PT - JOURNAL ARTICLE AU - Proietti, Massimiliano AU - Pickston, Alexander AU - Graffitti, Francesco AU - Barrow, Peter AU - Kundys, Dmytro AU - Branciard, Cyril AU - Ringbauer, Martin AU - Fedrizzi, Alessandro TI - Experimental test of local observer independence AID - 10.1126/sciadv.aaw9832 DP - 2019 Sep 01 TA - Science Advances PG - eaaw9832 VI - 5 IP - 9 4099 - http://advances.sciencemag.org/content/5/9/eaaw9832.short 4100 - http://advances.sciencemag.org/content/5/9/eaaw9832.full SO - Sci Adv2019 Sep 01; 5 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.