An analysis of the quantum measurement problem is presented which is a modest modification of the standard one often called the Copenhagen interpretation. The starting assumption is that QM is universal, and that all evolutions are unitary. We must also assume that the set of evolutions is restricted to allow the existence of stable structures for the equipment in our laboratory, including the measurement devices. The devices need some macroscopic properties to make the outcomes of the experiments stable data. There should not exist superdevices capable of retrieving the information needed to reverse these evolutions. As a result the quantum coherences complementary to the outcomes of experiments will typically be inaccessible and there is no need to remove them by decoherence. We need devices designed such that the outcome of an experiment is indicated by a pointer with the property that it has a definite but random final state. We then find that the outcomes of experiments can be considered factual data, and random in a classical sense. They can be considered to happen in a single world.
An analysis of the quantum measurement problem is presented which is a modest modification of the standard one often called the Copenhagen interpretation. The starting assumption is that QM is universal, and that all evolutions are unitary. We must also assume that the set of evolutions is restricted to allow the existence of stable structures for the equipment in our laboratory, including the measurement devices. The devices need some macroscopic properties to make the outcomes of the experiments stable data. There should not exist superdevices capable of retrieving the information needed to reverse these evolutions. As a result the quantum coherences complementary to the outcomes of experiments will typically be inaccessible and there is no need to remove them by decoherence. We need devices designed such that the outcome of an experiment is indicated by a pointer with the property that it has a definite but random final state. We then find that the outcomes of experiments can be considered factual data, and random in a classical sense. They can be considered to happen in a single world.
In this paper, I attempt a personal account of my understanding of the measurement problem in quantum mechanics, which has been largely in the tradition of the Copenhagen interpretation.I assume that (i) the quantum state is a representation of knowledge of a (real or hypothetical) observer relative to her experimental capabilities; (ii) measurements have definite outcomes in the sense that only one outcome occurs; (iii) quantum theory is universal and the irreversibility of the measurement process is only "for all practical purposes".These assumptions are analyzed within quantum theory and their consistency is tested in Deutsch's version of the Wigner's friend gedanken experiment, where the friend reveals to Wigner whether she observes a definite outcome without revealing which outcome she observes.The view that holds the coexistence of the "facts of the world" common both for Wigner and his friend runs into the problem of the hidden variable program.The solution lies in understanding that "facts" can only exist relative to the observer.
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.
One of the main goals of no-collapse interpretations of quantum mechanics is to show how results of measurements are objectified, or, in a more poignant terminology, how in a measurement situation facts arise. One needs to show how in their dynamical evolution the measured system and, perhaps more importantly, the measuring apparatus acquire, or at least appear to acquire, the definite properties that are otherwise explained by the collapse of the state. This can be seen as a special case of the problem whether quantum mechanics alone can account for the classical behaviour of macroscopic systems; in particular, the permanent definiteness of at least those properties of macroscopic systems that are directly observable, such as the position of the pointer of a measuring apparatus. In other words, whether quantum mechanics can account for macrofacts of the familiar kind.
Within the last decade, significant progress has been made towards a consistent and complete reformulation of the Copenhagen interpretation (an interpretation consisting in a formulation of the experimental aspects of physics in terms of the basic formalism; it is consistent if free from internal contradiction and complete if it provides precise predictions for all experiments). The main steps involved decoherence (the transition from linear superpositions of macroscopic states to a mixing), Griffiths histories describing the evolution of quantum properties, a convenient logical structure for dealing with histories, and also some progress in semiclassical physics, which was made possible by new methods. The main outcome is a theory of phenomena, viz., the classically meaningful properties of a macroscopic system. It shows in particular how and when determinism is valid. This theory can be used to give a deductive form to measurement theory, which now covers some cases that were initially devised as counterexamples against the Copenhagen interpretation. These theories are described, together with their applications to some key experiments and some of their consequences concerning epistemology.
It is a fundamental and widely accepted assumption that a measure-ment result exists universally, and in particular, it exists for every ob-server, independently of whether the observer makes the measurement or knows the result. In this paper, we will argue that, based on an analysis of protective measurements, this assumption is rejected by the many-worlds interpretation of quantum mechanics, and worlds, if they indeed exist ac-cording to the interpretation, can only exist relative to systems which are decoherent with respect to the measurement result. In standard quantum mechanics, it is postulated that when the wave func-tion of a quantum system is measured by a macroscopic device, it no longer follows the linear Schrödinger equation, but instantaneously collapses to one of the wave functions that correspond to definite measurement results. However, this collapse postulate is ad hoc, and the theory does not tell us why and how a definite measurement result appears. This measurement problem is widely
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.