In the last two decades, the scientific community has witnessed a surge in activity, interesting results, and notable progress in our conceptual understanding of computing and information based on the laws of quantum theory. One of the significant aspects of these developments has been an integration of several fields of inquiry that not long ago appeared to be evolving, more or less, along narrow disciplinary paths without any major overlap with each other. In the resulting body of work, investigators have revealed a deeper connection among the ideas and techniques of (apparently) disparate fields. As is evident from the title of this volume, logic, mathematics, physics, computer science and information theory are intricately involved in this fascinating story. The inquisitive reader might focus, perhaps, on the marriage of the most unlikely and intriguing fields of quantum theory and logic and ask: Why quantum logic? By many, “logic” is deemed to be panacea for faulty intuition. It is often associated with the rules of correct thinking and decision-making, but not necessarily in its most sublime role as a deep intellectual subject underlying the validity of mathematical structures and worthy of investigation and discovery in its own right. Indeed, within the realm of the classical theories of nature, one may encounter situations that defy comprehension, should one hold to the intuition developed through experiencing familiar macroscopic scenarios in our routine impressions of natural phenomena. One such example is a statement within the special theory of relativity that the speed of light is the same in all inertial frames. It certainly defies the common intuition regarding the observation of velocities of familiar objects in relative motion. One might be tempted to dismiss it as contrary to observation. However, while analyzing natural phenomena for objects moving close to the speed of light and, therefore, unfamiliar in the range of velocities we are normally accustomed to, logical deductions based on the postulates of the special relativity theory lead to the correct predictions of experimental observations. There exists an undeniable interconnection between the deepest theories of nature and mathematical reasoning, famously stated by Eugene Wigner as the unreasonable efficacy of mathematics in physical theories.
In the last two decades, the scientific community has witnessed a surge in activity, interesting results, and notable progress in our conceptual understanding of computing and information based on the laws of quantum theory. One of the significant aspects of these developments has been an integration of several fields of inquiry that not long ago appeared to be evolving, more or less, along narrow disciplinary paths without any major overlap with each other. In the resulting body of work, investigators have revealed a deeper connection among the ideas and techniques of (apparently) disparate fields. As is evident from the title of this volume, logic, mathematics, physics, computer science and information theory are intricately involved in this fascinating story. The inquisitive reader might focus, perhaps, on the marriage of the most unlikely and intriguing fields of quantum theory and logic and ask: Why quantum logic? By many, “logic” is deemed to be panacea for faulty intuition. It is often associated with the rules of correct thinking and decision-making, but not necessarily in its most sublime role as a deep intellectual subject underlying the validity of mathematical structures and worthy of investigation and discovery in its own right. Indeed, within the realm of the classical theories of nature, one may encounter situations that defy comprehension, should one hold to the intuition developed through experiencing familiar macroscopic scenarios in our routine impressions of natural phenomena. One such example is a statement within the special theory of relativity that the speed of light is the same in all inertial frames. It certainly defies the common intuition regarding the observation of velocities of familiar objects in relative motion. One might be tempted to dismiss it as contrary to observation. However, while analyzing natural phenomena for objects moving close to the speed of light and, therefore, unfamiliar in the range of velocities we are normally accustomed to, logical deductions based on the postulates of the special relativity theory lead to the correct predictions of experimental observations. There exists an undeniable interconnection between the deepest theories of nature and mathematical reasoning, famously stated by Eugene Wigner as the unreasonable efficacy of mathematics in physical theories.
No scientific theory has caused more puzzlement and confusion than quantum theory. Beginning in 1900, the theory developed in fits and starts and found a consistent mathematical framing only when John von Neumann published his Mathematische Grundlagen der Quantenmechanik in 1932 [12]. But even today, we struggle to understand the world as pictured by quantum theory. Physics is supposed to help us to understand the world, and yet quantum theory makes it seem a very strange place. One might be tempted to push aside our puzzlement as the result of our clinging to a primitive worldview. But our puzzlement is not merely a psychological obstacle; it is also an obstacle to the development of physics itself. This obstacle is encountered primarily in our attempts to unify quantum theory and the general theory of relativity. As argued persuasively by Chris Isham (who is represented in this volume), Lee Smolin [10], and others, the primary obstacle between us and future physics is our own failure to understand the conceptual foundations of current physical theories. How, then, are we to make conceptual progress? What is the process by which we find a new perspective, a perspective in which previously puzzling phenomena find a place in an intelligible—and perhaps beautiful—structure? I do not wish to make prescriptions or to claim that conceptual progress can be achieved in only one way. But this book begins with the Ansatz that conceptual progress might be achieved through free creations of the human intellect.
