Shalizi, Cosma Rohilla and Moore, Cristopher (2003) *What Is a Macrostate? Subjective Observations and Objective Dynamics.* [Preprint]

## Abstract

We consider the question of whether thermodynamic macrostates are objective consequences of dynamics, or subjective reflections of our ignorance of a physical system. We argue that they are both; more specifically, that the set of macrostates forms the unique maximal partition of phase space which 1) is consistent with our observations (a subjective fact about our ability to observe the system) and 2) obeys a Markov process (an objective fact about the system's dynamics). We review the ideas of computational mechanics, an information-theoretic method for finding optimal causal models of stochastic processes, and argue that macrostates coincide with the ``causal states'' of computational mechanics. Defining a set of macrostates thus consists of an inductive process where we start with a given set of observables, and then refine our partition of phase space until we reach a set of states which predict their own future, i.e. which are Markovian. Macrostates arrived at in this way are provably optimal statistical predictors of the future values of our observables.

Item Type: | Preprint |

Keywords: | Statistical mechanics, thermodynamics, macroscopic state, entropy, computational mechanics, causal state, memory effects, Markov processes, hydrodynamics, spin glasses, coarse graining, cellular automata, hysteresis, aging, nonequilibrium behavior, path dependence |

Subjects: | General Issues > Reductionism/Holism Specific Sciences > Physics > Statistical Mechanics/Thermodynamics |

Depositing User: | Cosma Rohilla Shalizi |

Date Deposited: | 03 Apr 2003 |

Last Modified: | 07 Oct 2010 11:11 |

Item ID: | 1119 |

URI: | http://philsci-archive.pitt.edu/id/eprint/1119 |
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