Lenny was frustrated—not a good sign considering his size and strength—and his head hurt. “George, I can’t remember all this stuff! Forces, masses, Newton’s equations, momentum, energy. You told me that I didn’t need to memorize stuff to do physics. Can’t you make it just one thing to remember?”
“Okay, Lenny. Calm down. I’ll make it simple. All you have to remember is that the action is always stationary.”
The principle of least action—really the principle of stationary action—is the most compact form of the classical laws of physics. This simple rule (it can be written in a single line) summarizes everything! Not only the principles of classical mechanics, but electromagnetism, ...
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