1124 NOTICES OF THE AMS VOLUME 45, NUMBER 9 I n the twentieth century, the quest for deeper understanding of the laws of nature has largely revolved around the development of two great theories: namely, general relativity and quantum mechanics. General relativity is, of course, Einstein’s theory according to which gravitation results from the curvature of space and time; the mathematical framework is that of Riemannian geometry. While previously spacetime was understood as a fixed arena, given ab initio, in which physics unfolds, in general relativity spacetime evolves dynamically, according to the Einstein equations. Part of the problem of physics, according to this theory, is to determine, given the initial conditions as input, how spacetime will develop in the future. The influence of general relativity in twentiethcentury mathematics has been clear enough. Learning that Riemannian geometry is so central in physics gave a big boost to its growth as a mathematical subject; it developed into one of the most fruitful branches of mathematics, with applications in many other areas. While in physics general relativity is used to understand the behavior of astronomical bodies and the universe as a whole, quantum mechanics is used primarily to understand atoms, molecules, and subatomic particles. Quantum theory has had a much more complex history than general relativity, and in some sense most of its influence on mathematics belongs to the twenty-first century. The quantum theory of particles—which is more commonly called nonrelativistic quantum mechanics—was put in its modern form by 1925 and has greatly influenced the development of functional analysis, and other areas. But the deeper part of quantum theory is the quantum theory of fields, which arises when one tries to combine quantum mechanics with special relativity (the precursor of general relativity, in which the speed of light is the same in every inertial frame but spacetime is still flat and given ab initio). This much more difficult theory, developed from the late 1920s to the present, encompasses most of what we know of the laws of physics, except gravity. In its seventy years there have been many milestones, ranging from the theory of “antimatter”, which emerged around 1930, to a more precise description of atoms, which quantum field theory provided by 1950, to the “standard model of particle physics” (governing the strong, weak, and electromagnetic interactions), which emerged by the early 1970s, to new predictions in our own time that one hopes to test in present and future accelerators. Quantum field theory is a very rich subject for mathematics as well as physics. But its development in the last seventy years has been mainly by physicists, and it is still largely out of reach as a rigorous mathematical theory despite important efforts in constructive field theory. So most of its impact on mathematics has not yet been felt. Yet in many active areas of mathematics, problems are Edward Witten is professor of physics at the Institute for Advanced Study. His e-mail address is witten@ias.edu.
Among philosophers in particular the principle of complementarity has been considered an unreasonable limitation of natural science, and Niels Bohr’s exposition of the inevitability of a statistical description when accounting for atomic phenomena has been regarded as a renouncement directly opposed to the very spirit of scientific investigation. Indeed, fundamental limitations in the range of applicability of the causal spacetime description of physical phenomena do exist, but quantum physics is precisely the generalization of the conceptual frame of classical physics which has converted these limitations into a springboard for investigations which are among the most fruitful in the history of science. Far from raising a barrier for scientific investigation, quantum physics has extended the scope of natural science to a degree that was unimaginable before its advent. Natural science now encompasses such different objects as the constituents of atoms, processes in remote galaxies, and psycho-chemical phenomena in man. Biochemistry and psychopharmacology, for instance, both rely on one of the most important results of quantum physics: the periodical system of the elements, which is fundamental to all modern chemistry. Furthermore atomic physics and chemistry are basic sciences for technology and thus contribute to the continuous development of instruments of observation for all branches of natural science.
