Recent experience of the great recession of 2008 has renewed one of the oldest debates in economics: whether economics could ever become a scientific discipline like physics. This paper proves that economics is truly a branch of physics by establishing for the first time a fundamental equation of economics (FEOE), which is similar to many fundamental equations governing other subfields of physics, for example, Maxwell’s Equations for electromagnetism. From recently established physics laws of social science (PLSS), this paper derives a fundamental equation of economics, which is the one mathematic equation that governs all observed economic phenomena. FEOE establishes a common entry point to solve all economic problems without any exception. We show that establishing FEOE clarifies many open questions regarding the foundation of economics. For example, the number one question for all economists ought to be what can be forecasted and what cannot be forecasted in economics. Without FEOE and PLSS, this number one question cannot be answered scientifically within the existing framework of economics. While FEOE re-affirms many existing economic theories, we also have found that many other popular economic theories are not compatible with FEOE, and we conclude that FEOE comes with its own version of microeconomics and macroeconomics. In microeconomics, the framework of laws of supply and demand and market equilibrium, which is traditionally assumed by most economists as the foundation of economics, is replaced by a new model called indeterministic supply demand pricing (ISDP) model. ISDP model is far more precise and universal mathematical abstraction of market reality than the framework of Marshall’s market equilibrium and laws of supply and demand. In macroeconomics, a new macroeconomic model called indeterministic balance sheet plus (IBS+) model can be derived from FEOE. Unlike the popular DSGE and Agent-based Computational Economic (ACE) models, the IBS+ model is universally applicable in any kind of economy, empirically falsifiable, making forecasts with reasonable accuracy, truthful abstraction of reality, capturing macroeconomic dynamics accurately, and most importantly based on a sound theoretical foundation. In conclusion, this paper shows that FEOE provides a solid physics foundation for both theoretical and practical economics. Therefore, after establishing the fundamental equation of economics in this paper, there should be no doubt that economics is simply a branch of quantum physics in parallel with chemistry and optics. Over last four hundred years, there are many schools of thoughts emerged in economics while there is only one school of thought by Newton-Einstein-Bohr survived the experimental and theoretical scrutiny in physics over the same period. The logic conclusion is that there must be only one school of thought allowed in economics as a subfield of physics.