https://home.cern/news/news/physics/century-quantum-mechanics Skip to main content CERN Accelerating science * + Sign in * Directory home Toggle navigation * About CERN At CERN, we probe the fundamental structure of particles that make up everything around us. We do so using the world's largest and most complex scientific instruments. Know more + Who we are o Our Mission o Our Governance o Our Member States o Our History o Our People + What we do o Fundamental research o Contribute to society o Environmentally responsible research o Bring nations together o Inspire and educate o Fast facts and FAQs + Key Achievements Key achievements submenu + The Higgs Boson + The W boson + The Z boson + The Large Hadron Collider + The Birth of the web + Antimatter * News Featured news, updates, stories, opinions, announcements Students from Belgium, Canada, Mexico, Turkiy... At CERN Press release 9 July, 2025 Elusive romance of top-quark pairs observed a... Physics Press release 8 July, 2025 Slovenian flag raised at CERN At CERN News 3 July, 2025 First-ever collisions of oxygen at the LHC Accelerators News 1 July, 2025 Latest news + News o Accelerators o At CERN o Computing o Engineering o Experiments o Knowledge sharing o Physics + Events + CERN Community o News and announcements o Official communications o Events + Scientists o News + Press Room Press Room submenu + Media News + Resources + Contact * Science Science img The research programme at CERN covers topics from kaons to cosmic rays, and from the Standard Model to supersymmetry Know more + Physics o Antimatter o Dark matter o The early universe o The Higgs boson o The Standard Model o + More + Accelerators o CERN's accelerators o The Antiproton Decelerator o The Large Hadron Collider o High-Luminosity LHC o + More + Engineering o Accelerating: radiofrequency cavities o Steering and focusing: magnets and superconductivity o Circulating: ultra-high vacuum o Cooling: cryogenic systems o Powering: energy at CERN o + More + Computing o The CERN Data Centre o The Worldwide LHC Computing Grid o CERN openlab o Open source for open science o The birth of the web o + More + Experiments o ALICE o ATLAS o CMS o LHCb o + More * Resources Featured resources CERN Courier May/Jun 2025 Courier Physics 1 July, 2025 High-Luminosity LHC images Image Accelerators 20 June, 2018 LHC Facts and Figures Brochure Knowledge sharing 10 May, 2022 See all resources + By Topic o Accelerators o At CERN o Computing o Engineering o Experiments o Knowledge sharing o Physics + By format o 360 image o Annual report o Brochure o Bulletin o Courier o Image o Video o + More + By audience o CERN community o Educators o General public o Industry o Media o Scientists o Students o + More * search E.G. BIRTH OF WEB, LHC PAGE 1, BULLETIN... E.G. BIRTH OF WEB, LHC... Search [ ]Search * | en + o en o fr News News Topic: Physics --------------------------------------------------------------------- A century of quantum mechanics On 9 July 1925, in a letter to Wolfgang Pauli, Werner Heisenberg revealed his new ideas, which were to revolutionise physics 9 July, 2025 | By Davide De Biasio Part of a letter written by hand on the left linked to the B&W old photo of 3 young men sitting in front of a lake. Left: The first lines of the letter sent by Werner Heisenberg to Wolfgang Pauli on 9 July 1925, explaining his efforts to interpret quantum physics differently. (Copyright: Heisenberg Society) Right: Wolfgang Pauli, Werner Heisenberg and Enrico Fermi take a break by Lake Como during the 1927 International Congress of Physicists, where the new quantum mechanics was discussed in depth. (Image: Wolfgang Pauli Archive, CERN) 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 overcome the limits of classical computing in describing extreme environments and complex systems. Theoretical and philosophical considerations, too, are far from exhausted. Despite its empirical power, there is still no consensus about quantum theory's true meaning. What guides the emergence of our classical world? Is the wavefunction a real entity, a representation of the observer's knowledge or an artifact we should abandon altogether? Should we think of measurement apparatus and observers as quantum objects? Heisenberg himself was cautious yet hopeful, writing to Pauli: "Perhaps people who can do more, will be able to make sense of it." A century on, physicists are still working to fulfil that dream. Whatever the last word may be, one thing is certain: the conversation sparked on Helgoland is far from over. * Read the new issue of the CERN Courier magazine and its special report on 100 years of quantum physics. * Explore the Wolfgang