T OPOLOGICAL MATTER IS FUNDAMENTALLY QUANTUM mechanical, with no correspondent in the classical world. Therefore, it would be quite natural to begin our journey with a review of topics that are particularly relevant to understanding the nature of topological quantum states, such as Berry phases and anyon physics. Yet, even before discussing those topics, pausing to think about some fundamental aspects of quantum theory would be extremely worthwhile. For several decades after the triumph of quantum mechanics in the 1920s and early 1930s, thinking about the foundations of the theory and challenging its standard interpretation were viewed by a majority of physicists as a waste of time, if not plain heresy. The judgment of the physics community on the value of such an endeavor has drastically changed after the groundbreaking work of John Bell and the subsequent experimental tests during the 1970s and 1980s. Today, studying the foundations of quantum mechanics has turned into a flourishing field intimately linked with the explosive rise of quantum information theory. Yet, the subject still has a rather marginal place in standard quantum mechanics textbooks, which focus on the formalism and on the recipes necessary for doing calculations. This chapter is intended for the reader who is less familiar with the problems brought to the fore by quantum information theory and the work on the foundations of quantum mechanics as a brief introduction to these fields and a summary of the basic terminology. But why should one bother about such things when discussing the physics of topological quantum matter? Basically, because the two areas are intimately connected: on the one hand, quantum computation is a prominent item on the list of possible applications for topological quantum states, on the other hand, key concepts in quantum theory, such as quantum entanglement, play a central role in understanding topological matter. Hence, starting the journey with a thought about the foundations of quantum mechanics should enable us to grasp the delicate root that makes the quantum world utterly strange and beautiful, then follow it as it grows out of the sphere of few particle physics into the realm of many-body systems. 1.1 THE QUANTUM MEASUREMENT PROBLEM There is a certain uneasiness about quantum mechanics that every student of this field has probably experienced in some measure. And it is not just the insecurity that results from losing the firm ground of classical intuition; special relativity also deals with this type of problem and does not generate similar feelings. Nor is it at the center of any controversy similar in scale and depth with, for example, the debate [455] between Einstein and Bohr on the foundations of quantum theory. To better understand the source of the problem, let us recall the “standard” way of presenting quantum theory, i.e., starting from certain principles or postulates [126, 440] — a typical approach that can be found in most of the commonly used textbooks. More specifically, let us focus on the key postulates that define the state vectors and their evolution, as well as the observables and the measurements.
Though the truths of logic and pure mathematics are objective and independent of any contingent facts or laws of nature, our knowledge of these truths depends entirely on our knowledge of the laws of physics. Recent progress in the quantum theory of computation has provided practical instances of this, and forces us to abandon the classical view that computation, and hence mathematical proof, are purely logical notions independent of that of computation as a physical process. Henceforward, a proof must be regarded not as an abstract object or process but as a physical process, a species of computation, whose scope and reliability depend on our knowledge of the physics of the computer concerned.
Abstract Quantum physics represents one of the most significant intellectual achievements of modern science, reshaping humanity’s understanding of nature at its most fundamental level. It provides the conceptual and mathematical foundations for explaining atomic structure, radiation, and the behavior of particles, forces, and materials, while underpinning many contemporary technologies. Yet its development was marked by profound conceptual disruption. From its earliest formulations, quantum theory challenged classical assumptions about determinism, causality, and certainty, generating paradoxes and disagreements even among its founders. In exploring the universe, quantum physics focuses on the smallest constituents of matter such as quarks, electrons, and photons, revealing a domain that consistently defies everyday intuition. Within this realm, particles exhibit wave–particle duality, physical outcomes are described probabilistically, and measurement plays a constitutive role in determining observable reality. Although such phenomena may appear extraordinary, physicists have rendered them intelligible through rigorous theoretical frameworks and precise mathematical formalism. Within a scientific context, the “truth” of quantum mechanics does not imply metaphysical finality, but rather its exceptional capacity to describe, explain, and predict experimental results with remarkable accuracy. Importantly, this scientific account need not contradict spiritual or religious perspectives. Instead, by unveiling the coherence and complexity of the universe, quantum physics can complement scriptural narratives that emphasize meaning and purpose. This study investigates the scientific truth and narrates the story of universe on the basis of quantum physics and explores its influences on technological development and reshaping the civilization.