The View from Within: What Can Embedded Observers (Not) Learn?

Abstract: Physics is usually done from a third-person perspective, as if the world were described from outside. Yet, observers are themselves physical systems within the world they observe. Here, we investigate this tension using a toy model of classical particles. Observers are physical systems characterised by a choice of (i) a manifest variable, whose value constitutes their empirical record, and (ii) ready states, which provide initial information. We ask what such observers (subjects) can learn about the world through interactions that establish a correlation with another system (the object). For each combination of the three types of learning (about the past, the future, or both), manifest variables, and ready states, we determine what the subject can learn. This shows that many subjects face an epistemic horizon---a limitation to what they can learn about the world---even though the model is classical and deterministic. For example, a subject with a complete manifest variable---one whose ontic state is the empirical record---and partial initial information can learn at most half the object's variables, recovering Spekkens' knowledge-balance principle, i.e. an analogue of Heisenberg's uncertainty principle. More generally, we find that subjects face more severe epistemic horizons when predicting than when retrodicting, and that learning by repeatable measurements can be more limited than either prediction or retrodiction alone. Our work provides language and tools to study how the first-person perspective can differ from the third-person perspective beyond the toy model studied here, and invites explanations of the inherent uncertainty in quantum theory from the standpoint of embedded observers.