Schrodinger's Cat

The measurement problem is addressed from the viewpoint that it is the distinguishability between the state preparation and its quantum ensemble, i.e. the set of states with which it has a non-zero overlap, that is at the heart of the difference between classical and quantum measurements. The measure for the degree of distinguishability between pairs of quantum states, i.e. the quantum fidelity, is for this purpose generalized, by the application of the superposition principle, to the setting where there exists an (...) arbitrary-dimensional quantum ensemble. (shrink)

In a recent paper, Zukowski and Markiewicz showed that Wigner’s Friend (and, by extension, Schrodinger’s Cat) can be eliminated as physical possibilities on purely logical grounds. I validate this result and demonstrate the source of the contradiction in a simple experiment in which a scientist S attempts to measure the position of object |O⟩ = |A⟩S +|B⟩S by using measuring device M chosen so that |A⟩M ≈ |A⟩S and |B⟩M ≈ |B⟩S. I assume that the measurement occurs by quantum amplification (...) without collapse, in which M can entangle with O in a way that remains reversible by S for some nonzero time period. This assumption implies that during this “reversible” time period, |A⟩M ̸= |A⟩S and |B⟩M ̸= |B⟩S – i.e., the macroscopic pointer state to which M evolves is uncorrelated to the position of O relative to S. When the scientist finally observes the measuring device, its macroscopic pointer state is uncorrelated to the object in position |A⟩S or |B⟩S, rendering the notion of “reversible measurement” a logical contradiction. (shrink)

The Schrodinger's Cat and Wigner's Friend thought experiments, which logically follow from the universality of quantum mechanics at all scales, have been repeatedly characterized as possible in principle, if perhaps difficult or impossible for all practical purposes. I show in this paper why these experiments, and interesting macroscopic superpositions in general, are actually impossible in principle. First, no macroscopic superposition can be created via the slow process of natural quantum packet dispersion because all macroscopic objects are inundated with decohering interactions (...) that constantly localize them. Second, the SC/WF thought experiments depend on von Neumann-style amplification to achieve quickly what quantum dispersion achieves slowly. Finally, I show why such amplification cannot produce a macroscopic quantum superposition of an object relative to an external observer, no matter how well isolated the object from the observer, because: the object and observer are already well correlated to each other; and reducing their correlations to allow the object to achieve a macroscopic superposition relative to the observer is equally impossible, in principle, as creating a macroscopic superposition via the process of natural quantum dispersion. (shrink)

The universality assumption (“U”) that quantum wave states only evolve by linear or unitary dynamics has led to a variety of paradoxes in the foundations of physics. U is not directly supported by empirical evidence but is rather an inference from data obtained from microscopic systems. The inference of U conflicts with empirical observations of macroscopic systems, giving rise to the century-old measurement problem and subjecting the inference of U to a higher standard of proof, the burden of which lies (...) with its proponents. This burden remains unmet because the intentional choice by scientists to perform interference experiments that only probe the microscopic realm disqualifies the resulting data from supporting an inference that wave states always evolve linearly in the macroscopic realm. Further, the nature of the physical world creates an asymptotic size limit above which interference experiments, and verification of U in the realm in which it causes the measurement problem, seem impossible for all practical purposes if nevertheless possible in principle. This apparent natural limit serves as evidence against an inference of U, providing a further hurdle to the proponent’s currently unmet burden of proof. The measurement problem should never have arisen because the inference of U is entirely unfounded, logically and empirically. (shrink)

I show in this paper why the universality of quantum mechanics at all scales, which implies the possibility of Schrodinger's Cat and Wigner's Friend thought experiments, cannot be experimentally confirmed, and why macroscopic superpositions in general cannot be observed or measured, even in principle. Through the relativity of quantum superposition and the transitivity of correlation, it is shown that from the perspective of an object that is in quantum superposition relative to a macroscopic measuring device and observer, the observer is (...) already sufficiently well correlated to the measuring device that once the object correlates to the measuring device, there is no time period in which the observer can perform an appropriate interference experiment to show that the measuring device is in a superposition. (shrink)

I accept that McTaggart's A-series and B-series are not inter-reducible and that both are needed for a complete temporal description of a physical system. I consider the Wigner's Friend thought experiment. The A-series are associated with each (quantum) system, and relativity is associated with the B-series. I consider temporal evolution through this 'hybrid' time. We may define the rate of temporal flow as 1 B-series second per A-series second.

