A Different Kind of Theory of Everything

In 1964, during a lecture at Cornell University, the physicist Richard Feynman articulated a profound mystery about the physical world. He told his listeners to imagine two objects, each gravitationally attracted to the other. How, he asked, should we predict their movements? Feynman identified three approaches, each invoking a different belief about the world. The first approach used Newton’s law of gravity, according to which the objects exert a pull on each other. The second imagined a gravitational field extending through space, which the objects distort. The third applied the principle of least action, which holds that each object moves by following the path that takes the least energy in the least time. All three approaches produced the same, correct prediction. They were three equally useful descriptions of how gravity works.

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