Gabriele Coci
University of Catania, Department of Physics and Astronomy “E. Majorana”
INFN Laboratori Nazionali del Sud, Catania
An Introduction
The year 2025 has been declared the International Year of Quantum Mechanics (IYQM). This year marks the centenary of the publication of the scientific papers that laid the foundations of quantum theory. For this occasion, many universities and research institutions are hosting interesting events in various forms — like those listed on the Italian Physical Society (SIF) page — that retrace the historical development of quantum mechanics and explore its impact in the modern society.
The Interdisciplinary Encyclopedia of Religion and Science (INTERS) presents a special issue for the IYQM, featuring a selection of key historical articles on the birth of quantum mechanics, alongside texts where its connections with the natural, philosophical, and social sciences are discussed highlighting their interdisciplinary significance.
The notion of quantum was introduced in 1900, by Max Planck to solve the “ultraviolet catastrophe” in the calculation of the black-body radiation spectrum — a major flaw in classical electromagnetism theory. In his seminal paper, Planck proposed that radiation is emitted not continuously, but in discrete units, the quanta of energy, laying the groundwork for quantum mechanics. In 1905, Albert Einstein extended Planck’s idea by proposing that light is absorbed in discrete packets, later called photons. This bold idea explained the photoelectric effect - where light striking on a metal surface causes electrons to be emitted - a phenomenon that classical wave theory could not account for. Although Einstein’s insight was crucial to the birth of quantum mechanics, he later questioned many of its interpretations. Interestingly, he was awarded the 1921 Nobel Prize not for his revolutionary work on relativity, but for his explanation of the photoelectric effect.
Exactly in 1925, Werner Heisenberg developed a new theoretical framework – called matrix mechanics – to support the atomic models of Niels Bohr and Arnold Sommerfeld, which proposed that bound electrons occupy discrete stationary orbits to ensure atom’s stability. Instead of using classical concepts like position and trajectory, Heisenberg focused only on observable quantities, representing atomic transitions with arrays of numbers (matrices). Collaborating with Max Born and Pascual Jordan, Heisenberg refined the mathematical structure of his theory, highlighting the non-commutative nature of observables – a feature which led him to the formulation of the uncertainty principle.
Between 1925 and 1926, Erwin Schrödinger, developed a wave-based model of quantum mechanics inspired by the work of Louis de Broglie, who proposed that wave matter could be associated to particles like electrons which can exhibit interference behavior. He formulated a differential equation — the Schrödinger equation — that describes how a particle’s state evolves over time through a continuous wavefunction. It was Max Born that later interpreted this wavefunction probabilistically. Together with Heisenberg’s matrix mechanics, Schrödinger’s wave mechanics completed the foundational structure of quantum theory, introducing key concepts like superposition — famously illustrated by the Schrödinger’s cat thought experiment.
Subsequently, many leading scientists worked to unify quantum mechanics by demonstrating the equivalence between its two main formulations - matrix mechanics and wave mechanics. Unlike the classical equivalence between geometrical and wave optics, or Newton’s and Huygens’ views on light, this task posed deeper theoretical challenges. As Louis de Broglie noted in his 1937 essay Matter and Light, resolving the contradictions introduced by quantum theory required a comprehensive framework capable of describing both matter and light, and eventually, all fundamental particles and forces. Some of these core issues were later addressed by integrating quantum mechanics with special relativity, giving rise to quantum field theory. Pioneered by Paul Dirac (1928) and further developed by 
Richard Feynman, Julian Schwinger and Sin-Itiro Tomonaga (1965 Nobel Prize in Physics), this approach laid the foundation for the Standard Model of Particle Physics — currently the most successful theory which describes the elementary constituents of matter and their interactions, with the notable exception of gravitational force. Despite several approaches attempt to solve this crucial problem in modern physics, gravity remains separate from the quantum world and currently a Grand Unified Theory is still unattained. For this reason, many questions arise, which can be summarized in the following one: Does quantum mechanics still have unresolved gaps that could provide a deeper understanding of the Universe’s origin and Nature in general?
