  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 seemingly different realities. The question whether
these realities can be reconciled in an observer-independent way has long
remained inaccessible to empirical investigation, until recent no-go-theorems
constructed an extended Wigner's friend scenario with four observers that
allows us to put it to the test. In a state-of-the-art 6-photon experiment, we
realise this extended Wigner's friend scenario, experimentally violating the
associated Bell-type inequality by 5 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.
