Zenith Grant Awardee
Caslav Brukner
IQOQI Vienna, Austrian Academy of Sciences
Co-Investigators
Veronika Baumann, IQOQI Vienna, Austrian Academy of Sciences
Project Title
Conscious Encapsulated Observers
Project Summary
Wigner’s-friend experiments are thought experiments that challenge the logical consistency of quantum theory and highlight the quantum measurement problem. These experiments extend the quantum superposition principle to observers and ask what would happen, if such an observer in superposition – called Wigner’s friend – is measured by a so-called superobserver –Wigner. The friend and Wigner would disagree on how to describe the friend’s measurement based on standard quantum theory, which in certain circumstances can be turned into an actual contradiction. This, however, involves many subtleties and issues involving modelling the friend’s mental states and cognition, as well as the communication between her and Wigner. In this project we want to address these issues and investigate the subtleties. On the one hand, we will model the communication between Wigner and his friend such that they can exchange classical messages. On the other hand, we will consider the operations on the friend’s mental state and investigate how they can influence her cognitive functions. By combining these two approaches we will gain insight into whether and to what extent Wigner’s-friend experiments put limits on the compatibility between the superposition of the friend’s mental states and her free will or modes of consciousness.
Technical Abstract
In the famous Wigner's-friend thought experiment an observer – the friend – would describe her interaction with another quantum system as a measurement and, therefore, apply the collapse postulate. A so-called superobserver – Wigner – however, describes the friend as a quantum system and, therefore, her interaction with other quantum systems unitarily. Attributing consciousness and agency to the friend and enabling her and Wigner to communicate allows for in principle observable contradictions for certain types of the stateupdate rule. In this project we want to consider explicit models of classical communication between observers and superobservers as well as operations performed on the observer’s memory. The internal operations on these memory states can be thought of as part of the friend’s cognitive process. We aim to classify what types of communication and operations lead to empirically observable contradictions. Succeeding in this would allow us to formulate prescriptions for when to use the collapse postulate of quantum theory and give insight into in how far a memory in coherent superposition is compatible with cognition and consciousness. Additionally, we want to describe Wigner's-friend experiments in the context of quantum reference frames in order to resolve potential contradictions by referring to different perspectives.
QSpace Latest
PressRelease: Precision experiment puts pressure on quantum collapse theories
Quantum mechanics, the theory governing the microscopic world, is famously counterintuitive. A particle can exist in a superposition of multiple states, such as different positions, until a measurement is performed. At that point, the wavefunction describing that particle appears to ‘collapse’ to a single outcome. This puzzle lies at the heart of the measurement problem, famously illustrated by Schrödinger’s cat, suspended between life and death until observed. The XENONnT detector, which was designed to be sensitive to rare physics events, has tightened constraints on one family of possible solutions to the measurement problem, known as ‘collapse theories.’ The work, which was partially funded by FQxI, was reported in Physical Review Letters in March 2026. Image credit: XENON Collaboration.