scholarly journals A no-go theorem for the persistent reality of Wigner’s friend’s perception

2021 ◽  
Vol 4 (1) ◽  
Author(s):  
Philippe Allard Guérin ◽  
Veronika Baumann ◽  
Flavio Del Santo ◽  
Časlav Brukner
Keyword(s):  
Quantum Mechanics ◽  
Quantum Theory ◽  
Thought Experiment ◽  
Joint Probability ◽  
Joint Probability Distribution ◽  
Initial State ◽  
Interpretations Of Quantum Mechanics ◽  
Present Information ◽  
Wigner’S Friend ◽  
Measured System

AbstractThe notorious Wigner’s friend thought experiment (and modifications thereof) has received renewed interest especially due to new arguments that force us to question some of the fundamental assumptions of quantum theory. In this paper, we formulate a no-go theorem for the persistent reality of Wigner’s friend’s perception, which allows us to conclude that the perceptions that the friend has of her own measurement outcomes at different times cannot “share the same reality”, if seemingly natural quantum mechanical assumptions are met. More formally, this means that, in a Wigner’s friend scenario, there is no joint probability distribution for the friend’s perceived measurement outcomes at two different times, that depends linearly on the initial state of the measured system and whose marginals reproduce the predictions of unitary quantum theory. This theorem entails that one must either (1) propose a nonlinear modification of the Born rule for two-time predictions, (2) sometimes prohibit the use of present information to predict the future—thereby reducing the predictive power of quantum theory—or (3) deny that unitary quantum mechanics makes valid single-time predictions for all observers. We briefly discuss which of the theorem’s assumptions are more likely to be dropped within various popular interpretations of quantum mechanics.

scholarly journals CAUSAL QUANTUM MECHANICS TREATING POSITION AND MOMENTUM SYMMETRICALLY

1995 ◽  
Vol 10 (08) ◽  
pp. 709-716 ◽  
Author(s):  
S. M. ROY ◽  
VIRENDRA SINGH
Keyword(s):  
Quantum Mechanics ◽  
Quantum Theory ◽  
Wigner Distribution ◽  
Probability Distributions ◽  
Joint Probability ◽  
Positive Definite ◽  
Joint Probability Distribution ◽  
Phase Space Density ◽  
Usual Quantum ◽  
Position Probability

De Broglie and Bohm formulated a causal quantum mechanics with a phase space density whose integral over momentum reproduces the position probability density of the usual statistical quantum theory. We propose a causal quantum theory with a joint probability distribution such that the separate probability distributions for position and momentum agree with the usual quantum theory. Unlike the Wigner distribution the suggested distribution is positive-definite and obeys the Liouville condition.

scholarly journals Experimental test of local observer independence

2019 ◽  
Vol 5 (9) ◽  
pp. eaaw9832 ◽  
Author(s):  
Massimiliano Proietti ◽  
Alexander Pickston ◽  
Francesco Graffitti ◽  
Peter Barrow ◽  
Dmytro Kundys ◽  
...  
Keyword(s):  
Quantum Mechanics ◽  
Scientific Method ◽  
Free Choice ◽  
Empirical Investigation ◽  
State Of The Art ◽  
Thought Experiment ◽  
Type Inequality ◽  
Repeated Measurements ◽  
Wigner’S Friend ◽  
Local Observer

The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics the objectivity of observations is not so clear, most markedly exposed in Wigner’s eponymous thought experiment where two observers can experience seemingly different realities. The question whether the observers’ narratives can be reconciled has only recently been made accessible to empirical investigation, through recent no-go theorems that construct an extended Wigner’s friend scenario with four observers. In a state-of-the-art six-photon experiment, we realize this extended Wigner’s friend scenario, experimentally violating the associated Bell-type inequality by five standard deviations. If one holds fast to the assumptions of locality and free choice, this result implies that quantum theory should be interpreted in an observer-dependent way.

scholarly journals POPPER'S THOUGHT EXPERIMENT REINVESTIGATED

2012 ◽  
Vol 10 (03) ◽  
pp. 1250033 ◽  
Author(s):  
CHRIS D. RICHARDSON ◽  
JONATHAN P. DOWLING
Keyword(s):  
Quantum Mechanics ◽  
Quantum Theory ◽  
Thought Experiment

Popper's original thought experiment probed some fundamental and subtle rules of quantum mechanics. He claimed that quantum mechanics was incomplete and devised an experiment to prove it. Two experiments have directly and indirectly tested Popper's hypothesis, and they provide some evidence that Popper's prediction may have been correct. The equations governing these two experiments and Popper's thought experiment will be derived from basic quantum principles. The experimental constants will be inputted and it will show that the two experiments reinforce each other and agree completely with quantum theory.

scholarly journals Consistent description of microscopic phenomena by macroscopic observers in quantum theory

