It is well known, that the causal Schrödinger evolution of a quantum state is not compatible with the reduction postulate, even when decoherence is taken into account. The violation of the causal evolution, introduced by the standard reduction postulate distinguishes certain systems (as the measurement devices), whose states are very close to statistical mixtures (as the ones resulting from the process of decoherence). In these systems, this violation has not any observable effect. In arbitrary quantum systems, the transition from the initial density matrix to a diagonal matrix predicted by the standard reduction postulate, would lead to a complete breakdown of the Schrödinger evolution, and consequently would destroy all the predictive power of quantum mechanics. What we show here, is that there is a modified version of the postulate that allows to treat all the quantum mechanical systems on equal footing. The standard reduction postulate may be considered as a good approximation, useful for practical purposes, of this modified version which is consistent with the Schrödinger evolution and via decoherence with the experimental results. Within this approach, the physical role played by the reduction postulate is as a tool for the computation of relative probabilities and consequently for the determination of the probabilities of consistent histories.
It is well known, that the causal Schrödinger evolution of a quantum state is not compatible with the reduction postulate, even when decoherence is taken into account. The violation of the causal evolution, introduced by the standard reduction postulate distinguishes certain systems (as the measurement devices), whose states are very close to statistical mixtures (as the ones resulting from the process of decoherence). In these systems, this violation has not any observable effect. In arbitrary quantum systems, the transition from the initial density matrix to a diagonal matrix predicted by the standard reduction postulate, would lead to a complete breakdown of the Schrödinger evolution, and consequently would destroy all the predictive power of quantum mechanics. What we show here, is that there is a modified version of the postulate that allows to treat all the quantum mechanical systems on equal footing. The standard reduction postulate may be considered as a good approximation, useful for practical purposes, of this modified version which is consistent with the Schrödinger evolution and via decoherence with the experimental results. Within this approach, the physical role played by the reduction postulate is as a tool for the computation of relative probabilities and consequently for the determination of the probabilities of consistent histories.
The problem of measurement in quantum mechanics is reanalyzed within a general, strictly probabilistic framework (without reduction postulate). Based on a novel comprehensive definition of measurement the natural emergence of objective events is demonstrated and their formal representation within quantum mechanics is obtained. In order to be objective an event is required to be observable or readable in at least two independent, mutually non-interfering ways with necessarily agreeing results. Consistency in spite of unrestricted validity of reversibility of the evolution or the superposition principle is demonstrated and the role played by state reduction, in a properly defined restricted sense, is discussed. Some general consequences are pointed out.
The two main types of quantum measurements von Neumann's postulate of the reduction of the wave function answers the question of what happens to the quantum object during the measurement. However, the reduction postulate does not answer the question of how the measuring device must be designed to realize the desired measurement. To answer this second question, one must understand the connection between the measuring device as an ordinary physical system, and the nature of the desired measurement (the quantity to be measured, the desired precision, and the choice of how the measurement is to perturb the quantum object). The Schrödinger equation cannot tell us the connection between the design of the measuring device and the nature of the measurement, because the Schrödinger equation neither governs nor describes the process of measurement. More specifically: The evolution of the wave function as described by the Schrödinger equation has two key features: it is a reversible evolution (from the final state in principle one can always evolve back to the initial one), and it is a deterministic evolution (the final state is determined uniquely by the initial one). By contrast, the reduction of the wave function in a measurement is irreversible (once the measurement is finished and the information has been extracted from it, it is impossible to return to the pre-measurement state), and it is nondeterministic (one cannot predict, before the measurement, the post-measurement state of the measured object).
One of the fundamental conceptual difficulties in quantum mechanics arises from the fact that two of the axioms of the theory concerning the evolution of a microscopic system are contradictory. The first axiom assumes that the state of a quantum system evolving undisturbed by any observer obeys a linear deterministic law expressed by the SchrSdinger equation. The second one (wave-packet reduction principle) states that, when the quantum system interacts with an apparatus devised to measure some of its dynamical variables, its state changes abruptly and stochastically, jumping into a normalized eigenstate of the measured variable. This evolution is not deterministic nor linear and cannot be accounted for by the SchrSdinger equation. As a consequence the internal coherence of the theory becomes questionable. On one side the influence of the apparatus on the quantum system cannot be described as an interaction of the quantum constituents of the measuring macroscopic apparatus with the measured quantum system. On the other side, the impossibility of considering a macroscopic object as made by quantum constituents, i .e . the difficulty of quantum mechanics in describing the behaviour of macroscopic systems, is even more dramatic if we consider the evolution of the closed system consisting of the macroscopic apparatus and of the microscopic system. In such a case the appearance of superpositions of macr0scopically distinguishable states related to the apparatus cannot be avoided. These superpositions contradict both our classical experience and wave-packet reduction principle, which provides for statistical mixtures.
