In physics, we like theories that are simple and broad-ranging. By “simple,” physicists usually mean a mathematical theory that rests on as few postulates as possible; by “broad-ranging,” we mean theories that can describe a wide class of phenomena, even when apparently not related. A quintessential example is Einstein’s theory of general relativity. Resting on a handful of simple principles, it successfully describes planetary orbits in this (and any) solar system, black holes, gravitational waves, and the expansion of the universe.

When theories are simple and broad-ranging, physicists call them “beautiful.” Nobel laureates Steven Weinberg and Frank Wilczek have compared such theories with Mozart’s musical compositions, masterful and perfect constructions where, as if by divine revelation, every note is where it should be: Take one out and the composition crumbles. Likewise, beautiful theories have a mathematical integrity that seems to be revealing something deep about nature, a sort of hidden code of Creation: From the very large to the very small, the universe has many layers, each built upon its own mathematical description. Are these not parts of a larger composition, a single unifying tune resonating through all of nature?

So hope those who pursue a final theory, a theory that would weave together the many layers of physical reality into one mathematical wholeness. We can call this the ultimate Platonic dream, the quest for a single simple and broad-ranging theory of physics. Indeed, during the past four decades, the search for such a theory has inspired many of the brightest physicists in the world. But today we are seeing the limits of this Platonic thrust to mathematize nature, due to a lack of experimental validation and several theoretical obstacles—including the possibility of multiple universes and the troubling questions they pose.