Principle of Least Action

What is the quickest route to go from A to B? Most often it is just a straight line. However, when you are on a terrain, with different elevation, relative to the sea level, the quickest route might not be a straight line anymore. The mathematics of variational calculus allows us to calculate this optimal path. It turns out that electrons and light also follow the most optimal path to go from A to B. This is called the principle of least action, which forms the foundation of modern physics.

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