In a joint paper Jeff Bub and Itamar Pitowski argued that the quantum state represents `the credence function of a rational agent [...] who is updating probabilities on the basis of events that occur'. In the famous thought experiment designed by Wigner, Wigner's friend performs a measurement in an isolated laboratory which in turn is measured by Wigner. Here we consider Wigner's friend as a rational agent and ask what her `credence function' is. We find experimental situations in which the friend can convince herself that updating the probabilities on the basis of events that happen solely inside her laboratory is not rational and that conditioning needs to be extended to the information that is available outside of her laboratory. Since the latter can be transmitted into her laboratory, we conclude that the friend is entitled to employ Wigner's perspective on quantum theory when making predictions about the measurements performed on the entire laboratory, in addition to her own perspective, when making predictions about the measurements performed inside the laboratory.
In a joint paper Jeff Bub and Itamar Pitowski argued that the quantum state represents the credence function of a rational agent [...] who is updating probabilities on the basis of events that occur'. In the famous thought experiment designed by Wigner, Wigner's friend performs a measurement in an isolated laboratory which in turn is measured by Wigner. Here we consider Wigner's friend as a rational agent and ask what her credence function' is. We find experimental situations in which the friend can convince herself that updating the probabilities on the basis of events that happen solely inside her laboratory is not rational and that conditioning needs to be extended to the information that is available outside of her laboratory. Since the latter can be transmitted into her laboratory, we conclude that the friend is entitled to employ Wigner's perspective on quantum theory when making predictions about the measurements performed on the entire laboratory, in addition to her own perspective, when making predictions about the measurements performed inside the laboratory.
Wigner’s friend experiment and its modern extensions display the ambiguity of the quantum mechanical description regarding the assignment of quantum states. While the friend applies the state-update rule to the system upon observing an outcome of her measurement in a quantum system, Wigner describes the friend’s measurement as a unitary evolution, resulting in an entangled state for the composite system of the friend and the system. In this respect, Wigner is often referred to as a “superobserver,” who has the supreme technological ability to keep the friend’s laboratory coherent. As such, it is often argued that he has the “correct” description of the state. Here, we show that the situation is more symmetrical than is usually thought: There are different types of information that each of the observers has that the other fundamentally cannot have—they reside in different “bubbles” (in Cavalcanti’s terminology). While this can explain why the objectivity of the state assignment is only relative to the bubble, we consider more elaborated situations in the form of a game in which the players can switch between bubbles. We find that, in certain circumstances, observers may be entitled to adopt and verify the state assignment from another bubble if they condition their predictions on information that is in principle available to them.
Wigner’s friend scenarios—in which external agents describe a closed laboratory containing a friend making a measurement—highlight the difficulties of quantum theory when accounting for measurements. The problem is to accommodate for unitary evolution from the point of view of the external agent with the measurements or other operations carried out by the friend. Here, we show in the context of a relativistic thought experiment that an operation that may be accounted for unitarily in a given reference frame cannot be described unitarily in a different reference frame. This result, based on the frame dependence of the state update in relativistic contexts, could point to some fundamental inadequacy when attempting to model actions taken by a complex agent as unitary operations.
Wigner-friend scenarios -- in which external agents describe a closed laboratory containing a friend making a measurement -- highlight the difficulties inherent to quantum theory when accounting for measurements. In non-relativistic scenarios, the difficulty is to accommodate unitary evolution for a closed system with a definite outcome obtained by the friend. In relativistic scenarios the tensions between quantum theory and relativity induce additional constraints. A generic property of relativistic scenarios is the frame-dependence of state update upon a measurement. Based on a definite example, we will show that this property leads to inconsistent accounts for outcomes obtained in different reference frames. We will further argue that these results point to some fundamental inadequacy when attempting to model actions taken by a complex agent as unitary operations made on simple wavefunctions.
There has been a surge of recent interest in the Wigner's Friend paradox, sparking several novel thought experiments and no-go theorems. The main narrative has been that Wigner's Friend highlights a counterintuitive feature that is unique to quantum theory, and which is closely related to the quantum measurement problem. Here, we challenge this view. We argue that the gist of the Wigner's Friend paradox can be reproduced without assuming quantum physics, and that it underlies a much broader class of enigmas in the foundations of physics and philosophy. To show this, we first consider several recently proposed Extended Wigner's Friend scenarios, and demonstrate that some of their implications for the absoluteness of observations can be reproduced by classical thought experiments that involve the duplication of agents. Crucially, some of these classical scenarios are technologically much easier to implement than their quantum counterparts. Then, we argue that the essential structural ingredient of all these scenarios is a feature that we call "Restriction A": that a physical theory cannot give us a probabilistic description of the observations of all agents. Finally, we argue that this difficulty is at the core of other puzzles in the foundations of physics and philosophy, and demonstrate this explicitly for cosmology's Boltzmann brain problem. Our analysis suggests that Wigner's Friend should be studied in a larger context, addressing a frontier of human knowledge beyond quantum foundations: to obtain reliable predictions for experiments in which these predictions can be privately but not intersubjectively verified.
Quantum theory cannot consistently describe the use of itself », Nature Communications, 9, 3711, 2018 11 Wigner's friend's thought experiment involves two observers.The first (the friend) is locked up in a laboratory, performing a quantum measurement on a physical system whose quantum state is a superposition of two distinct eigenstates of an observable.When the friend has finished the measurement, he obtains as a result one of the two eigenvalues of the observable, and he assigns to his system a new state identical to the eigenstate corresponding to this measured eigenvalue.The second observer (Wigner) is located outside the laboratory, and he only knows one thing about what is happening inside: that his friend is performing the measurement described above.Under such conditions, he is forced to attribute to everything contained in the laboratory (including his friend) a quantum state that extends the superposition beyond the physical system, to his friend himself.The
There has been an upsurge of interest in the consequences for quantum physics of the so-called Wigner's Friend Paradox. In its original formulation, the paradox has been turned inside out, and virtually every aspect of it has been looked into. Consequently, it is becoming widely accepted that we can find the potentially puzzling consequences of Wigner's thought experiment only in light of its many-parties extensions. Nonetheless, this contribution returns to the source. Reframing the question as an inference problem, we advance a radically Bayesian interpretation that shows no contradiction between Wigner's and Wigner's Friend's descriptions-neither classically nor quantumly. Therefore, with no paradoxical conclusion. In doing so, we flesh out and expose previously untouched aspects of Wigner's thought experiment, in particular, the fact that compatibility and agreement are fundamental to our understanding of it. Also, by conservatively extending Wigner's original setup and incorporating what we call the 'benefit of the doubt', we see how either Wigner's or his Friend's description can be driven to match one another's -- an impossibility if either agent does not keep an open mind.