2021 ◽  
Author(s):  
Alexey Kryukov
Keyword(s):  
Quantum Mechanics ◽  
Quantum Theory ◽  
Quantum Physics ◽  
Random Noise ◽  
Thought Experiments ◽  
Newtonian Physics ◽  
Measurement Results ◽  
Modern Physics ◽  
Measured System ◽  
Physical Phenomena

Abstract Quantum mechanics is the foundation of modern physics that is thought to be applicable to all physical phenomena, at least in principle. However, when applied to macroscopic bodies, the theory seems to be inconsistent. Wigner's friend and related thought experiments demonstrate that accounts by different observers described by the rules of quantum mechanics may be contradictory. Although still highly debated, such experiments seem to demonstrate an incompatibility of quantum mechanics with the usual rules of logic. Alternatively, one of the hidden assumptions in the thought experiments must be wrong. For instance, the argument is invalidated if macroscopic observers cannot be considered as physical systems described by the rules of quantum theory. Here we prove that there is a way to apply the rules of quantum mechanics to macroscopic observers while avoiding contradictory accounts of measurement by the observers. The key to this is the random noise that is ever present in nature and that represents the uncontrollable part of interaction between measured system and the surroundings in classical and quantum physics. By exploring the effect of the noise on microscopic and macroscopic bodies, we demonstrate that accounts of Wigner, the friend and other agents all become consistent. Our result suggests that the existing attempts to modify the Schrodinger equation to account for measurement results may be misguided. More broadly, the proposed mechanism for modeling measurements underlies the phenomenon of decoherence and is shown to be sufficient to explain the transition to Newtonian physics in quantum theory.

Spooky predictions at a distance: reality, complementarity and contextuality in quantum theory

2019 ◽  
Vol 377 (2157) ◽  
pp. 20190089 ◽  
Author(s):  
Arkady Plotnitsky
Keyword(s):  
Quantum Mechanics ◽  
Quantum Theory ◽  
Theme Issue ◽  
Interpretations Of Quantum Mechanics

This article brings together reality, complementarity and contextuality in quantum theory. It clarifies Bohr's concept of complementarity by considering the non-realist epistemology and the corresponding interpretations of quantum mechanics, based in the concept of ‘reality without realism’. Finally, as its main novel contribution, it establishes the connections between complementarity and contextuality. This article is part of the theme issue ‘Contextuality and probability in quantum mechanics and beyond’.

scholarly journals Generalized probability rules from a timeless formulation of Wigner's friend scenarios

Quantum ◽  
2021 ◽  
Vol 5 ◽  
pp. 524
Author(s):  
Veronika Baumann ◽  
Flavio Del Santo ◽  
Alexander R. H. Smith ◽  
Flaminia Giacomini ◽  
Esteban Castro-Ruiz ◽  
...  
Keyword(s):  
Measurement Problem ◽  
Consistency Condition ◽  
Joint Probability ◽  
The Other ◽  
Joint Probability Distribution ◽  
Conditional Probabilities ◽  
Unitary Evolution ◽  
Standard Quantum ◽  
Wigner’S Friend ◽  
Subsequent Measurement

The quantum measurement problem can be regarded as the tension between the two alternative dynamics prescribed by quantum mechanics: the unitary evolution of the wave function and the state-update rule (or "collapse") at the instant a measurement takes place. The notorious Wigner's friend gedankenexperiment constitutes the paradoxical scenario in which different observers (one of whom is observed by the other) describe one and the same interaction differently, one –the Friend– via state-update and the other –Wigner– unitarily. This can lead to Wigner and his friend assigning different probabilities to the outcome of the same subsequent measurement. In this paper, we apply the Page-Wootters mechanism (PWM) as a timeless description of Wigner's friend-like scenarios. We show that the standard rules to assign two-time conditional probabilities within the PWM need to be modified to deal with the Wigner's friend gedankenexperiment. We identify three main definitions of such modified rules to assign two-time conditional probabilities, all of which reduce to standard quantum theory for non-Wigner's friend scenarios. However, when applied to the Wigner's friend setup each rule assigns different conditional probabilities, potentially resolving the probability-assignment paradox in a different manner. Moreover, one rule imposes strict limits on when a joint probability distribution for the measurement outcomes of Wigner and his Friend is well-defined, which single out those cases where Wigner's measurement does not disturb the Friend's memory and such a probability has an operational meaning in terms of collectible statistics. Interestingly, the same limits guarantee that said measurement outcomes fulfill the consistency condition of the consistent histories framework.

scholarly journals Making Mistakes Saves the Single World of the Extended Wigner’s Friend Experiment

2021 ◽  
Author(s):  
Szymon Łukaszyk
Keyword(s):  
Quantum Theory ◽  
Thought Experiment ◽  
Physical Space ◽  
Entangled State ◽  
Freedom Of Choice ◽  
Identity Of Indiscernibles ◽  
Quantum Domain ◽  
Wigner’S Friend ◽  
Ugly Duckling ◽  
Single World

