Search

According to QBism, quantum states, unitary evolutions, and measurement operators are all understood as personal judgments of the agent using the formalism. Meanwhile, quantum measurement outcomes are understood as the personal experiences of the same agent. Wigner's conundrum of the friend, in which two agents ostensibly have different accounts of whether or not there is a measurement outcome, thus poses no paradox for QBism. Indeed the resolution of Wigner's original thought experiment was central to the development of QBist thinking. The focus of this paper concerns two very instructive modifications to Wigner's puzzle: One, a recent no-go theorem by Frauchiger and Renner (Nat Commun 9:3711, 2018), and the other a thought experiment by Baumann and Brukner (Quantum, Probability, Logic: The Work and Influence of Itamar Pitowsky, Springer, Cham, 2020). We show that the paradoxical features emphasized in these works disappear once both friend and Wigner are understood as agents on an equal footing with regard to their individual uses of quantum theory. Wigner's action on his friend then becomes, from the friend's perspective, an action the friend takes on Wigner. Our analysis rests on a kind of quantum Copernican principle: When two agents take actions on each other, each agent has a dual role as a physical system for the other agent. No user of quantum theory is more privileged than any other. In contrast to the sentiment of Wigner's original paper, neither agent should be considered as in "suspended animation." In this light, QBism brings an entirely new perspective to understanding Wigner's friend thought experiments. [ABSTRACT FROM AUTHOR]

In this paper I continue the investigation in Viaggiu (Class Quantum Gravity 35:215011, 2018), Viaggiu (Phys Scr 94:125014, 2019) concerning my proposal on the nature of the cosmological constant. In particular, I study both mathematically and physically the quantum Planckian context and I provide, in order to depict quantum fluctuations and in absence of a complete quantum gravity theory, a semiclassical solution where an effective inhomogeneous metric at Planckian scales or above is averaged. In such a framework, a generalization of the well known Buchert formalism (Buchert in Gen Relativ Gravit 33:1381, 2001) is obtained with the foliation in terms of the mean value s (t ^) of the time operator t ^ in a maximally localizing state { s } of a quantum spacetime (Doplicher et al. in Commun Math Phys 172:187, 1995; Doplicher in Space-time and fields: a quantum texture, in Karpacz, new developments in fundamental interaction theories, arXiv:hep-th/0105251, 2001; Bahns et al. in Advances in algebraic quantum field theory, Springer, Cham; Tomassini and Viaggiu in Class Quantum Gravity 28:075001, 2011) and in a cosmological context (Tomassini and Viaggiu in Class Quantum Gravity 31:185001, 2014). As a result, after introducing a decoherence length scale L D where quantum fluctuations are averaged on, a classical de Sitter universe emerges with a small cosmological constant depending on L D and frozen in a true vacuum state (lowest energy), provided that the kinematical backreaction is negligible at that scale L D . Finally, I analyse the case with a non-vanishing initial spatial curvature R showing that, for a reasonable large class of models, spatial curvature and kinematical backreation Q are suppressed by the dynamical evolution of the spacetime. [ABSTRACT FROM AUTHOR]