The issue of measurement has been part of the discussion of quantum mechanics. In addition, this has been complicated by the argument over reality and when and where it arises in the quantum system. One of the arguments of A. Einstein, with B. Podolsky and N. Rosen was for the existence of a reality in the system before the measurement. Modern approaches to time-resolved studies require the fast lasers simply because they provide the finest time scale over which measurements can be made at the present time. There are problems with the ideas, in that such measurements might be thought to be incompatible with the standard Copenhagen–Gottingen viewpoint. While the earlier measurements were simple spin measurements, more modern approaches have concentrated on a continuous measurement in which one integrates the weak signal over time. One problematic aspect of continuous measurements is the time evolution of the system, where what is being measured may be a function of time.
The issue of measurement has been part of the discussion of quantum mechanics. In addition, this has been complicated by the argument over reality and when and where it arises in the quantum system. One of the arguments of A. Einstein, with B. Podolsky and N. Rosen was for the existence of a reality in the system before the measurement. Modern approaches to time-resolved studies require the fast lasers simply because they provide the finest time scale over which measurements can be made at the present time. There are problems with the ideas, in that such measurements might be thought to be incompatible with the standard Copenhagen–Gottingen viewpoint. While the earlier measurements were simple spin measurements, more modern approaches have concentrated on a continuous measurement in which one integrates the weak signal over time. One problematic aspect of continuous measurements is the time evolution of the system, where what is being measured may be a function of time.
The basic thesis of this book is that there is no quantum measurement problem. I do not mean by this that the apparent paradoxes of quantum mechanics arise because we fail to recognize ‘that quantum theory does not describe physical reality’ (Fuchs and Peres 2000a). Quantum theory describes reality just fine, like any other scientific theory worth taking seriously: describing (and explaining) reality is what the scientific enterprise is about. Nor do I mean that we have found solutions to the problem, ways to modify or supplement quantum mechanics so that the paradoxes are removed. I don’t actually think that we have found any very satisfactory solutions in this sense (at least not in the relativistic domain) but that isn’t my theme: even if we did have these modifications, they would be ways of changing from quantum physics, which supposedly does have a measurement problem, to some other theory, which supposedly doesn’t.
All alternative interpretations have at their roots some form of disgust at the Copenhagen–Gottingen philosophy, and all have a desire to seek an alternative truth for the understanding of quantum mechanics. The alternative view is of an intrinsic reality prior to the measurement, as discussed by A. Einstein, and by most of the founders of the wave theory of quantum mechanics. A key point of Neils Bohr’s arguments with Einstein, particularly with regard to the Einstein–B. Podolsky–N. Rosen type of experiments, was that one could not make multiple measurements on the same particle, or quantum system. In his view, the measurement itself defined the quantum system. With the arrival of the Schrodinger equation, quantum mechanics had a causal, deterministic equation of motion for the wave function. Perturbation theory, based upon the Schrodinger equation, provided transition details that undermined the assertions of quantum jumps.
The Einstein-Podolsky-Rosen argument for the incompleteness of quantum mechanics involves two assumptions: one about locality and the other about when it is legitimate to infer the existence of an element-of-reality. Using one simple thought experiment, we argue that quantum predictions and the relativity of simultaneity require that both these assumptions fail, whether or not quantum mechanics is complete. EPR’s (1935) argument for the incompleteness of QM turns upon the following sufficient condition: “If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity.“ EPR apply this condition in a case where knowledge gained from measuring one quantum system allows one to predict with certainty a value for a physical quantity on another.
Q UANTUM MECHANICS, TOGETHER WITH RELATIVITY, are the basis of most of what we know of the very large, the very small, and the very fast.And in all of these areas our knowledge is extensive.It would be hard to say how good these theories are; arithmetic, one might say, is good mathematics.For all that, there is a problem, a strange and amorphous difficulty: it is the problem of measurement.This problem has become much more interesting, and much more pressing, as a result of some recent developments.Hitherto the difficulty has been so peculiar, and so formless, that physicists have on the whole thought it a matter of philosophy.But that is not quite right (or that is what I shall argue).It is certainly philosophical, but it is also a matter of methodology, a question of what physics is to do; and now the matter has come to seem quite pressing.In this sense the measurement problem has become a problem for physics.But what is the problem of measurement?As a first stab, we can say this: on the basis of quantum mechanics, there is a difficulty in accounting for the
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.