This paper proposes an interpretation of time that is an 'A-theory' in that it incorporates both McTaggart's A-series and his B-series. The A-series characteristics are supposed to be 'ontologically private' analogous to qualia in the problem of other minds, such as in the Inverted Spectrum thought experiment, and is given a definition. The main idea is then that the experimenter and the cat do not share the same A-series characteristics, e.g. the same 'now', to some extent. So there is no (...) single time at which the cat gets ascribed different states, one by the experimenter and one by the cat. Also it is proposed one may define an ontologically private 'unit of becoming' that coordinatizes the future/present/past A-series spectrum as well as allow one to calculate rates of becoming with seconds. The latter are taken to measure differences in B-series times. (shrink)

This paper proposes an interpretation of time that is an 'A-theory' in that it incorporates both McTaggart's A-series and his B-series. The A-series characteristics are supposed to be 'ontologically private' analogous to qualia in the Inverted Spectrum thought experiment and is given a definition. It is proposed one may define a 'unit of becoming' that coordinatizes the future/present/past spectrum as well as allowing one to calculate the rates of becoming. We give a picture of this interpretation and discuss how it (...) relates to the Schrodinger's Cat paradox. (shrink)

This paper proposes an interpretation of time that is an 'A-theory' in that it incorporates both McTaggart's A-series and his B-series. The A-series characteristics are supposed to be 'ontologically private' analogous to qualia in the Inverted Spectrum thought experiment and is given a definition. The main idea is that the experimenter and the cat do not share the same A-series characteristics. So there is no single time at which the cat gets ascribed different states. It is proposed one may define (...) a 'unit of becoming' that coordinatizes the future/present/past spectrum as well as allowing one to calculate the rates of becoming. We give a picture of this interpretation and discuss how it relates to the Schrodinger's Cat 'paradox'. Also relativity is briefly considered. (shrink)

A theory of time was proposed in "A theory of time", an early version of which is on PhilPapers. The idea was that the A-series features of a physical system are ontologically private, and this was given a mathematical definition. Also B-series features are ontologically public. This brief note is a detailed rumination on path-integrals and Schrodinger's Cat, in this theory.

This paper proposes an interpretation of time that is an 'A-theory' in that it incorporates both McTaggart's A-series and his B-series. The A-series characteristics are supposed to be 'ontologically private' analogous to qualia in the problem of other minds and is given a definition. The main idea is that the experimenter and the cat do not share the same A-series characteristics, e.g the same 'now'. So there is no single time at which the cat gets ascribed different states. It is (...) proposed one may define a 'unit of becoming' that coordinatizes the future/present/past 'private' spectrum as well as allowing one to calculate the rates of becoming. Relativity is briefly considered. (shrink)

This project retraces activations of Kierkegaard in the development of polit­ical theology. It suggests alternative modes of states of exception than those attributed to him by Schmitt, Taubes and Agamben. Several Kierkegaardian themes open themselves to 'something like pure potential' in Agamben, namely: living death, animality, criminality, auto-constitution, modification, liturgy, love and certain articulations of improbabilities. Attention is drawn to a modal ontology and auto-constitution at work in Kierkegaard's writings, as well as a complicated and indissociable operation between killing and (...) letting-live in legalist exceptionalism, comparable to similar functions found in Foucault regarding the biopowers and necropolitics of territorial and governmental apparatuses. It closes in consideration of Kierkegaard's critique of enumeration, large numbers, and statistical probability alongside contemporary tele-technoscientific social controls via the online datafication of people by surveillance or platform capitalisms. After Kierkegaard, such apparatuses are perhaps suspect as calculated to tranquilize humanity into more docile subhumans as it fools folk into becoming part of its numbers. (*Accompanying file includes only front matter, abstract, and endnotes*). (shrink)

It is revealed the invalidity of the idea that famous Schrodinger's cat thought experiment can be a quantum touchstone. The arguments are: (i) the probabilistic incorrectness in the (over)rating of the subject, (ii) the possibility of imagining non-quantum scenarios but completely similar to that experiment (iii) lack of ratified practical tests having genuine essence (i.e., non-counterfeit). So, the aforesaid experiment appears as a simplistic thought exercise without any notable significance for quantum physics.

After a brief presentation of Feynman diagrams, we criticizise the idea that Feynman diagrams can be considered to be pictures or depictions of actual physical processes. We then show that the best interpretation of the role they play in quantum field theory and quantum electrodynamics is captured by Hughes' Denotation, Deduction and Interpretation theory of models, where “models” are to be interpreted as inferential, non-representational devices constructed in given social contexts by the community of physicists.

