Arrow of time

The apparent asymmetry between past and future, marked by irreversible processes and, in thermodynamics, increasing entropy.

Second law of thermodynamics: The principle that the total entropy of an isolated system does not decrease over time. It gives the thermodynamic arrow its direction: toward greater entropy.
Entropy: A thermodynamic state quantity associated with energy dispersal and the number of microscopic configurations compatible with a macroscopic state. Entropy supplies the quantity whose typical increase marks the thermodynamic arrow.
Cosmological arrow of time: The observed direction of cosmic evolution, from the early universe toward its present expansion. Cosmic history supplies the low-entropy boundary condition often invoked to explain the thermodynamic arrow.
Ludwig Boltzmann: An Austrian physicist who developed the statistical interpretation of thermodynamics and the kinetic theory of gases. His work connected entropy increase with the statistical behavior of microscopic systems.
Statistical mechanics: The framework that explains macroscopic physical behavior through the statistical properties of microscopic states. It explains how overwhelmingly probable entropy growth can produce macroscopic irreversibility.
Time-reversal symmetry: A symmetry under reversing the direction of time in a physical law or process. Many fundamental laws have this symmetry even though everyday processes do not.
Radiation arrow of time: The observed asymmetry in which electromagnetic radiation typically spreads outward from sources rather than converging on them. Its familiar direction appears connected to the universe's special past conditions.
Arthur Eddington: A British astronomer and physicist who popularized the phrase “the arrow of time” in the twentieth century. He gave the thermodynamic direction its influential name and linked it to cosmology.
Boltzmann entropy: A measure of a macrostate's multiplicity, proportional to the logarithm of the number of microscopic states compatible with it. Its connection between multiplicity and entropy underlies the statistical account of the arrow.
Microscopic reversibility: The property that microscopic dynamics permit a time-reversed trajectory when velocities and other time-odd variables are reversed. It makes the emergence of macroscopic irreversibility a central explanatory problem.

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