When you throw a stick to the lake, your dog will run to fetch it. However, it doesn’t always jump into the lake directly. If the stick is close to the shore, the dog will run along the shore first to be closer to the stick. Then it will jump into the water. It is faster for a dog to run on the shore than to swim in the lake. In general, dogs will take the path that minimizes the time to fetch the stick. How amazing they are! Dogs are so smart.

Light travels slower in water than in air, just like dogs move slower in water than on land. When light enter water from air, its path bends in a way that minimizes the time it takes from one point to another. How amazing it is! Light is so smart.

In general, any particle, when traveling through space, will minimize a physical quantity called action. This pattern is called the Least Action Principle. It seems all matter in the world is guided by this mysterious and universal economic principle. Nobody knows for sure why this is the case. But some great scientists, such as Planck, think the Least Action Principle is more fundamental than other physical laws. Many great works are inspired by this principle. For example, Feynman’s path integral method is an extension of the Least Action Principle to the quantum world.

Very often, the best way to understand science is through an economic principle, such as the Least Action Principle. Similarly, the best way to understand economics is through scientific theory, such as the thermodynamic theory. That is why I wrote the book, The Unity of Science and Economics.