La mécanique quantique est une théorie physique contemporaine réputée pour ses défis au sens commun et ses paradoxes. Depuis bientôt un siècle, plusieurs interprétations de la théorie ont été proposées par les physiciens et les philosophes, offrant des images quantiques du monde, ou des ontologies, radicalement différentes. L'existence d'un hasard fondamental, ou d'une multitude de mondes en-dehors du nôtre, dépend ainsi de l'interprétation adoptée. Après avoir discuté de la définition de l'interprétation d'une théorie physique, ce livre présente trois principales interprétations (...) quantiques, empiriquement équivalentes : l'interprétation dite orthodoxe, l'interprétation de Bohm, et l'interprétation des mondes multiples. Des textes d'Albert & Galchen, ainsi que de Mermin, présentent le concept de non-localité et invitent à une analyse de l'argument d'Einstein-Podolsky-Rosen et du théorème de Bell. (shrink)

The primary quantum mechanical equation of motion entails that measurements typically do not have determinate outcomes, but result in superpositions of all possible outcomes. Dynamical collapse theories (e.g. GRW) supplement this equation with a stochastic Gaussian collapse function, intended to collapse the superposition of outcomes into one outcome. But the Gaussian collapses are imperfect in a way that leaves the superpositions intact. This is the tails problem. There are several ways of making this problem more precise. But many authors dismiss (...) the problem without considering the more severe formulations. Here I distinguish four distinct tails problems. The first (bare tails problem) and second (structured tails problem) exist in the literature. I argue that while the first is a pseudo-problem, the second has not been adequately addressed. The third (multiverse tails problem) reformulates the second to account for recently discovered dynamical consequences of collapse. Finally the fourth (tails problem dilemma) shows that solving the third by replacing the Gaussian with a non-Gaussian collapse function introduces new conflict with relativity theory. (shrink)

Book Review of: Dow We Really Understand Quantum Mechanics? by Franck Laloe.

We propose a technical reformulation of the measurement problem of quantum mechanics, which is based on the postulate that the final state of a measurement is classical; this accords with experimental practice as well as with Bohr’s views. Unlike the usual formulation (in which the post-measurement state is a unit vector in Hilbert space), our version actually opens the possibility of admitting a purely technical solution within the confines of conventional quantum theory (as opposed to solutions that either modify this (...) theory, or introduce unusual and controversial interpretative rules and/or ontologies).To that effect, we recall a remarkable phenomenon in the theory of Schrödinger operators (discovered in 1981 by Jona-Lasinio, Martinelli, and Scoppola), according to which the ground state of a symmetric double-well Hamiltonian (which is paradigmatically of Schrödinger’s Cat type) becomes exponentially sensitive to tiny perturbations of the potential as ħ→0. We show that this instability emerges also from the textbook wkb approximation, extend it to time-dependent perturbations, and study the dynamical transition from the ground state of the double well to the perturbed ground state (in which the cat is typically either dead or alive, depending on the details of the perturbation).Numerical simulations show that adiabatically arising perturbations may (quite literally) cause the collapse of the wave-function in the classical limit. Thus, at least in the context of a simple mathematical model, we combine the technical and conceptual virtues of decoherence (which fails to solve the measurement problem but launches the key idea that perturbations may come from the environment) with those of dynamical collapse models à la grw (which do solve the measurement problem but are ad hoc), without sharing their drawbacks: single measurement outcomes are obtained (instead of merely diagonal reduced density matrices), and no modification of quantum mechanics is needed. (shrink)

