Why directed percolation is rarely seen in experiments
In plain words
Theory says any spreading process with a single absorbing state and short-range interactions should follow directed percolation laws, yet clean experimental observations are few. What in real systems spoils the expected behavior is not fully understood.
Precise statement
Identify which conditions of the Janssen-Grassberger conjecture (a single absorbing state, short-range interactions, no extra conservation laws or quenched disorder) fail in candidate experiments (catalytic surface reactions, cyclically sheared suspensions, granular flows, transitional shear flows), and quantify how each deviation shifts the measured exponents. An answer is a predictive criterion for when DP exponents are observed.
What would settle it
A set of experiments in which controlled removal of each violating ingredient moves measured exponents onto DP values as predicted.
Status in the literature
DP exponents were measured in electroconvection of nematic liquid crystals (arXiv:0706.4151, 2007) and at the transition to turbulence in shear flows (arXiv:1504.03304, 2015); most other candidates deviate.
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