The problem of measurement taken at face value shows clearly that there is an inconsistency inside the quantum formalism.The phenomenon of decoherence is often presented as a solution to it.A widely debated question is to decide between two different interpretations.The first one is to consider that the decoherence process has the effect to actually project a superposed state into one of its classically interpretable component, hence doing the same job as the reduction postulate.For the second one, decoherence is only a way to show why no macroscopic superposed state can be observed and so, to explain the classical appearance of the macroscopic world, while the quantum entanglement between the system, the apparatus and the environment never disappears.In this case, explaining why only one single definite outcome is observed remains to do.In this paper, we examine arguments for and against both interpretations and defend a position according to which the outcome that is observed is relative to the observer in close parallel to the Everett interpretation.
This paper presents a revised unified account of the Total Wave Modified Schrödinger Equation (TWMSE), a proposed deterministic collapse framework in which quantum state reduction is investigated as a possible dynamical consequence of interaction between a system wavefunction and observer fields associated with physical interactions. The purpose of this revision is to distinguish clearly between mathematical structures that have been explicitly formulated, consequences that follow from the minimal written dynamics, and stronger experimental proposals that remain to be derived. A central result is that a spatially homogeneous scalar collapse term can modify the global amplitude of a quantum state but cannot, by itself, generate spatial localization or branch selection. After normalization, the common scalar factor cancels from the spatial probability distribution. Likewise, a spatially homogeneous multiplicative metric can restore a conserved physical norm but does not by itself produce a measurable spatial probability bias. The paper also corrects the treatment of the time dependent metric. Conservation of the physical norm requires the full time dependent metric condition, G˙+ℏi(H†G−GH)=0, rather than the time independent pseudo Hermitian relation alone. For the minimal scalar TWMSE model, the corresponding metric compensates exactly for the non Hermitian amplitude evolution, while also revealing a long time invertibility issue when the collapse functional possesses a nonzero mean. These results change the status of several experimental signatures proposed in the earlier unified TWMSE account. In particular, a nonzero Sorkin parameter, a second harmonic in Mach Zehnder interferometry, associated detection time estimates, and an already established branch dependent history bias are not treated in this revised paper as derived predictions of the minimal homogeneous TWMSE dynamics. They are retained instead as open derivation and experimental targets that require a genuinely spatially structured, operator level, or branch sensitive probability dynamics. The two system history experiment is consequently reformulated as a falsifiability target. A completed TWMSE model would need to demonstrate mathematically how different observer field histories could produce different normalized outcome statistics for systems having the same final standard quantum state, while remaining distinguishable from ordinary Hamiltonian evolution, decoherence, systematic effects, and competing collapse models. The paper further identifies the principal mathematical requirements for completing the framework, including branch sensitive collapse dynamics, existence and consistency of the time dependent metric, derivation of the physical probability law, strong coupling phase dynamics, accumulated interference and memory, multi particle consistency, no signalling, relativistic completion, conservation laws, parameter identification, and derivation or revision of the earlier interferometric proposals. This paper represents the author's current technical position on these questions and supersedes the experimental interpretation of the earlier technical unified account, Zenodo DOI 10.5281/zenodo.19537024, where those claims exceed what follows from the minimal written dynamics. The earlier technical paper remains part of the public development record. Its plain English companion, Zenodo DOI 10.5281/zenodo.19537305, likewise remains part of that record and should be read in light of the present revision. The objective of this revised account is therefore narrower but stronger: to state explicitly what TWMSE presently derives, what remains conditional, what remains open, and what must be demonstrated before the framework can yield a distinctive, normalized, causal, and experimentally testable prediction. Revision notice: This publication represents the author's current technical position on the experimental status of TWMSE. It supersedes the interpretation of experimental claims in Zenodo DOI 10.5281/zenodo.19537024 where those claims exceed what follows from the minimal homogeneous dynamics. The plain English companion to that earlier account, Zenodo DOI 10.5281/zenodo.19537305, should likewise be read in light of this revision. Both earlier records remain publicly available as part of the development history.
Left on its own, a quantum state evolves deterministically under the Schrödinger equation, forming superpositions. Upon measurement, however, a stochastic process governed by the Born rule collapses it to a single outcome. This dual evolution of quantum states – the core of the Measurement Problem – has puzzled physicists and philosophers for nearly a century. Yet, amid the cacophony of competing interpretations, the problem today is not as impenetrable as it once seemed. This paper reviews the current status of the Measurement Problem, distinguishing between what is well understood and what remains unresolved. We examine key theoretical approaches, including decoherence, many-worlds interpretation, objective collapse theories, hidden-variable theories, dualistic approaches, deterministic models, and epistemic interpretations. To make these discussions accessible to a broader audience, we also reference curated online resources that provide high-quality introductions to central concepts.