Prologue: Stormclouds : London, April 1900 -- Quantum of action: The most strenuous work of my life : Berlin, December 1900 ; Annus Mirabilis : Bern, March 1905 ; A little bit of reality : Manchester, April 1913 ; la Comédie Française : Paris, September 1923 ; A strangely beautiful interior : Helgoland, June 1925 ; The self-rotating electron : Leiden, November 1925 ; A late erotic outburst : Swiss Alps, Christmas 1925 -- Quantum interpretation: Ghost field : Oxford, August (...) 1926 ; All this damned quantum jumping : Copenhagen, October 1926 ; The uncertainty principle : Copenhagen, February 1927 ; The 'Kopenhagener geist' : Copenhagen, June 1927 ; There is no quantum world : Lake Como, September 1927 -- Quantum debate: The debate commences : Brussels, October 1927 ; An absolute wonder : Cambridge, Christmas 1927 ; The photon box : Brussels, October 1930 ; A bolt from the blue : Princeton, May 1935 ; The paradox of Schrödinger's cat : Oxford, August 1935 -- Interlude: The first war of physics : Christmas 1938-August 1945 -- Quantum fields: Shelter Island : Long Island, June 1947 ; Pictorial semi-vision thing : New York, January 1949 ; A beautiful idea : Princeton, February 1954 ; Some strangeness in the proportion : Rochester, August 1960 ; Three quarks for Muster Mark! : New York, March 1963 ; The 'God particle' : Cambridge, Massachusetts, Autumn 1967 -- Quantum particles: Deep inelastic scattering : Stanford, August 1968 ; Of charm and weak neutral currents : Harvard, February 1970 ; The magic of colour : Princeton/Harvard, April 1973 ; The November revolution : Long Island/Stanford, November 1974 ; Intermediate vector bosons : Geneva, January/June 1983 ; The standard model : Geneva, September 2003 -- Quantum reality: Hidden variable : Princeton, Spring 1951 ; Bertlmann's socks : Boston, September 1964 ; The Aspect experiments : Paris, September 1982 ; The quantum eraser : Baltimore, January 1999 ; Lab cats : Stony Brook/Delft, July 2000 ; The persistent illusion : Vienna, December 2006 -- Quantum cosmology: The wavefunction of the universe : Princeton, July 1966 ; Hawking radiation : Oxford, February 1974 ; The first superstring revolution : Aspen, August 1984 ; Quanta of space and time : Santa Barbara, February 1986 ; Crisis? What crisis? : Durham, Summer 1994 -- A quantum of solace? : Geneva, March 2010. (shrink)

One of the most prospective directions of study of C.G. Jung’s synchronicity phenomenon is reviewed considering the latest achievements of modern science. The attention is focused mainly on the quantum entanglement and related phenomena – quantum coherence and quantum superposition. It is shown that the quantum non-locality capable of solving the Einstein-Podolsky-Rosen paradox represents one of the most adequate physical mechanisms in terms of conformity with the Jung’s synchronicity hypothesis. An attempt is made on psychophysiological substantiation of synchronicity within the (...) context of molecular biology. An original concept is proposed, stating that biological molecules involved in cell division during mitosis and meiosis, particularly DNA may be considered material carriers of consciousness. This assumption may be formulated on the basis of phenomenology of Jung’s analytical psychology. (shrink)

The following introduction offers a broad survey of the history of quantum physics. It then outlines the position of each contributor in this Special Focus Section concerning the collapse of the quantum wave function and defines three important terms (Hilbert space, Schrödinger’s cat, and decoherence) used in discussing this topic.

Science has made a mighty advance since it originated in ancient Greece more than 2500 years ago. Yet we still live in Plato's cave today; we think everything around us moves continuously, but continuous motion is merely a shadow of real motion. This book will lead you to walk out the cave along a logical and comprehensible road. After passing Zeno's arrow, Newton's inertia, Einstein's light, and Schrodinger's cat, you will reach the real world, where every thing in the universe, (...) whether it is an atom or a ball or even a star, ceaselessly jumps in a random and discontinuous way. In a famous metaphor, God does play dice with the universe. Discovering motion is not continuous but discontinuous and random is like finding the Earth is not at rest but moving. The new discovery may finally solve Zeno's paradoxes and the quantum puzzle, and it will lead to a profound shift in our world view. (shrink)

The most successful theory in all of science--and the basis of one third of our economy--says the strangest things about the world and about us. Can you believe that physical reality is created by our observation of it? Physicists were forced to this conclusion, the quantum enigma, by what they observed in their laboratories. Trying to understand the atom, physicists built quantum mechanics and found, to their embarrassment, that their theory intimately connects consciousness with the physical world. Quantum Enigma explores (...) what that implies and why some founders of the theory became the foremost objectors to it. Schrodinger showed that it "absurdly" allowed a cat to be in a "superposition" simultaneously dead and alive. Einstein derided the theory's "spooky interactions." With Bell's Theorem, we now know Schrodinger's superpositions and Einstein's spooky interactions indeed exist. Authors Bruce Rosenblum and Fred Kuttner explain all of this in non-technical terms with help from some fanciful stories and bits about the theory's developers. They present the quantum mystery honestly, with an emphasis on what is and what is not speculation. Physics' encounter with consciousness is its skeleton in the closet. Because the authors open the closet and examine the skeleton, theirs is a controversial book. Quantum Enigma's description of the experimental quantum facts, and the quantum theory explaining them, is undisputed. Interpreting what it all means, however, is controversial. Every interpretation of quantum physics encounters consciousness. Rosenblum and Kuttner therefore turn to exploring consciousness itself--and encounter quantum physics. Free will and anthropic principles become crucial issues, and the connection of consciousness with the cosmos suggested by some leading quantum cosmologists is mind-blowing. Readers are brought to a boundary where the particular expertise of physicists is no longer a sure guide. They will find, instead, the facts and hints provided by quantum mechanics and the ability to speculate for themselves. (shrink)

