= Information velocity
An <information velocity> must specify what counts as a newly distinguishable message and how it is detected. In a causal model, receivers cannot distinguish two otherwise identical preparations before their difference can arrive. <Front velocity> therefore bounds the arrival of genuinely new changes when the model has a finite <domain of dependence>. <Group velocity> can estimate the delay of a weakly distorted narrow <wave packet>, but it is not a universal speed of information: <damping>, amplification and reshaping can move the peak without advancing the first possible response. A Gaussian peak shift alone does not test <relativistic causality>.<Wave-Front Propagation by Richard Fitzpatrick>[equations 871–873 and discussion after equation 887: earliest response versus phase propagation]{ref}
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