The empirical record supplies several physical asymmetries compatible with the structural claim, though it does not by itself establish a metaphysically fundamental direction of passage. The second law of thermodynamics (formalized by Clausius -(Rudolf Clausius, 1865) and given its statistical foundation by Boltzmann -(Ludwig Boltzmann, 1877)) establishes that the entropy of an isolated system (entropy: the count, in logarithmic form, of microscopic configurations compatible with a system's macroscopic description, equivalent on a statistical reading to the system's degree of disorder) tends toward its maximum, and spontaneous entropy reversal is never observed at scales relevant to physical processes. The entropic gradient is central to ordinary measurement, memory, computation, and life. Cosmological observation adds evidence of large-scale expansion: Hubble's -(Edwin Hubble, 1929) recession data and the accelerating-expansion measurements of Riess et al. -(Adam G. Riess et al., 1998) and Perlmutter et al. -(Saul Perlmutter et al., 1999) describe an important time-asymmetric feature of our observed cosmic history. Expansion alone, however, does not establish a fundamental temporal direction or a universal cosmological arrow. Quantum mechanics supplies a third relevant asymmetry, though one whose source needs careful naming. Unitary Schrödinger evolution (the equation governing the deterministic, information-preserving evolution of a quantum state in the absence of measurement) is formally reversible; the irreversibility associated with measurement enters through decoherence, the effectively irreversible loss of phase coherence (the loss of the precise quantum-mechanical relationships between possible outcomes that distinguish quantum from classical statistics) into uncontrolled environmental degrees of freedom (Zurek, W. H., 2003); (Erich Joos, H. Dieter Zeh, Claus Kiefer, Domenico Giulini, Joachim Kupsch, and Ion-Olimpiu Stamatescu, 2003), which is itself a thermodynamic process. The quantum arrow does not enter as a separate principle. It enters at the same interface the classical arrow enters: wherever a system becomes coupled to a much larger one. These phenomena do not supply a single empirical proof that succession is metaphysically directed. They exhibit physical asymmetries that the structural account must accommodate and that are consistent with its claim. Whether the cosmological trajectory asymptotically approaches a maximum-entropy state or recurs at the largest scale is the further question [§7.6](<7.6 Singularity.md>) takes up.


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← [7.2.3 What is Predicted](<7.2.3 What is Predicted.md>) | [7.2.5 What is Concluded](<7.2.5 What is Concluded.md>) →
