Magnifying the Quantum World by Dave Munger

In the 1870s, when Max Planck was still a young German university student, his professor Philipp von Jolly discouraged him from continuing to pursue physics, reportedly saying that nothing was left to discover in the field except for a few minor details.

Undaunted, Planck became a professor of physics at the University of Berlin, and by 1900 had developed a theory that would turn physics upside-down: Electromagnetic energy could only be emitted in discrete packets, or “quanta.” The field of quantum mechanics was born, and its ramifications continue to echo through physics today. Indeed, modern quantum researchers aren’t just filling in minor details; they’re still adding in leaps and bounds to our knowledge of how the world fundamentally works.

Planck’s breakthrough came out of his studies of “black bodies,” idealized objects that perfectly absorb and then re-emit electromagnetic radiation. In reality, nothing can absorb light so perfectly, but many real-world objects, like a hunk of iron, absorb and emit electromagnetic radiation similarly to a black body. As an iron ingot is heated, it begins to emit electromagnetic radiation, energy that travels on a spectrum of frequencies. When it’s quite hot, the ingot turns red—and as its temperature rises further, the ingot will progressively turn orange, then yellow, then white. These are only the frequencies we can see—the ingot, of course, is emitting invisible electromagnetic radiation too, in frequencies like infrared. Planck studied this “black-body spectrum,” and precisely measured how changing temperature affected the radiation a black body emitted. In his work, he came to realize that the emitted radiation didn’t smoothly increase with temperature, but in fact changed in sudden steps. Planck never quite understood the implications of his discovery, but Einstein and other physicists soon began to see its reach. Their conclusion: Everything in the universe—energy, light, particles, and all the macroscopic objects they form and influence—is somehow quantized, and subject to strange probabilistic behavior that defies classical explanations. In the quantum world, objects can be in multiple places at the same time, can simultaneously harbor mutually exclusive states, and can pop in and out of existence spontaneously. Even Richard Feynman, the Nobel-Prize-winning physicist who arguably had a better grasp of quantum mechanics than anyone else in the 20th century, quipped that no one really understood it.