Physics-Inspired Gravity Model

Trade policies, particularly tariffs, are powerful instruments with profound and complex implications for the global economic landscape. This chapter investigates these dynamics by introducing and extending the gravity model of trade, a workhorse of empirical international economics. Inspired by Newton’s law of universal gravitation, the model fundamentally explains bilateral trade flows as a function of economic mass and resistance factors like geographic distance and policy barriers. Moving beyond its traditional linear specification, we argue that capturing the full, real-world impact of trade policy requires incorporating nonlinear dynamics. Such enhancements allow the model to account for critical asymmetries; for instance, the dampening effect of a new tariff on trade can be far more pronounced than the stimulating effect of its removal, with these effects varying substantially across different economic contexts and industrial sectors. By bridging a classic economic model with modern nonlinear econometric techniques, this chapter provides a sophisticated framework for analyzing the multifaceted consequences of trade and protectionism. In the end of the chapter a project work is given which readers must try to implement on their own to demonstrate their compete learning by integrating the theoretical frameworks, empirical methodologies, and analytical techniques covered throughout this chapter. This hands-on project will challenge you to construct a gravity model dataset, estimate the impact of trade policies using Poisson pseudo-maximum likelihood (PPML) regression, perform counterfactual analysis to simulate a major trade policy shock, and, finally, link the resulting changes in trade flows to macroeconomic outcomes using panel data techniques. By completing this end-to-end analysis, you will not only reinforce your conceptual understanding but also develop the practical skills necessary to conduct rigorous, policy-relevant trade analysis.