Quantum mechanics is certainly a strange place to start an investigation into the nature of the relationship between science and society. However, this article begins by integrating its history and proceeds through current conceptions of it by some physicists to examine quantum mechanics as an intersection of both science and knowledge. A famous thought experiment by Einstein, Podolsky, and Rosen pointed to an area that few physicists still research–a paradigm gone awry. Yet the vast majority of physiciststoday do not agonize over this uncertainty. Physics today presses forward with indominable speed and precision. But by questioning a small sample of physicists doing physics about their practice, this study aims at an everyday physicist’s understanding of the matter; a paradox that lies between the results of experiment and their interpretation or place as knowledge in society. For Einstein, Podolsky, and Rosen, the paradox was that we can only determine the spin of an entangled particle–it only becomes real–after measuring the spin of its partner particle. For a few contemporary physicists, the problem is how to understand the fact that a measured particle “teleports” its state instantaniously to another point in space. Practicing physicist resolve this, to put it simply, by just using it. Quantum mechanics, as with physics, is incredibly accurate–accepting it as truth yields results that physicists and society can use. By interrogating this process in its historicity and current formulation, this article sheds some light on the way physics works as knowledge. It provides a possibility for turning the scientific eye toward the very meaning of science.
Quantum mechanics is certainly a strange place to start an investigation into the nature of the relationship between science and society. However, this article begins by integrating its history and proceeds through current conceptions of it by some physicists to examine quantum mechanics as an intersection of both science and knowledge. A famous thought experiment by Einstein, Podolsky, and Rosen pointed to an area that few physicists still research–a paradigm gone awry. Yet the vast majority of physiciststoday do not agonize over this uncertainty. Physics today presses forward with indominable speed and precision. But by questioning a small sample of physicists doing physics about their practice, this study aims at an everyday physicist’s understanding of the matter; a paradox that lies between the results of experiment and their interpretation or place as knowledge in society. For Einstein, Podolsky, and Rosen, the paradox was that we can only determine the spin of an entangled particle–it only becomes real–after measuring the spin of its partner particle. For a few contemporary physicists, the problem is how to understand the fact that a measured particle “teleports” its state instantaniously to another point in space. Practicing physicist resolve this, to put it simply, by just using it. Quantum mechanics, as with physics, is incredibly accurate–accepting it as truth yields results that physicists and society can use. By interrogating this process in its historicity and current formulation, this article sheds some light on the way physics works as knowledge. It provides a possibility for turning the scientific eye toward the very meaning of science.
This article presents a general discussion of several aspects of our present understanding of quantum mechanics. The emphasis is put on the very special correlations that this theory makes possible: They are forbidden by very general arguments based on realism and local causality. In fact, these correlations are completely impossible in any circumstance, except for very special situations designed by physicists especially to observe these purely quantum effects. Another general point that is emphasized is the necessity for the theory to predict the emergence of a single result in a single realization of an experiment. For this purpose, orthodox quantum mechanics introduces a special postulate: the reduction of the state vector, which comes in addition to the Schrödinger evolution postulate. Nevertheless, the presence in parallel of two evolution processes of the same object (the state vector) may be a potential source for conflicts; various attitudes that are possible to avoid this problem are discussed in this text. After a brief historical introduction, recalling how the very special status of the state vector has emerged in quantum mechanics, various conceptual difficulties are introduced and discussed. The Einstein–Podolsky–Rosen (EPR) theorem is presented with the help of a botanical parable, in a way that emphasizes how deeply the EPR reasoning is rooted into what is often called “scientific method.” In another section the Greenberger–Horne–Zeilinger argument, the Hardy impossibilities, as well as the Bell–Kochen–Specker theorem are introduced in simple terms. The final two sections attempt to give a summary of the present situation: One section discusses nonlocality and entanglement as we see it presently, with brief mention of recent experiments; the last section contains a (nonexhaustive) list of various attitudes that are found among physicists, and that are helpful to alleviate the conceptual difficulties of quantum mechanics.
