  Does quantum theory apply at all scales, including that of observers? New
light on this fundamental question has recently been shed through a resurgence
of interest in the long-standing Wigner's friend paradox. This is a thought
experiment addressing the quantum measurement problem -- the difficulty of
reconciling the (unitary, deterministic) evolution of isolated systems and the
(non-unitary, probabilistic) state update after a measurement. Here, by
building on a scenario with two separated but entangled friends introduced by
Brukner, we prove that if quantum evolution is controllable on the scale of an
observer, then one of 'No-Superdeterminism', 'Locality' or 'Absoluteness of
Observed Events' -- that every observed event exists absolutely, not relatively
-- must be false. We show that although the violation of Bell-type inequalities
in such scenarios is not in general sufficient to demonstrate the contradiction
between those three assumptions, new inequalities can be derived in a
theory-independent manner, that are violated by quantum correlations. This is
demonstrated in a proof-of-principle experiment where a photon's path is deemed
an observer. We discuss how this new theorem places strictly stronger
constraints on physical reality than Bell's theorem.