La meccanica quantistica è una delle più grandi conquiste intellettuali del xx secolo. Le sue leggiregolano il mondo atomico e subatomico e si riverberano su una miriade di fenomeni del mondomacroscopico, dalla formazione dei cristalli alla superconduttività, dalle proprietà dei fluidi a bassatemperatura agli spettri di emissione di una candela che brucia o di una supernova che esplode, daimeccanismi di combustione della fornace solare ai principi di base delle nanotecnologie. Non c’èquasi nulla nel mondo che ci circonda su cui non (...) soffi l’alito delle leggi quantistiche. Tuttavia, per come è usualmente presentata nei libri di testo, la meccanica quantistica è sostanzialmenteun’insieme di regole per calcolare le distribuzioni di probabilità dei risultati di qualunqueesperimento (nel dominio di validità della meccanica quantistica). In quanto tale, non ci forniscedirettamente una descrizione della realtà. Una descrizione della realtà, cioè un’ ontologia , dovrebbedirci che cosa c’è nel mondo e come si comporta, quali sono i processi che si realizzano a livellomicroscopico e, di conseguenza, fornirci una spiegazione del formalismo quantistico. (shrink)

Introduction The digital age has revolutionized the way we think, communicate, work, and enjoy life. Technology has increased the pace of life at an ...

Bohmian mechanics is a quantum theory with a clear ontology. To make clear what we mean by this, we shall proceed by recalling first what are the problems of quantum mechanics. We shall then briefly sketch the basics of Bohmian mechanics and indicate how Bohmian mechanics solves these problems and clarifies the status and the role of of the quantum formalism.

This book comprises all of John Bell's published and unpublished papers in the field of quantum mechanics, including two papers that appeared after the first edition was published. It also contains a preface written for the first edition, and an introduction by Alain Aspect that puts into context Bell's great contribution to the quantum philosophy debate. One of the leading expositors and interpreters of modern quantum theory, John Bell played a major role in the development of our current understanding of (...) the profound nature of quantum concepts. First edition Hb (1987): 0-521-33495-0 First edition Pb (1988): 0-521-36869-3. (shrink)

David Lewis's untimely death on 14 October 2001 deprived the philosophical community of one of the outstanding philosophers of the 20th century. As many obituaries remarked, Lewis has an undeniable place in the history of analytical philosophy. His work defines much of the current agenda in metaphysics, philosophical logic, and the philosophy of mind and language. This volume, an expanded edition of a special issue of the Australasian Journal of Philosophy, covers many of the topics for which Lewis was well (...) known, including possible worlds, counterpart theory, vagueness, knowledge, probability, essence, fiction, laws, conditionals, desire and belief, and truth. Many of the papers are by very established philosophers; others are by younger scholars including many he taught. The volume also includes Lewis's Jack Smart Lecture at the Australian National University, "How Many Lives has Schrodinger's Cat?," published here for the first time. Lewisian Themes will be an invaluable resource for anyone studying Lewis's work and a major contribution to the many topics that he mastered. (shrink)

In 'How Many Lives Has Schrödinger's Cat?' David Lewis argues that the Everettian no-collapse interpretation of quantum mechanics is in a tangle when it comes to probabilities. This paper aims to show that the difficulties that Lewis raises are insubstantial. The Everettian metaphysics contains a coherent account of probability. Indeed it accounts for probability rather better than orthodox metaphysics does.

