics, Physics as Nature's Economics. Cambridge University Press. (Tier 1)
- Walras, L. (1874). Éléments d'économie politique pure. (Tier 1)
- Research on the limitations of equilibrium models in economics has been conducted by multiple scholars, including Beinhocker's The Origin of Wealth (2006) and Arthur's work on complexity economics. (Tier 2)