100 years on, quantum mechanics is redefining reality—with us at the cente

sciencehabit writes: Standing in a garden on the remote German island of Helgoland one day in June, two theoretical physicists quibble over who—or what—constructs reality. Carlo Rovelli, based at Aix-Marseille University, insists he is real with respect to a stone on the ground. He may cast a shadow on the stone, for instance, projecting his existence onto their relationship. Chris Fuchs of the University of Massachusetts Boston retorts that it’s preposterous to imagine the stone possessing any worldview, seeing as it is a stone. Although allied in their belief that reality is subjective rather than absolute, they both leave the impromptu debate unsatisfied, disagreeing about whether they agree.

Such is the state of theoretical quantum mechanics, scientists’ deepest description of the atomic world. The theory was developed 100 years ago on Helgoland, where a 23-year-old Werner Heisenberg retreated to escape a bout of hay fever—and to reimagine what an atom looks like. The leading picture at the time featured electrons hopping in discrete, or quantized, leaps of energy between fixed orbits around the nucleus. It explained the behavior of hydrogen but failed for bigger atoms. On blustery walks and cold swims in the North Sea, Heisenberg abandoned the simplistic orbital picture, instead developing a new mathematical language that would work for any atom. Later in 1925, Erwin Schrödinger conjured up a complementary lens—his eponymous wave equation—which describes the positions of electrons in probabilistic terms.

Within a few years, their calculations would reveal a disturbingly fuzzy picture of reality, one in which certain properties are inherently unknowable and others take on different values depending on how they’re measured. “What we observe is not nature itself, but nature exposed to our method of questioning,” Heisenberg wrote after winning the 1932 Nobel Prize in Physics.

This year, hundreds of physicists convened on Helgoland to commemorate the birth of quantum mechanics. It has certainly earned its keep over the past century, not only by predicting experimental outcomes with immaculate precision, but also by enabling technologies such as lasers, transistors, and atomic clocks. Yet even today, scientists struggle to interpret what the theory implies about nature. Central to the confusion is how the act of measurement pins down the indeterminate behavior of atoms. The standard framing has an unsettling anthropocentric flavor, suggesting humans play some special role in shaping the universe. Now, bolstered by a string of recent experiments, theorists such as Fuchs and Rovelli are leaning into the discomfort, emphasizing how observers do indeed create the world they inhabit. What’s at stake is nothing less than reality itself.

“We don’t need to fix quantum mechanics to make it compatible with what we observe; we need to recognize that there are alternative ways of looking at the world,” says Alyssa Ney, a philosopher of physics at the Ludwig Maximilian University of Munich. Quantum theory compels physicists to “make room for different notions of what it means to be real.”