Quantum theory is a phenomenally successful cornerstone of physics. But a recent poll among quantum physicists shows that there is still little agreement on what the theory tells us about physical reality. Yet precious bits of consensus are emerging. As always, Albert Einstein was ahead of his time. Quantum theory was still in its infancy, its formalism a mere sketch, and its implications only beginning to be realized. Yet Einstein, in 1917, already spotted what would trouble him throughout his lifetime. He concluded that the nascent theory, which he had helped initiate, seemed to leave “time and direction of elementary processes,” such as the spontaneous emission of radiation from a molecule, “to chance” 1. Einstein's conclusion was visionary. It was also incredibly bold, for it suggested a radical departure from the powerful, centuries-old deterministic and causal worldview of classical physics. But Einstein also immediately expressed his discomfort with the idea of fundamental chance. In fact, his worries were just the seed of what would turn into one of the longest and most heated debates in the history of physics. What does quantum theory mean, and what does it tell us about the nature of reality? Some of the brightest minds in physics locked horns over these issues. Besides Einstein, there were Niels Bohr, Erwin Schrödinger, Werner Heisenberg, Pascual Jordan, Carl Friedrich von Weizsäcker, Wolfgang Pauli, and Richard Feynman, just to name a few. Ask anyone today working on foundational questions in quantum theory and you are likely to hear that there is still no consensus on many of these questions—all the while, of course, everybody seems to be in perfect agreement on how to apply the quantum formalism when it comes to making experimental predictions. Remarkably, there has never been any serious attempt to find out what the current opinions of the community really are and how they may be correlated. We jumped on the opportunity and polled 33 participants at a 2011 conference on “Quantum Physics and the Nature of Reality” (Fig. 1) 2. (We hasten to add that our poll is to be understood as merely a snapshot and certainly cannot claim to be representative.) As soon as we had collected the questionnaires, the respondents eagerly asked to hear the results. Evidently, we had not been the only curious party. What did we find? The fundamental questions, it seems, still elicit radically different responses. Take the quantum state—represented by the wavefunction—as an illustration. The wavefunction is the central mathematical symbol of quantum mechanics. It allows us to make incredibly accurate predictions, be it in atomic, molecular, or solid-state physics. But how are we to interpret it? Is it itself a physical property, or does it merely represent our knowledge? Our poll revealed a tie between these two positions. And not only that: a third of respondents preferred a mix of both viewpoints—perhaps a sign that a more nuanced view, or even a synthesis of views, may be on the horizon. Only 3% of those polled thought that quantum states never refer to a single system but only to ensembles of systems. Perhaps this departure from the ensemble view—a view that, we suspect, remains more popular in the physics community at large—has to do with the fact that experiments with single quantum systems have become routine in today's quantum-information laboratories. Sharp disagreement was in evidence on another deep question. Abraham Pais, Einstein's biographer, recalled that “during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it” 3. Putting the issue differently, are the physical properties of objects well defined before we actually measure them? Half of the respondents in our poll believed that they sometimes were, while the other half said that they never were. These two positions correspond to radically different ways of understanding the function of measurement: either as simply ascertaining—at least sometimes—of what is already there, or as an act of genuine creation. The role of measurement is also at the heart of another perennial favorite of foundational discussions, the measurement problem. The measurement problem highlights a curious fact about quantum theory: while the act of measurement assumes a fundamental role, it is not described—not without interpretive contortions, anyway—by the dynamical law of quantum theory, the Schrödinger equation. For decades, the measurement problem was seen by many as the main stumbling block for quantum theory, sparking a storm of rivaling attempts to deal with this perceived issue. Today, the waters seem to have calmed and attention shifted elsewhere, as only less than a fourth of respondents consider the measurement problem “a severe difficulty threatening quantum mechanics.” Changing attitudes are also evident when it comes to the issue of chance Einstein had brought to light in 1917. Today, the question of the randomness of quantum events no longer seems to be all that contentious. Most of our respondents accepted randomness as irreducible or a fundamental concept in nature (or both). Less than one-tenth of respondents thought that quantum randomness was merely apparent rather than fundamental, and no one believed in a hidden determinism governing quantum events. We wish we could hear Einstein's opinion today. Why have attitudes and priorities changed? It's hard to say, but we think there may be several driving forces. The rapid ascent of quantum information science has not only led to applications such as secure cryptography and the promise of lightning-fast quantum computers, but has also shed new light on foundational problems and handed us new tools for addressing them. Our poll shows that these tools are considered a welcome addition, with three-fourths seeing quantum information as a breath of fresh air for foundational investigations. There is also optimism about the future technological development of quantum information, with more than 75% of respondents expecting a working and useful quantum computer to be realized within the next fifty years. Such optimism might be rooted in the astonishing experimental progress we have witnessed over the past decades, another driving force behind changing