Recently fresh life has been breathed into the old trope of Wigner’s Friend by generalizing it to settings which emphasize multiple agents and the quantum entanglement that arises between them. This allows further considerations to be added—such as Bell inequality violations, GHZ states, or the Hardy paradox—to help sharpen the analysis. Much of this work was ushered in by an influential paper of Frauchiger and Renner first posted two years ago, a modification of which appeared in Nature Communications only recently (making a splash in the media and the blogosphere). As the authors put what they set out to show: “[W]e 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.”
Well, the issue of whether they actually do show this is far from settled, and there has been a spate of discussion around the subject since. The main thing that has become clear is that this is a fruitful setting for exploring what various interpretations of quantum theory necessarily entail. For instance, one might argue that quantum interpretations along the lines of QBism and Rovelli’s relationalism have more ground to be taken seriously than previously thought. In any case, there is much that remains to be explored.
The workshop will take place in the basement of the UMass Center on Beacon Hill located at One Beacon St., Boston, MA 02108. See map below for directions.