RT Journal Article SR Electronic T1 Experimental test of local observer independence JF Science Advances JO Sci Adv FD American Association for the Advancement of Science SP eaaw9832 DO 10.1126/sciadv.aaw9832 VO 5 IS 9 A1 Proietti, Massimiliano A1 Pickston, Alexander A1 Graffitti, Francesco A1 Barrow, Peter A1 Kundys, Dmytro A1 Branciard, Cyril A1 Ringbauer, Martin A1 Fedrizzi, Alessandro YR 2019 UL http://advances.sciencemag.org/content/5/9/eaaw9832.abstract AB 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 markedly exposed in Wigner’s eponymous thought experiment where two observers can experience seemingly different realities. The question whether the observers’ narratives can be reconciled has only recently been made accessible to empirical investigation, through recent no-go theorems that construct an extended Wigner’s friend scenario with four observers. In a state-of-the-art six-photon experiment, we realize this extended Wigner’s friend scenario, experimentally violating the associated Bell-type inequality by five standard deviations. If one holds fast to the assumptions of locality and free choice, this result implies that quantum theory should be interpreted in an observer-dependent way.