attitudes. We are now at stage where almost perfect experimental control of single quantum systems has been achieved. The 2012 Nobel Prize in physics was awarded for just such experiments. This has put at our fingertips the ability to actually carry out the thought experiments—or approximations thereof, at any rate—that the founders of quantum theory had dreamed up. Take Schrödinger's cat. In 1935, Schrödinger wanted to illustrate what seemed to him an absurd prediction of the new theory: that the strange quantum superpositions of the atomic world can be amplified to everyday objects. Schrödinger's object of choice was a cat in a specifically prepared box. Quantum theory then suggested that the state of the cat should become transformed into a bizarre quantum superposition of being simultaneously alive and dead. But what Schrödinger had intended as a “burlesque” 4 thought experiment is now approaching experimental reality. Over the last decades, researchers have become increasingly skilled in breeding small versions of Schrödinger's cat. While they don't involve actual cats, they do already involve large biological molecules. Our respondents, it seems, were inspired by such developments. A solid two-thirds majority declared that they don't expect any fundamental limit in this enterprise of observing larger and larger objects in superpositions of macroscopically distinct states. No limit, then, for quantum theory—something that Bohr had already suggested when he, in some of his early thought experiments, considered apparatuses that included macroscopic parts behaving in quantum-mechanical ways. There are other new trends. Why do people seem to be much more at ease with relativity theory than with quantum theory? For one, both special and general relativity theory are grounded in a few simple physical principles. By contrast, quantum theory is lacking such a foundation and is instead based on a rather abstract mathematical body. Today, several researchers are out to change this situation, trying to derive quantum theory from a small set of physical or information-theoretic principles. They hope that in this way, we will understand why we have this peculiar theory to begin with. A majority in our poll regarded such “reconstructions” of quantum theory as giving useful insights, indicating once more that any new tool that promises fresh insights is embraced with an open mind. To be sure, many (albeit not all) of the questions we asked in the poll are interpretive questions. This means that unless we should find out some day that quantum theory is in fact not always valid—in which case we would have to rethink the whole game—experiments arguably can't play the role of the arbiter. Some physicists will conclude from this that we need not bother with such questions, and relegate them to philosophy. We think this is too quick. Many ideas in physics didn't come out of thin air, but came about when someone started looking at a problem with a certain philosophical disposition. In this sense, we may think of the plurality of views in quantum mechanics as productive. Every interpretation highlights some aspect of quantum theory, is particularly suited for explaining particular phenomena, and may inspire new technologies—such as the quantum computer—or even new physics. What, then, does the future hold? Half of the respondents in our poll believe that in fifty years from now, we will probably still be coming together for conferences on the fundamental questions of quantum theory. But what kinds of questions will be on the agenda? Will we still be worrying about the same problems as today? Or will these problems have been solved—or found to be less relevant—and our focus shifted to new questions once again? Who knows. But the location of the conference where we conducted the poll may tell a story. It was the place of a monastery, and a central position in the battle between Catholics and Protestants during the seventeenth century. Many of the questions that were hotly debated back then are no longer of interest to many people today, and other, new questions have since taken center stage in our society. Maybe some of the battles over the interpretation of quantum theory will meet a similar fate.
The difficulties of the old quantum theory were largely dispelled by the development of quantum mechanics in the 1920s. First Heisenberg (1925) developed the rather abstract matrix mechanics and soon after Schro¨dinger (1926; see also Eckart, 1926) developed the physically more appealing wave mechanics. Subsequently he showed them to be mathematically equivalent. The physicists were greatly relieved; now they had once again a mathematical formalism that enabled the results of experiments to be calculated. It was possible to move forward. It is certainly a very elegant and indeed beautiful formalism, and does enable the results of many physical measurements to be calculated with great accuracy. It is used daily by atomic and nuclear physicists; there is no other way to make calculations. From the earliest days there has been intense debate about the interpretation of quantum mechanics, and it shows no signs of being settled; if anything it is becoming more intense over the years. As Penrose has remarked, no one understands quantum mechanics. Feynman (1967; see also Sykes, 1994) also admitted that ‘nobody understands quantum mechanics’, and Einstein once remarked: ‘I have thought a hundred times as much about the quantum problems as I have about general relativity theory’ (Pais, 1994, p. 57). It should therefore be evident that the most that can be done here is to try to show where the real difficulties lie, and how the failure to recognize that quantum mechanics is an essentially statistical theory has led to a series of confusing quantum paradoxes. It is just these paradoxes that have often been used to support theological speculations. It is useful to ask what ‘understanding quantum mechanics’ actually means. Certainly it means being able to calculate the results of many measurements. The same applies to Newton’s theory of gravitation, which we understand in the sense that we can use it as a basis of calculation, although we do not really understand the