Science Puzzle
Boltzmann Brains
According to statistical mechanics, in a universe that has existed for an infinite or sufficiently long time, random thermal fluctuations could spontaneously assemble any configuration of matter, including a fully-formed, conscious brain complete with memories of a life it never lived.
Such a spontaneously appearing observer is called a Boltzmann Brain. If Boltzmann Brains are possible, they would vastly outnumber observers who evolved through billions of years of cosmic and biological history. So why do we observe a universe that looks like it evolved, rather than just being a momentary fluctuation?
The Answer
This is the Boltzmann Brain problem, and it is a genuine problem in modern cosmology with no fully accepted solution. It arises from trying to reconcile statistical mechanics with an eternal or very long-lived universe.
Several responses have been proposed. If the universe is not eternal but began at the Big Bang, then evolved observers may vastly outnumber Boltzmann Brains simply because there is not enough time for random fluctuations to produce them in large numbers. The rarity of the Big Bang state is itself the puzzle, however.
Other physicists argue that Boltzmann Brain production can be suppressed by the structure of vacuum fluctuations in certain cosmological models. Some invoke the anthropic principle: a Boltzmann Brain would have inconsistent memories and hallucinations, while we observe a remarkably consistent universe, which is evidence we are not fluctuations.
The real value of the Boltzmann Brain argument is that it reveals a tension in cosmological thinking between the laws of probability, the arrow of time, and the conditions at the beginning of the universe.
The principle: Statistical mechanics and the Boltzmann Brain paradox. In a sufficiently old universe, random fluctuations should produce observers far more often than evolution. Why we observe an evolved universe is an open cosmological question.