Causal Asymmetry in a Quantum World (Phys. Rev. X 8, 0310)

The fundamental laws of physics work in the same way whether time moves forward or backward. Yet, while a glass can fall and scatter shards across the floor, glass shards never gather together and leap back onto the counter to form a complete glass. The source of this temporal asymmetry is one of the deepest mysteries in physics. We tackle this problem by combining two different disciplines, computational and quantum mechanics. Our results illustrate that the asymmetry could emerge from forcing classical causal explanations on observations in a fundamentally quantum world.

Computational mechanics asks the following question: Given a sequence of observations, how many past causes must we postulate to explain future behaviour? This quantity is asymmetric when the time is reversed. There is an unavoidable memory overhead cost for modelling a process in the “less-natural” temporal direction—one must pay a price to enforce explanations adhering to a less-favoured order of events.

We show that quantum models always mitigate this overhead. Not only can we construct quantum models that need less past information than optimal classical counterparts, these models can always be reprogrammed to model the time-reversed process without additional memory cost. This remains true even for observational data where this classical overhead diverges, such that all classical models for the less-natural temporal direction require unbounded memory.

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