gravitational force itself. Very often, especially in modern physics, the mathematical formalism does not give us the sort of intuitive physical feeling for what is going on that we often obtain from classical mechanics. Perhaps we can never achieve more than this for quantum mechanics. In classical mechanics we can assume that a property that is measured objectively exists prior to the interaction of the measuring apparatus with the observed system. Quantum mechanics, however, ‘is incompatible with the proposition that measurement discovers some unknown but pre-existing property’ (Peres, 1997, p. 14). There is an important distinction between mysteries and absurdities. We may be forced to live with mysteries, but there is no reason why we should put up with absurdities such as the wave-particle duality, tunnelling through potential barriers, acausal events and the collapse of the wave function. These are the quantum paradoxes that will be discussed later on. They arise within the Copenhagen interpretation of quantum mechanics and are solved by other interpretations. These alternative interpretations enable us to make conjectures about what is going on, even though we cannot understand it in detail. This is acceptable and a stimulus to further research. It is certainly difficult enough to understand quantum mechanics, but there is no need to try to visualize logical absurdities in the effort to do so. Quantum mechanics is a mathematical formalism that makes it possible to calculate the results of some experiments. Anything more than this, such as questions about the interpretation of what is measured and whether it tells us what is actually going on in the system being studied, belongs to philosophy. Thus we are concerned here with what is essentially a philosophical debate; if it were just a matter of physics the debate would have been settled long ago. One of the difficulties is that it invokes the results of experiments and uses the language of quantum mechanics. Inevitably this language uses philosophical terms and then it is easy to give the impression that the philosophical propositions are in some way entailed by the physical results. These in turn can influence theological thinking. In order to be quite clear which arguments are justified and which are not, it is essential to separate the three levels of discourse: physical, philosophical and theological. Our task is to relate these together, and this immediately raises the traditional problems of the philosophy of science, including the relation of experiment to theory, and of theory to explanation. We begin by discussing experiments, distinguishing between those that are real and those that are imaginary.
For over nine decades, quantum mechanics has profoundly shaped our understanding of nature, offering explanations for the stability of atoms, the formation of mole cules, the properties of materials, and phenomena such as light or radioactivity. It is essential to modern technologies—from microchips to lasers—and underpins the physics of elementary particles. Yet, despite its successes, it remains a deeply mysterious theory, confronting a central enigma: why does the quantum world behave so differently from everyday reality? Elements such as superposition and quantum entanglement—which allow particles to exist simultaneously in multiple states or remain interconnected regardless of distance—contradict classical world intuitions. This gap between microscopic and macroscopic behavior has given rise to what is known as the “measurement problem,” raising fundamental questions about the nature of reality. Diverging interpretations have led to two major schools of thought: anti-realism, promoted by Niels Bohr, which holds that reality depends on the observer and their interaction with measurement instruments; and realism, supported by Albert Einstein, which argues that quantum mechanics is incomplete and that eality exists independently of observation. Bohr’s complementary philosophy and Heisenberg’s uncertainty principle were central to the formulation of the “Copenhagen interpretation,” which became dominant in the 20th century. However, the persistent opposition of scientists like Einstein and Schrödinger inspired generations of physicists to seek a unification between quantum mechanics and relativity, culminating in new theories such as loop quantum gravity and string theory. Thus, the debate between realism and anti-realism remains alive, and philosophy continues to play a crucial role in exploring the new foundations of physics.
The measurement problem in quantum mechanics first cropped up during the 1930s during a famous series of debates between Einstein and Bohr concerning the physical adequacy – or ‘completeness’ – of the orthodox (Copenhagen) theory.1 As we have seen, it is a topic central to Hanson’s thinking not only with regard to quantum mechanics but also for its wider bearing on the scope and limits of scientific knowledge, the theory-laden character of observations, and the underdetermination of theory by evidence.2 Thus it is frequently adduced in support of his proto-Kuhnian claim that scientists working within different paradigms must be thought of as observing different things or inhabiting ‘different worlds’.3 What I propose to do now is look in more detail at the arguments surrounding this topic and, especially, at various proposals – from Einstein down – for a realist construal of the quantum data (i.e., the empirical measurement-results) that would offer an alternative to Hanson’s idea of the uncertainty principle as placing an absolute limit on our knowledge of objects and events in the microphysical domain. Having set out his arguments already in chapters 4 and 5, I shall not make any further detailed reference to Hanson’s work but assume that the reader who has come this far will have them clearly in mind. Thus my purpose in the remainder of this book is to raise the question: what becomes of the paradigm-relativist approach to issues in the history and philosophy of science if it derives so much of its suasive force from an interpretation of quantum mechanics that has proved increasingly open to challenge?
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.