This theoretical (and philosophical) challenge is just one side of the story. The world today differs greatly from that of quantum mechanics’ founders. Technological advancements, from the advent of personal computers to the rise of powerful global servers, which can handle an enormous amount of digital data, have drastically transformed society. Many of these innovations, such as Magnetic Resonance Imaging (MRI), semiconductor circuits and above all lasers(1964 Nobel Prize in Physics awarded to Charles Townes, Nikolay Basov and Alexander Prokhorov), directly stem from quantum principles. From the first developments in quantum electronics, through laser spectroscopy, to the methods of trapping and cooling atoms with lasers to explore novel quantum phenomena at macroscopic level, there have been numerous studies that have led to breakthrough discoveries. Most recently, in 2018, the Nobel prize was awarded toArthur Askin, Gérard Mourou and Donna Strickland for the development of a high-intensity ultra-short optical pulse.
However, coming back to the theoretical question, one of the most intriguing and least understood aspects of quantum mechanics is entanglement. It is defined as the phenomenon where two quantum systems become linked, such that the state of one instantly affects the other, regardless of distance. This defies classical concepts of locality and cannot be compare by any classical analogies. In 1935, the Einstein-Podolsky-Rosen (EPR) paradox questioned the completeness of quantum mechanics, suggesting the existence of hidden variables which could explain entanglement as a manifestation of missing information. In 1964, John Bell showed that no local hidden variable theory could match all of quantum mechanics’ predictions and in his pivotal paper derived inequalities that would be violated by quantum systems. In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John Clauser, and Anton Zeilinger for their pioneering experiments with entangled photons, confirming the violation of Bell inequalities and advancing quantum information science. Therefore, entanglement has been proved to be a real phenomenon and nowadays it is at the basis of worldwide effort towards the advancements in quantum computing, communication and information.
Due to its profound complexity and deeply counterintuitive predictions, quantum mechanics has long been a subject of philosophical reflection. Its challenges to classical notions of reality, causality, and observation have sparked debates that reach beyond the physics sphere. Many interpretations touch on questions traditionally reserved for metaphysics and epistemology. At times, the discourse even crosses into the realm of the transcendent, blurring lines between science and theology.
In his article titled Quantum Mechanics (2002) written some years ago for INTERS, the English theoretical physicist and Anglican priest John Polkinghorne examined the controversial role of consciousness in quantum measurements. In particular, he focused on the profound implications arising from concepts such as the collapse of the wavefunction or the Many-World interpretation, which have been developed within quantum theory to explain the effect of the observer when measuring the state of a system. The same does the Spanish physicist and theologian Javier Sánchez Cañizares, who in his INTERS entry Quantum Mechanics. Philosophical and Theological Implications (2019), after synthesizing the main steps from the classical paradigm to the quantum formulation with its various interpretations, also analyzes its theological implications, emphasizing how quantum mechanics offers many points of dialogue between science and religion. These are just some examples of how the enigmatic nature of quantum phenomena invites theological inquiry into the nature of existence and consciousness. Certainly, in the philosophical debate — which, unlike the scientific one, is more speculative and not experimentally falsifiable — one must be cautious of excesses, to avoid falling into acts of mysticism or in a flood of quantum flapdoodle as the American physicist Stephen Barr calls them in his text, Destiny and Quantum Mechanics (2007), quoting his fellow countryman and colleague Murray Gell-Mann, the theorist who coined the name quark to refer to the fundamental building blocks of matter and postulate their existence.
In conclusion, quantum mechanics remains a challenging field, both in its theoretical complexity and philosophical implications. However, this should not discourage those interested in studying it. On the contrary, it should inspire all of them to appreciate its “beauty” and share that with others eager to understand it. As Heisenberg himself wrote in the his Lecture, The Meaning of Beauty in the Exact Natural Sciences, delivered in 1970 at the Bavarian Academy of Fine Arts in 1970: “The beauty of nature finds itself reflected also in the beauty of the natural sciences.”
Let the IYQM serve as an opportunity to rediscover the depth of this theory through key foundational texts — those suggested within this introduction, along with many others not included for brevity. As quantum mechanics continues to drive major advancements not only in theoretical and applied physics but also in the philosophical and epistemological debate on knowledge and consciousness, it stands as a cornerstone of human thought, especially in the new century, where its technological progress is rapidly growing.
