Entropy as an arrow of time

The thermodynamic arrow of time is the observed direction in which entropy tends to increase, distinguishing records of the past from expectations about the future.

Second law of thermodynamics: The law that the total entropy of an isolated system does not decrease over time. It states the macroscopic direction that defines the thermodynamic arrow.
Thermodynamic entropy: A state function that quantifies energy dispersal and the number of microscopic configurations consistent with a system's macroscopic state. Its physical meaning underlies the claim that time has a thermodynamic direction.
Radiation arrow of time: The observed asymmetry in which electromagnetic radiation spreads outward from sources rather than converging on them. It is often explained through boundary conditions related to the thermodynamic arrow.
Weyl curvature hypothesis: Roger Penrose's proposal linking the universe's low initial gravitational entropy to vanishing Weyl curvature. It offers a gravitational account of the exceptionally low-entropy early universe.
Statistical mechanics: The framework that derives macroscopic behavior from the statistical properties of microscopic states. It explains entropy increase through the relative abundance of higher-entropy states.
Time-reversal symmetry: A symmetry in which physical laws remain unchanged when the direction of time is reversed. Its survival in microscopic laws makes the macroscopic arrow a puzzle.
Psychological arrow of time: The experienced direction of time in which people remember the past and anticipate the future. Memory formation consumes energy and is associated with entropy production.
Gravitational entropy: The entropy associated with gravitational degrees of freedom, including the structure of spacetime and matter distribution. Its early-universe behavior is central to explaining why the thermodynamic arrow began pointing forward.
Boltzmann entropy: An entropy measure proportional to the logarithm of the number of microscopic states compatible with a macroscopic state. Its multiplicity interpretation explains why systems typically evolve toward higher entropy.
Past hypothesis: The proposal that the universe began in an exceptionally low-entropy state. A low-entropy boundary condition supplies the direction that the second law alone cannot select.

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