This work presents a general framework to evaluate the observational accessibility of Planck-suppressed propagation effects using time-of-flight measurements of astrophysical messengers. Instead of proposing a fundamental theory of quantum gravity, we adopt a low-energy effective field theory (EFT) approach and focus on how such effects could be tested in practice. We introduce a generalized figure of merit defined by the combination of particle energy and propagation distance. This quantity captures the essential requirement for observable propagation-induced time delays: both high energy and large distance are needed simultaneously. For the quadratic case, we show that probing effects at the level of order-unity coefficients requires extremely large values of this figure of merit, together with sufficiently clean temporal structure from the source. By mapping the observability phase space, we identify distinct regimes defined by instrumental resolution, intrinsic source variability, and astrophysical limitations. Using GRB221009A, we derive a current constraint on the effective coefficient of the model. The central result of this work is that the dominant limitation in testing Planck-scale propagation effects is neither theoretical consistency nor detector sensitivity, but the scarcity of astrophysical sources capable of reaching the required regime with clean timing. This leads to a key conclusion: the real bottleneck in quantum gravity phenomenology is observational. Specifically, the lack of sources that simultaneously achieve extremely high energy, large propagation distance, and well-resolved temporal structure prevents access to the relevant regime. This conclusion is both honest and falsifiable. The discovery of suitable high-energy, high-redshift sources with clean timing would immediately open this observational window. Conversely, their continued absence strengthens the interpretation that the limitation is astrophysical rather than theoretical.
Eduardo Parra (2026) studied this question.