Quantum mechanics under the Copenhagen interpretation is one of the most experimentally well verified formalisms. However, it is known that the interpretation makes explicit reference to external observation or "measurement." One says that the Copenhagen interpretation suffers from the measurement problem. This deficiency of the interpretation excludes it as a viable fundamental formalism and prevents the use of standard quantum mechanics in discussions of quantum cosmology. Numerous alternative interpretations have been developed with the goals of reproducing its predictive success while obviating the measurement problem. While several interpretations make distinct, falsifiable, predictions, many claim to precisely reproduce the results of standard quantum mechanics. The sheer number of interpretations raises several issues. If the experimental predictions are identical, how are they to be assessed? On what grounds can an interpretation be said to trump another? Without recourse to experimental findings, one may continue to assess an interpretation on its logical structure, self-consistency, and simplicity (number and plausibility of its assumptions). We argue, and where possible, demonstrate, that all common interpretations have unresolved deficiencies. Among these deficiencies are failures to resolve the measurement problem, fine-tuning problems, logical/mathematical inconsistencies, disagreement with experiment, and others. Shortcomings as severe as these call into question the viability of any of the common interpretations. When appropriate, we indicate where future work may resolve some of these issues.
Quantum mechanics under the Copenhagen interpretation is one of the most experimentally well verified formalisms. However, it is known that the interpretation makes explicit reference to external observation or "measurement." One says that the Copenhagen interpretation suffers from the measurement problem. This deficiency of the interpretation excludes it as a viable fundamental formalism and prevents the use of standard quantum mechanics in discussions of quantum cosmology. Numerous alternative interpretations have been developed with the goals of reproducing its predictive success while obviating the measurement problem. While several interpretations make distinct, falsifiable, predictions, many claim to precisely reproduce the results of standard quantum mechanics. The sheer number of interpretations raises several issues. If the experimental predictions are identical, how are they to be assessed? On what grounds can an interpretation be said to trump another? Without recourse to experimental findings, one may continue to assess an interpretation on its logical structure, self-consistency, and simplicity (number and plausibility of its assumptions). We argue, and where possible, demonstrate, that all common interpretations have unresolved deficiencies. Among these deficiencies are failures to resolve the measurement problem, fine-tuning problems, logical/mathematical inconsistencies, disagreement with experiment, and others. Shortcomings as severe as these call into question the viability of any of the common interpretations. When appropriate, we indicate where future work may resolve some of these issues.
The interpretation of quantum mechanics has been controversial since the introduction of quantum theory in the 1920s. Although the Copenhagen interpretation is commonly accepted, its usual formulation suffers from some serious drawbacks. Based mainly on Bohr's concepts, the formulation assumes an independent and essential validity of classical concepts running in parallel with quantum ones, and leaves open the possibility of their ultimate conflict. In this book, Roland Omnes examines a number of recent advances, which, combined, lead to a consistent revision of the Copenhagen interpretation. His aim is to show how this interpretation can fit all present experiments, to weed out unnecessary or questionable assumptions, and to assess the domain of validity where the older statements apply. The text offers a self-contained treatment of interpretation (in nonrelativistic physics) in a manner accessible to both physicists and students. Although some hard results are included, the concepts and mathematical developments are maintained at an undergraduate level. This book enables readers to check every step, apply the techniques to new problems and make sure that no paradox or obscurity
In the well known Copenhagen interpretation of Quantum mechanics, advocated by N. Bohr, the physical objects and the experimental results can be described only in a macroscopic language, leaving any possible microscopic description as unspeakable. This point of view has been deepened by C. Rovelli in the relational interpretation of Quantum mechanics. Most of the alternative interpretations, which try a detailed microscopic description of physical phenomena and of their evolution, have in common the explicit introduction of the wave function as the basic element of the theory. These interpretations require the notion of quantum state as the fundamental concept of the theory, which is the typical unspeakable physical element according to the Copenhagen interpretation. The two basic physical entities are intimately bound together by the integrity of the wave function. These interpretations are usually indicated as realistic. It is well known that the use of the wave function and its time evolution in the description of the physical processes leads unavoidably to some difficulties or so-called paradoxes. The measurement problem is at the center of these difficulties, mainly because it requires the introduction of the reduction process of the wave function, which is not included explicitly within the mathematical formalism of Quantum Mechanics. In this paper we build up and propose a model which goes beyond the standard formalism and which is able to solve the measurement problem and all the other difficulties which, in a way or in another, are related to it.
