Boltzmann brain
Physicists use the Boltzmann brain thought experiment as a reductio ad absurdum argument for evaluating competing scientific theories.
Over a sufficiently long time, random fluctuations could cause particles to spontaneously form literally any structure of any degree of complexity, including a functioning human brain.
[1][2] The idea is named after the physicist Ludwig Boltzmann (1844–1906), who, published a theory in 1896 that tried to account for the fact that the universe is not as chaotic as the budding field of thermodynamics seemed to predict.
[5][6] When applied to more recent theories about the multiverse, Boltzmann brain arguments are part of the unsolved measure problem of cosmology.
[8][12][13] A Boltzmann brain (or body or world) need not fluctuate suddenly into existence, argue Anthony Aguirre, Sean M. Carroll, and Matthew C. Johnson.
However, smaller structures that minimally meet the criteria are vastly and exponentially more common than larger structures; a rough analogy is how the odds of a single real English word showing up when one shakes a box of Scrabble letters are greater than the odds that a whole English sentence or paragraph will form.
[16] The average timescale required for the formation of a Boltzmann brain is vastly greater than the current age of the universe.
[17] A typical nucleated Boltzmann brain will cool off to absolute zero and eventually completely decay, as any isolated object would in the vacuum of space.
[8] Theoretically a Boltzmann brain can also form, albeit again with a tiny probability, at any time during the matter-dominated early universe.
A New Scientist journalist summarizes that "The starting point for our understanding of the universe and its behavior is that humans, not disembodied brains, are typical observers".
[8][17] Carroll believes that a better understanding of the measurement problem in quantum mechanics would show that some vacuum states have no dynamical evolution and cannot support nucleated brains, nor any other type of observer.
[22] Some cosmologists believe that a better understanding of the degrees of freedom in the quantum vacuum of holographic string theory can solve the Boltzmann brain problem.
[16] One caveat is that if the universe is a false vacuum that locally decays into a Minkowski or a Big Crunch-bound anti-de Sitter space in less than 20 billion years, then infinite Boltzmann nucleation is avoided.
(If the average local false vacuum decay rate is over 20 billion years, Boltzmann brain nucleation is still infinite, as the universe increases in size faster than local vacuum collapses destroy the portions of the universe within the collapses' future light cones).
Proposed hypothetical mechanisms to destroy the universe within that timeframe range from superheavy gravitinos to a heavier-than-observed top quark triggering "death by Higgs".
[25][26][13] If no cosmological constant exists, and if the presently observed vacuum energy is from quintessence that will eventually completely dissipate, then infinite Boltzmann nucleation is also avoided.
