Predictions of quantum theory have been confirmed experimentally in the microscopic domain with no known exceptions. This success motivates physicists to assume universal validity of the theory. To put the predictions of the quantum theory to the test in the domain of more complex systems researchers like e.g. Eugene Wigner have proposed carefully designed Gedankenexperiments revealing unexpected difficulties of the theory. Daniela Frauchiger and Renato Renner have recently suggested an extension of the Gedankenexperiment commonly known as Wigner's friend and arrived at a conclusion that one agent, upon observing a particular measurement outcome, must conclude that another agent has predicted the opposite outcome with certainty. Their analysis shows that quantum theory cannot consistently describe the use of itself. Here, we will study the mentioned Gedankenexperiments and introduce an approach leading to consistent predictions of quantum theory, independent of observer using the theory.
Predictions of quantum theory have been confirmed experimentally in the microscopic domain with no known exceptions. This success motivates physicists to assume universal validity of the theory. To put the predictions of the quantum theory to the test in the domain of more complex systems researchers like e.g. Eugene Wigner have proposed carefully designed Gedankenexperiments revealing unexpected difficulties of the theory. Daniela Frauchiger and Renato Renner have recently suggested an extension of the Gedankenexperiment commonly known as Wigner's friend and arrived at a conclusion that one agent, upon observing a particular measurement outcome, must conclude that another agent has predicted the opposite outcome with certainty. Their analysis shows that quantum theory cannot consistently describe the use of itself. Here, we will study the mentioned Gedankenexperiments and introduce an approach leading to consistent predictions of quantum theory, independent of observer using the theory.
Quantum theory provides an extremely accurate description of fundamental processes in physics. It thus seems likely that the theory is applicable beyond the, mostly microscopic, domain in which it has been tested experimentally. Here, we propose a Gedankenexperiment to investigate the question whether quantum theory can, in principle, have universal validity. The idea is that, if the answer was yes, it must be possible to employ quantum theory to model complex systems that include agents who are themselves using quantum theory. Analysing the experiment under this presumption, we find that one agent, upon observing a particular measurement outcome, must conclude that another agent has predicted the opposite outcome with certainty. The agents' conclusions, although all derived within quantum theory, are thus inconsistent. This indicates that quantum theory cannot be extrapolated to complex systems, at least not in a straightforward manner.
Abstract Quantum theory provides an extremely accurate description of fundamental processes in physics. It thus seems likely that the theory is applicable beyond the, mostly microscopic, domain in which it has been tested experimentally. Here, we propose a Gedankenexperiment to investigate the question whether quantum theory can, in principle, have universal validity. The idea is that, if the answer was yes, it must be possible to employ quantum theory to model complex systems that include agents who are themselves using quantum theory. Analysing the experiment under this presumption, we find that one agent, upon observing a particular measurement outcome, must conclude that another agent has predicted the opposite outcome with certainty. The agents’ conclusions, although all derived within quantum theory, are thus inconsistent. This indicates that quantum theory cannot be extrapolated to complex systems, at least not in a straightforward manner.
Quantum mechanics is one of our most successful physical theories; its predictions agree with experimental observations to an extremely high accuracy. However, the bare formalism of quantum theory does not provide straightforward answers to seemingly simple questions: for example, how should one model systems that include agents who are themselves using quantum theory? These foundational questions may be investigated with a theorist's tool – the thought experiment. Its purpose is to turn debates about the interpretation of quantum theory into actual physics questions. In this article, we give a state-of-the-art overview on quantum thought experiments involving observers, from the basic Wigner's friend to the recent Frauchiger-Renner setup, clarifying their interpretational significance and responding to objections and criticism on the way.
In April 2016, Daniela Frauchiger and Renato Renner published an article online in which they introduce a Gedankenexperiment that led them to conclude that single-world interpretation of quantum theory cannot be self-consistent. In a new version of the paper, published in September 2018, the authors moderate their original claim by concluding that quantum theory cannot be extrapolated to complex systems, at least not in a straightforward manner. The purpose of this article is to offer a careful reconstruction of the F-R argument, which allows us to show that: (i) the argument can be more clearly formulated with no reference to what subjects know or see, but rather only in terms of quantum propositions, (ii) in contrast to what some commentators suppose, the argument does not require the hypothesis of collapse to arrive to its conclusion, and (iii) the contradiction resulting from the F-R argument is inferred by making classical conjunctions between different and incompatible contexts. On the basis of this clarification, we will finally argue that the conclusion of the F-R argument is not as novel and original as its great impact might make us to suppose.
A number of no-go theorems have shown that Wigner's Friend scenarios combined with various metaphysical assumptions lead to contradictions in any version of quantum theory. We present an alternative constructive approach that only assumes that agents make properly qualified true statements. Quantum observers are modeled rigorously, although simplistically, using quantum circuits. Terminology is suggested to help avoid contradictions. Our methodology is applied to the Frauchiger-Renner paradox and results in statements by all agents that are both true and consistent. Quantum theory evades the no-go theorems because they make an incorrect implicit assumption about how quantum agents behave.
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.