Curvature Blindness: Material Inference in Stars and Black Hole Candidates
Gravitational observations can leave material properties undetermined in two different ways: distinct states can have the same total source, and distinct spacetime histories can have the same retained observations. We organize these inverse problems in general relativity through a property-level identifiability criterion, with applications to stars and black hole candidates. Two ideal massless gases illustrate equal-source ambiguity: their temperatures differ by a factor of $(8/29)^{1/4}$ and their entropies differ while their stress-energy tensors agree. A separate causal-fluid example starts with identical sources and geometric data but different constitutive responses; the two models' curvature histories separate at linear order in proper time and their scale factors at cubic order. For black hole candidates, the relevant distinction is between stationary exterior parameters, perturbing fluxes and constituent history. The framework provides a test for whether retained data identify a material property, separating instantaneous source equality, history-wide source equality and observational projection.