A Century of Quantum Mechanics

Summary

On 9 July 1925, in a letter to Wolfgang Pauli, Werner Heisenberg revealed his new ideas, which were to revolutionise physics Just 100 years ago, on 9 July 1925, Werner Heisenberg wrote a letter to his friend, colleague and fiercest critic, Wolfgang Pauli. A few weeks earlier, Heisenberg had returned from the North Sea outpost of Helgoland, where he had laid the foundations of modern quantum mechanics and changed our understanding of the atomic world. The letter, preserved in the Wolfgang Pauli Archive at CERN, reveals Heisenberg’s efforts to liberate physics from the semi-classical picture of atoms as planetary systems, with electrons in orbit around the nucleus. “All of my pitiful efforts are directed at completely killing off the concept of orbits – which, after all, cannot be observed – and replacing it with something more suitable,” he explains in his letter to Pauli. By sweeping away the old interpretation, Heisenberg could focus on building a more coherent model, based purely on what the experiments were observing. Attached to the letter was the draft of Heisenberg’s famous Umdeutung paper, which was received for publication a few weeks later, and which is often considered as the birth certificate of modern quantum theory. In the following months, Max Born, Pascual Jordan and Wolfgang Pauli himself helped turn Heisenberg’s work into matrix mechanics, the first mature formulation of quantum theory. Today, those early reflections underlie the most precise framework in the history of science: the Standard Model of particle physics. Experiments at CERN keep pushing it to extreme regimes, and time and again, it proves astonishingly accurate. To celebrate 100 years of quantum mechanics, the CERN Courier looks back at the impact of this theory and examines how it keeps delivering new puzzles, experimental ideas and technologies. For instance, quantum sensors may soon extend their reach from low- to high-energy applications, while quantum simulations could help overco...