Pauli Archive. +-------------------------------------------------------------------+ |Lieber Pauli... | | | |Read the translation of the letter sent by Werner Heisenberg to | |Wolfgang Pauli on 9 July 1925. The original letter is preserved in | |CERN's Wolfgang Pauli Archive. | | | |Letter written by hand in German (Copyright: Heisenberg Society) | | | | | | | |Dear Pauli, | | | |If you believe that I read your letter laughing mockingly, then you| |are gravely mistaken; quite the contrary - since Helgoland, my | |views on mechanics have become more radical with each passing day, | |and it is my firm conviction that Bohr's theory of the hydrogen | |atom, in its present form, is no better than Lande's theory of the | |Zeeman effect. | | | |However, on certain points we do not agree. (Your argument against | |mechanical orbits in H on account of the sum rules was already | |known to me; we discussed it with you once in Copenhagen, if I am | |not mistaken. And I fully agree, should you wish to deduce from it | |that m must take half-integer values.) | | | |But I do not know what you mean by orbits "falling into the | |nucleus." Surely we are agreed that even the kinematics of quantum | |theory is wholly different from that of classical mechanics (the hn| |relation!). I therefore see no geometrically intelligible or | |controllable meaning in the notion of "falling into the nucleus." | |It is, in fact, my sincere conviction that any interpretation of | |the Rydberg formula in terms of circular or elliptical orbits | |within classical geometry possesses not the slightest physical | |significance, and my entire pitiful effort is directed at | |exterminating the concept of orbits - after all, they cannot be | |observed - and replacing them with something more appropriate. | | | |For this reason, I take the liberty of simply sending you the | |manuscript of my work. I believe that at least the critical, that | |is to say, the negative portion contains real physics. I do feel | |terribly guilty, however, for having to ask you to return the | |manuscript within two or three days, as I should like either to | |complete it during the last days of my stay here - or to burn it. | | | |As for my own opinion of this scribbling, with which I am not at | |all satisfied: I am firmly persuaded of the value of the negative | |and critical part, but I regard the positive part as rather formal | |and poor. Still, perhaps those more capable than I may yet make | |something sensible of it. So I would ask that you concentrate | |primarily on the introduction as you read. | | | |Regarding the final point of your letter: I did not mean to say | |that the intensity of the 2536 line is 1/30 - that value has, after| |all, been measured. What I meant was rather this: the 2p2 - 2s | |transition, which is almost solely responsible for the splitting of| |the 2p2 level, when one attempts to interpret the Hanle splitting, | |appears to amount to roughly 1/30, which to my mind does not seem | |in agreement with the spectrum. | | | |Now then, I beg you once more for sharp criticism and the swift | |return of the paper! | | | |Many greetings to the entire Institute! | | | |W. Heisenberg | | | | | +-------------------------------------------------------------------+ quantum theorylibrary Related Articles Official launch of the Quantum Year Knowledge sharing News 5 February, 2025 It's beginning to look a lot like Quantum Computing Opinion Enrica Porcari 13 December, 2024 CERN welcomes International Year of Quantum S... Knowledge sharing News 12 June, 2024 View all news Also On Physics ATLAS closes in on rare Higgs decays Physics News 11 July, 2025 Elusive romance of top-quark pairs observed a... Physics Press release 8 July, 2025 LHCb weighs up the Z Physics News 6 June, 2025 ALICE eyes the cosmos Physics News 2 June, 2025 AI enhances Higgs boson's charm Physics News 19 May, 2025 ALICE detects the conversion of lead into gol... Physics News 8 May, 2025 ATLAS gets under the hood of the Higgs mechan... Physics News 10 April, 2025 CMS finds unexpected excess of top quarks Physics News 3 April, 2025 Symmetry between up and down quarks is more b... Physics News 28 March, 2025 View all news Follow Us Facebook icon Instagram icon TikTok icon X icon LinkedIn icon Youtube icon More Social Media Accounts Find us * Contact us * Getting here * CERN * Esplanade des Particules 1 * P.O. Box * 1211 Geneva 23 * Switzerland CERN & You * Doing business with CERN * Knowledge transfer * CERN's neighbours * CERN & Society Foundation * Partnerships * Alumni General Information * Careers * Visits * Privacy policy * Cookie Management Copyright (c) 2025 CERN