Boltzmann brain

A hypothetical conscious brain arising briefly from a rare fluctuation in a disordered universe. It illustrates how statistical mechanics can make observers with misleading memories seem possible.

Boltzmann fluctuation: A rare temporary decrease in entropy produced by random microscopic motion in a system near equilibrium. A brain-like fluctuation is the specific low-entropy event behind the hypothesis.
Ludwig Boltzmann: An Austrian physicist who developed statistical mechanics and gave entropy a statistical interpretation. His account of entropy made rare low-entropy fluctuations a cosmological possibility.
Ordinary observer: An observer whose existence results from a long physical history, such as biological evolution on a planet. The hypothesis contrasts fluctuation-born observers with observers embedded in ordinary cosmic history.
Cognitive instability: A problem arising when a theory predicts that most observers have perceptions or memories that do not track reality. A model dominated by Boltzmann brains undermines confidence in the observations supporting that model.
Thermal equilibrium: A state in which a system's macroscopic properties no longer change and no net heat flows. The hypothesis considers observers arising in a universe that has reached equilibrium.
Boltzmann's brain paradox: A cosmological problem in which fluctuation-born observers outnumber ordinary observers in some models. The paradox turns the hypothetical brain into a test of cosmological theories.
Cosmological measure problem: The problem of defining probabilities for events in cosmologies containing infinitely many regions or events. Different probability prescriptions can change whether Boltzmann brains dominate predicted observers.
Bayesian confirmation theory: An account of evidence in which beliefs are updated according to how strongly observations favor competing hypotheses. Unreliable fluctuation memories complicate the use of observations to confirm cosmological theories.
Entropy: A physical quantity measuring the number of microscopic configurations compatible with a system's macroscopic state. The improbability of a brain fluctuation is expressed through its unusually low entropy.
Arthur Eddington: A British astronomer and physicist who emphasized the observed direction of time and the low entropy of the early universe. His discussion of the thermodynamic arrow helped frame the cosmological puzzle.

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