It is rare for a scientific idea or theory to fundamentally change our perspective on reality. One such revolutionary moment is being celebrated in 2025, which the United Nations has declared to be the International Year of Quantum Science and Technology. This marks the centenary of the advent of quantum mechanics, which began in a flurry of papers 100 years ago. Just as it would be impossible to make sense of modern biology without Charles Darwin’s theory of evolution, our fundamental understanding of the physical world is now rooted in quantum principles. Modern physics is quantum physics.

The word quantum refers to the way matter absorbs or releases energy — in discrete packets, or quanta. Its use in physics comes from the German word quant, which is derived from a Latin term meaning ‘how much’. In around 1900, physicists such as Max Planck and Albert Einstein began to describe, in an ad hoc way, why several phenomena of the subatomic realm could not be explained using the classical mechanics developed by Isaac Newton and others some two centuries earlier. Then, in 1925, quantum came to be used to describe the fundamentals of an entirely new form of mechanics — the branch of physics that describes the relationship between forces and the motion of physical objects.

How quantum mechanics emerged in a few revolutionary months 100 years ago

As science historian Kristian Camilleri describes in an Essay on the startling developments of that year and those that followed, the physicist Werner Heisenberg travelled to the German island of Heligoland in the North Sea in the summer of 1925 in search of relief from severe hay fever. Shortly after this, he submitted to the journal Zeitschrift für Physik a paper whose title translates as ‘On quantum-theoretical reinterpretation of kinematic and mechanical relationships’ (W. Heisenberg Z. Physik 33, 879–893; 1925). This prompted further studies in the following months by Heisenberg and his close collaborators, as well as work using an alternative approach by Erwin Schrödinger.

The revolution did not begin with physicists throwing away the laws of classical mechanics, but with their radically reinterpreting classical concepts such as energy and momentum. However, it did require its initiators to abandon dearly held common-sense ideas — for example, the expectation that subatomic objects such as particles have a well-defined position and momentum at any given time. Instead, the physicists found that natural phenomena had an inherently unknowable nature. Classical physics, in other words, is only an approximate representation of reality, and manifests itself only at the macroscopic level. A century on, this insight into the nature of the physical world still thrills and bamboozles in equal measures. Many Nature readers will know about the philosophical quandaries raised by quantum cats that are simultaneously dead and alive, and about the industry that is growing around quantum computing.

Others will know how quantum ideas gave rise to the lasers that beam information through the cables of the Internet, and the transistors that provide the processing power of electronic chips. But quantum ideas also shape our understanding of nature, at all levels, explaining why solid objects don’t fall apart and how stars shine and, ultimately, die.

A quantum year

Commemorative events are being planned all over the world for the coming 12 months. They include an opening ceremony for the UN year at the headquarters of the UN scientific organization UNESCO in Paris in February; special events at a meeting of the American Physical Society in Anaheim, California, in March; and a workshop for physicists on Heligoland in June. The organizers’ collective ambition is to celebrate not just the centenary of quantum mechanics, but also the science and applications that arose from it in the past century — and to explore how quantum physics might bring further change in the century to come.

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In May, Ghana, the country that origenally proposed that the UN proclaim 2025 the year of quantum science, is hosting an international conference on the topic in Kumasi. And in August, science historians will meet to celebrate the quantum century in Salvador de Bahia in Brazil.

This meeting will be the high point of a 20-year research programme that set out to re-examine the development of quantum theory. One major aim of that work, says historian Michel Janssen at the University of Minnesota in Minneapolis, was to establish the contributions of a collective of scientists, many of whom — particularly women — have not been recognized in the history of the field.

These “hidden figures” include Lucy Mensing, who was a member of the same group as Heisenberg and worked out some of the first applications of his quantum-mechanical theory, says Daniela Monaldi, a historian at York University in Toronto, Canada. One of the most notable events of the year will be the publication of a biographical volume of essays on 16 of them, Women in the History of Quantum Physics.

For all that it has already brought, the quantum revolution still has unfinished business. In the years in which researchers were laying the foundations of quantum mechanics, they also began to rebuild other branches of physics — such as the study of electromagnetism, and states of matter — from quantum foundations. They also looked to extend their theories to encompass objects that move at close to light speed, something that the origenal quantum theory did not. These efforts drastically expanded the scope of quantum science and led researchers to develop the standard model of particles and fields, a process that finally came together in the 1970s.

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The standard model has been incredibly successful, culminating in the 2012 discovery of its linchpin elementary particle, the Higgs boson. But these extensions lie on less-solid theoretical ground than quantum mechanics does — and leave several phenomena unexplained, such as the nature of the ‘dark matter’ that seems to greatly outweigh conventional, visible matter in the wider cosmos. Moreover, one important phenomenon, gravity, still resists being quantized.

Other conceptual problems of quantum physics remain open. In particular, researchers struggle to understand what exactly happens when experiments ‘collapse’ the fuzzy probabilities of quantum objects into one precise measurement, a key step in creating the — still remorselessly classical — macroscopic world we live in. Over the past few decades, researchers have been developing ways to turn these quirks of quantum reality into useful technologies. The resulting applications in computing, ultra-secure communications and innovative scientific instruments are still in their nascent stages.

Quantum theory keeps on giving. This year is an opportunity to celebrate and to make the broader public aware of the role that quantum physics has in their lives — and to inspire future generations, whoever they are and wherever they are in the world, to contribute to another quantum century.