In your own world

There's a MIT Technology Review article about this quantum experiment, but it sometimes throws up a paywall now, which is annoying.

So I'll go with this article instead:

A Wild New Quantum Physics Experiment Suggests That Objective Reality May Not Exist After All

It's all to do with the Wigner's friend thought experiment (now turned into an actual experiment). 

The actual arXiv paper is available at this link.  The abstract:

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.

Actually, it's worth downloading the paper and reading the discussion at the end.

Modulo the potential loopholes and accepting the pho-tons’ status as observers, the violation of inequality (2)implies that at least one of the three assumptions of freechoice, locality, and observer-independent facts must fail.Since abandoning free choice and locality might not re-solve the contradiction [5], one way to accommodate ourresult is by proclaiming that “facts of the world” canonly be established by a privileged observer—e.g., onethat would have access to the “global wavefunction” inthe many worlds interpretation [17] or Bohmian mechan-ics [18]. Another option is to give up observer indepen-dence completely by considering facts only relative toobservers [19], or by adopting an interpretation such asQBism, where quantum mechanics is just a a tool thatcaptures an agent’s subjective prediction of future mea-surement outcomes [20]. This choice, however, requiresus to embrace the possibility that different observers ir-reconcilably disagree about what happened in an exper-iment.