"A century ago, physics had its Darwinian moment — a change in perspective that was as consequential for the physical sciences as the theory of evolution by natural selection was for biology.
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 [1]. 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.
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 original 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.
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.” [2]
1. What is the significance of Heisenberg's uncertainty principle?
The Heisenberg Uncertainty Principle is significant because it reveals a fundamental limit in quantum mechanics, showing that certain pairs of properties, like a particle's position and momentum, cannot be known with infinite precision simultaneously. This principle revolutionized physics by challenging the classical view of a deterministic universe and establishing that the act of measurement inherently influences the system being observed. Its significance lies in establishing a probabilistic view of quantum phenomena and enabling the development of new areas of physics.
Key significances of the Heisenberg Uncertainty Principle:
Foundation of quantum mechanics:
The principle is a cornerstone of modern physics, demonstrating that particles have wave-like properties and that measurements of properties like position and momentum are inherently uncertain.
Challenges classical determinism:
It replaced the classical idea that one could predict the future with certainty if all initial conditions were known. Instead, it suggests that at the quantum level, outcomes are a matter of probability.
Observer effect:
It highlights that the act of measuring a quantum system is not a passive observation; it inherently interacts with and alters the system. For example, to precisely measure a particle's position, one must use a form of energy that will unavoidably change its momentum.
Application to other quantities:
The principle extends beyond position and momentum to other "conjugate pairs," such as energy and time. This means there is a limit to how precisely you can know a system's energy and how long you have to measure it.
Explanation of atomic stability:
The principle helps explain why electrons do not spiral into the nucleus of an atom. The uncertainty in the electron's position and momentum keeps it from collapsing into a single point.
Real-world implications:
While the effects are negligible for large, everyday objects, the principle is crucial for understanding phenomena at the atomic and subatomic level, which in turn are fundamental to many technologies.
2. Nature 637, 251-252 (2025)
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