The quantum measurement problem remains a central unresolved issue in theoretical physics, arising from the incompatibility between unitary quantum dynamics and the emergence of definite measurement outcomes. Closely related is the question of the physical status of the wavefunction within contemporary quantum theory. This review critically surveys leading approaches to the measurement problem, focusing on their formal consistency, physical assumptions, and empirical implications. We examine the standard measurement framework, the role of environment-induced decoherence and quantum Darwinism, and the restrictions imposed by no-go theorems such as Bell, Kochen–Specker, and Pusey–Barrett–Rudolph. Interpretations including Everettian many-worlds, Bohmian mechanics, spontaneous collapse models, relational quantum mechanics, QBist approaches, and Copenhagen-type frameworks are assessed in terms of their ability to account for classical definiteness without sacrificing empirical adequacy. We also review experimental efforts probing macroscopic superpositions and collapse dynamics. The analysis indicates that, despite its predictive success, quantum theory lacks a complete physical account of outcome selection. This suggests that the measurement problem reflects a fundamental structural limitation of current theory, motivating either new physical principles or a redefinition of the wavefunction’s ontological role.
Quantum mechanics famously posits two distinct evolution rules for the state of a system: a deterministic, unitary time evolution under the Schr¨odinger equation, and a probabilistic “collapse” of the wavefunction upon measurement as dictated by the Born rule. The apparent inconsistency of these two dynamical laws – one continuous and one discontinuous – constitutes the quantum measurement problem. This foundational problem, which underlies the emergence of definite outcomes from quantum superpositions, has persisted since the theory’s inception. In this review, we present a comprehensive and rigorous examination of the measurement problem and survey the leading approaches that seek to resolve or circumvent it. We begin by formulating the measurement problem and its conceptual challenges. We then discuss in detail the major interpretations of quantum mechanics and theoretical frameworks addressing the problem: the Copenhagen interpretation, Everett’s many-worlds interpretation, de Broglie–Bohm pilot-wave theory, objective collapse models, the role of decoherence and environment-induced superselection, and epistemic approaches such as QBism. For each interpretation, we describe the core principles and mathematical formalism, assessing how (and whether) it attempts to solve the measurement problem. We also review modern developments and experiments relevant to quantum measurements, including tests of Bell inequalities, observations of decoherence and macroscopic superpositions, weak measurements and quantum eraser experiments, and thought experiments ‘a la Wigner’s friend. We highlight how these results inform the ongoing debate. Finally, we discuss open problems and challenges that remain in fully resolving the measurement problem.
Scientific progress does not always remove difficult conceptual questions about knowledge, reality, and observation. Quantum mechanics is one of the most successful theories in modern science, yet its development has raised persistent questions concerning measurement, observation, and quantum reality. Despite extensive theoretical and experimental progress, the measurement problem remains unresolved and continues to generate different interpretations. This paper examines what this unresolved problem reveals about scientific understanding and how people may approach unresolved scientific questions when established knowledge does not provide a final answer. The discussion uses a conceptual approach, drawing on selected historical and theoretical work and experiments related to the measurement and observer problem. The paper highlights how competing interpretations can remain open while established applications continue to work
When we are presented with apparently bizarre properties of microsystems governed by quantum mechanics (as in our discussion of the double-slit experiment in the previous chapter), we naturally ask whether it might be possible to find another theory, or at least another story to go with the equations, that would make more sense to us. We might seek a theory and a view of fundamental physical processes that would accord more with our classically based, everyday common sense. This typically takes the form of the question of the possibility of the existence of a more detailed description of atomic processes than that afforded by the wave function (that is, by the Copenhagen interpretation of the previous chapter). In this chapter we discuss some of the central interpretive problems of the Copenhagen version of quantum mechanics. Then, in the next two chapters, we look at the severe, general restrictions that are placed on any such completion of quantum mechanics and at one particular extension of quantum mechanics. First, though, we begin here with the struggle that ensued over the status of causality in quantum theory. THE COMPLETENESS OF QUANTUM MECHANICS Although the Heisenberg uncertainty principle (Section 20.4) and the lack of absolute predictive power are an inherent feature of quantum mechanics, there have been attempts to preserve an in principle completely deterministic structure for physics. Even classically, in a macroscopic sample of gas there are so many molecules that it would be hopeless, as a practical matter, to predict the future locations of all these molecules from Newton's laws of motion.