Abstract The Extended Wigner’s Friend thought experiment comprising a quantum system containing an agent who draws conclusions, upon observing the outcome of a measurement of a qubit prepared in two non-orthogonal versions by another agent led its authors to conclude that quantum theory cannot consistently describe the use of itself. It has also been proposed that this thought experiment is equivalent to coherent entangled state (Bell type) experiments. It is argued in this paper that the assumption of the freedom of choice of the first Wigner’s friend invalidates such equivalency. It is also argued that the assumption of locality (physical space) introduces superfluous identity of indiscernibles metric axiom, which is invalid in quantum domain and generally disproven by the Ugly duckling mathematical theorem.

scholarly journals Thought Experiment as a Logical Transformation in Transreal Logical Space

2019 ◽  
Author(s):  
Walter Gomide
Keyword(s):  
Quantum Mechanics ◽  
Quantum Theory ◽  
Thought Experiment ◽  
Thought Experiments ◽  
General Definition ◽  
Epr Paradox ◽  
Logical Space ◽  
Definition Of ◽  
American Physicist ◽  
The Ideal

In this article, from the concepts of formal causality and logical transformation, defined with transreal numbers, I intend to re-analyze the famous Einstein, Podolsky and Rosen paradox (the EPR paradox), according to which Quantum Mechanics is incomplete. In order to make such an analysis of the paradox, I present a general definition of thought experiments, in terms of the concept of logical transformation in a transreal logical space, and show that the EPR paradox, in broad outlines, bases the incomplete character of Quantum Mechanics on the fact of not having a formal causality between the ideal and concrete worlds of quantum theory - these concepts, the “ideal and concrete worlds”, by their turn, are inspired by the work of the American physicist Wolfgang Smith.

scholarly journals Quantum measurements and contextuality

2019 ◽  
Vol 377 (2157) ◽  
pp. 20190033 ◽  
Author(s):  
Robert B. Griffiths
Keyword(s):  
Quantum Mechanics ◽  
Probability Distribution ◽  
Quantum Physics ◽  
Joint Probability ◽  
Quantum Measurements ◽  
Joint Probability Distribution ◽  
Theme Issue ◽  
Different Types ◽  
Consistent Manner ◽  
Quantum Observables

In quantum physics, the term ‘contextual’ can be used in more than one way. One usage, here called ‘Bell contextual’ since the idea goes back to Bell, is that if A , B and C are three quantum observables, with A compatible (i.e. commuting) with B and also with C , whereas B and C are incompatible, a measurement of A might yield a different result (indicating that quantum mechanics is contextual) depending upon whether A is measured along with B (the { A ,  B } context) or with C (the { A ,  C } context). An analysis of what projective quantum measurements measure shows that quantum theory is Bell non-contextual: the outcome of a particular A measurement when A is measured along with B would have been exactly the same if A had, instead, been measured along with C . A different definition, here called ‘globally (non)contextual’ refers to whether or not there is (non-contextual) or is not (contextual) a single joint probability distribution that simultaneously assigns probabilities in a consistent manner to the outcomes of measurements of a certain collection of observables, not all of which are compatible. A simple example shows that such a joint probability distribution can exist even in a situation where the measurement probabilities cannot refer to properties of a quantum system, and hence lack physical significance, even though mathematically well defined. It is noted that the quantum sample space, a projective decomposition of the identity, required for interpreting measurements of incompatible properties in different runs of an experiment using different types of apparatus, has a tensor product structure, a fact sometimes overlooked. This article is part of the theme issue ‘Contextuality and probability in quantum mechanics and beyond’.

scholarly journals Consistent Descriptions of Quantum Measurement

2019 ◽  
Vol 49 (11) ◽  
pp. 1306-1324 ◽  
Author(s):  
Jianhao M. Yang
Keyword(s):  
Quantum Mechanics ◽  
Quantum System ◽  
Quantum Measurement ◽  
Composite System ◽  
Thought Experiment ◽  
Thought Experiments ◽  
Extended Version ◽  
Reasoning Process ◽  
Measurement Result ◽  
Wigner’S Friend

Abstract The Wigner’s friend type of thought experiments manifest the conceptual challenge on how different observers can have consistent descriptions of a quantum measurement event. In this paper, we analyze the extended version of Wigner’s friend thought experiment (Frauchiger and Renner in Nat Commun 3711:9, 2018) in detail and show that the reasoning process from each agent that leads to the no-go theorem is inconsistent. The inconsistency is with respect to the requirement that an agent should make use of updated information instead of outdated information. We then apply the relational formulation of quantum measurement to resolve the inconsistent descriptions from different agents. In relational formulation of quantum mechanics, a measurement is described relative to an observer. Synchronization of measurement result is a necessary requirement to achieve consistent descriptions of a quantum system from different observers. Thought experiments, including EPR, Wigner’s Friend and it extended version, confirm the necessity of relational formulation of quantum measurement when applying quantum mechanics to composite system with entangled but space-like separated subsystems.

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