Agency under indefinite causality: operational eternalism in higher-order quantum theory

Abstract: After two decades of research on indefinite causality, a philosophical lesson emerges: the tension between operational quantum theory and dynamical spacetime physics is unbridgeable if one believes both types of theories to be fundamental. We interpret this tension through operational eternalism, a stance analogous to the block-universe view but applied to information rather than geometry. Inputs and outputs are primary givens, while agents are secondary constructs arising from specific groupings of data. Agency is perspectival: from Alice's perspective Bob may not qualify as an observer, and vice versa. These results redefine the observer in the operational approach as a tool to avoid non-causality. They also provide a criterion for Wigner's friends as a class of causally compatible agents.