Experimental Rejection of Observer-Independence in the Quantum World

Bernardo mentioned this, have not looked deeply yet:

Experimental rejection of observer-independence in the quantum world

Proietti and co

The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics, the objectivity of observations is not so clear, most dramatically exposed in Eugene Wigner's eponymous thought experiment where two observers can experience fundamentally different realities. While observer-independence has long remained inaccessible to empirical investigation, recent no-go-theorems construct an extended Wigner's friend scenario with four entangled observers that allows us to put it to the test. In a state-of-the-art 6-photon experiment, we here realise this extended Wigner's friend scenario, experimentally violating the associated Bell-type inequality by 5 standard deviations. This result lends considerable strength to interpretations of quantum theory already set in an observer-dependent framework and demands for revision of those which are not.

Here's a bot summary:

A quantum experiment suggests there’s no such thing as objective reality

The breakthrough that Proietti and co have made is to carry this out. “In a state-of-the-art 6-photon experiment, we realize this extended Wigner’s friend scenario,” they say.

They use these six entangled photons to create two alternate realities—one representing Wigner and one representing Wigner’s friend. Wigner’s friend measures the polarization of a photon and stores the result. Wigner then performs an interference measurement to determine if the measurement and the photon are in a superposition.

The experiment produces an unambiguous result. It turns out that both realities can coexist even though they produce irreconcilable outcomes, just as Wigner predicted.  

That raises some fascinating questions that are forcing physicists to reconsider the nature of reality.

Although observer independence is accepted in QM, which I think is also somehow connected with observers different frames of reference in Relativity... I don''t think you can show there are two different realities in actual existence, if you haven't measured them to show they are both different realities... at which point, they are not two different realities, so it's not possible to observe such a situation.

Once you've prepared the state (a), you can't say anything much about what is going on until you measure it (c). There is just no practical information available about what is going on between (a) and (c).

Thanks - Yeah I am not sure what to think about it myself, beyond realizing I need to go back to my old textbooks relating to physics.

I also am not sure if "observer" here even needs to be conscious, IIRC it was Wheeler making a point to say he felt a machine might fulfill the role.

Some more commentary:

Quantum experiment suggests there really are ‘alternative facts’

The experiment could have immense implications for our understanding of the nature of quantum reality, which depends on how we interpret quantum theory. According to Fedrizzi and his colleagues, their work favours interpretations saying that the outcomes of experiments are subjective, such as quantum Bayesianism and Rovelli’s relational quantum mechanics.

In turn, it questions more mainstream views, such as the Copenhagen interpretation, which says that the properties of quantum systems don’t exist until observed, but then become objective facts, and the many worlds interpretation, which says that all possible measurement outcomes are real and objective, but each in a different world.

Renato Renner at the Swiss Federal Institute of Technology in Zurich, who last year published a thought experiment along similar lines, thinks photons might not count as observers. “The validity of the conclusions depends on whether one can reasonably claim that their experiment mimics ‘observers’,” he says.

But Rovelli is thrilled. “I do take it as a great piece of evidence directly supporting the relational interpretation. I agree in full with the way they interpret it,” he says. “It is fantastic that ‘ideal experiments’ of the past become real experiments of today.”

Since the QBism article is behind a paywall, more on QBism from the initiator of the idea Chris Fuchs

Also, Bernardo's Sci Am article connecting the Relational Interpretation to Idealism.

Here's that old video explaining Wigner's Friend:

Quantum experiment suggests there really are ‘alternative facts’

Can't comment on the merits of the work before reading more, but - ooh boy, is that an unfortunate choice by the headline writer.