Quantum theory is one the most important and successful theories of modern physical science. It has been estimated that its principles form the basis for about 30 per cent of the world's manufacturing economy. This is all the more remarkable because quantum theory is a theory that nobody understands. The meaning of Quantum Theory introduces science students to the theory's fundamental conceptual and philosophical problems, and the basis of its non-understandability. It does this with the barest minimum of jargon and (...) very little mathematics in the main text. Readers wishing to delve more deeply into the theory's mathematical subtleties can do so in an extended series of appendices. The book brings the reader up to date with the results of new experimental tests of quantum weirdness and reviews the latest thinking on alternative interpretations, the frontiers of quantum cosmology, quantum gravity and potential application of this weirdness in computing, cryptography and teleportation. (shrink)

By way of an example, Lewis imagines your being invited to join Schrödinger’s cat in its box for an hour. This box will either fill up with deadly poison fumes or not, depending on whether or not some radioactive atom decays, the probability of decay within an hour being 50%. The invitation is accompanied with some further incentive to comply (Lewis sets it up so there is a significant chance of some pretty bad but not life-threatening punishment if you don’t (...) get in the box). Lewis argues that the many minds theory implies that you should get in the box with the cat, despite this making it 50% likely you will die. (shrink)

The recent debates concerning divine action in the context of quantum mechanics are examined with particular reference to the work of William Pollard, Robert J. Russell, Thomas Tracy, Nancey Murphy, and Keith Ward. The concept of a quantum mechanical “event” is elucidated and shown to be at the center of this debate. An attempt is made to clarify the claims made by the protagonists of quantum mechanical divine action by considering the measurement process of quantum mechanics in detail. Four possibilities (...) for divine influence on quantum mechanics are identified and the theological and scientific implications of each discussed. The conclusion reached is that quantum mechanics is not easily reconciled with the doctrine of divine action. (shrink)

We criticize the bare theory of quantum mechanics -- a theory on which the Schrödinger equation is universally valid, and standard way of thinking about superpositions is correct.

This paper offers a defense of backwards in time causation models in quantum mechanics. Particular attention is given to Cramer's transactional account, which is shown to have the threefold virtue of solving the Bell problem, explaining the complex conjugate aspect of the quantum mechanical formalism, and explaining various quantum mysteries such as Schrödinger's cat. The question is therefore asked, why has this model not received more attention from physicists and philosophers? One objection given by physicists in assessing Cramer's theory was (...) that it is not testable. This paper seeks to answer this concern by utilizing an argument that backwards causation models entail a fork theory of causal direction. From the backwards causation model together with the fork theory one can deduce empirical predictions. Finally, the objection that this strategy is questionable because of its appeal to philosophy is deflected. (shrink)

In this fascinating and accessible book, physicist Victor J. Stenger guides the lay reader through the key developments of quantum mechanics and the debate over its apparent paradoxes. In the process, he critically appraises recent metaphysical fads. Dr. Stenger's knack for elucidating scientific ideas and controversies in language that the nonspecialist can comprehend opens up to the widest possible audience a wealth of information on the most important findings of contemporary physics.

We discuss two recent attempts two solve Schrodinger's cat paradox. One is the modal interpretation developed by Kochen, Healey, Dieks, and van Fraassen. It allows for an observable which pertains to a system to possess a value even when the system is not in an eigenstate of that observable. The other is a recent theory of the collapse of the wave function due to Ghirardi, Rimini, and Weber. It posits a dynamics which has the effect of collapsing the state of (...) macroscopic systems. We argue that the modal interpretation cannot account for non-accurate measurements and that both accounts have the consequence that in ordinary measurement situations the observables that ends up well defined are not quite the ones that we want to be well defined. (shrink)

The purpose of this paper is to review and clarify the quantum “measurement problem.” The latter originates in the ambivalent nature of the “observer”: Although the observer is not described by the Schrödinger equation, it should nevertheless be possible to “quantize” him and include him in the wave function if quantum theory is universally valid. The problem is to prove that no contradiction may arise in these two conflicting descriptions. The proof invokes the notion of irreversibility. The validity of the (...) latter is questionable, because the standard rationale for classical irreversibility, namely mixing and coarse graining, does not apply to quantum theory. There is no chaos in a closed, finite quantum system. However, when a system is large enough, it cannot be perfectly isolated from its “environment,” namely from external (or even internal) degrees of freedom which are not fully accounted for in the Hamiltonian of that system. As a consequence, the long-range evolution of such a quantum system is essentially unpredictable. It follows that the notion of irreversibility is a valid one in quantum theory and the “measurement problem” can be brought to a satisfactory solution. (shrink)