A Boltzmann brain is a hypothesized self-aware entity which arises due to random fluctuations out of a state of chaos. The idea is named for physicist Ludwig Boltzmann, who had advanced an idea that the known universe arose as a random fluctuation, similar to process through which Boltzmann brains might arise.
Boltzmann brains are often referred to in the context of the “Boltzmann brain paradox” or “problem”.
The concept arises from the need to explain why we observe such a large degree of organization in the universe. The second law of thermodynamics states that the entropy in the universe will always increase. We may think of the most likely state of the universe as one of high entropy, closer to uniform and without order. So why is the observed entropy so low?
Boltzmann proposed that we and our observed low-entropy world are a random fluctuation in a higher-entropy universe. Even in a near-equilibrium state, there will be stochastic fluctuations in the level of entropy. The most common fluctuations will be relatively small, resulting in only small amounts of organization, while larger fluctuations and their resulting greater levels of organization will be comparatively more rare. Large fluctuations would be almost inconceivably rare, but this can be explained by the enormous size of the universe and by the idea that if we are the results of a fluctuation, there is a “selection bias”: We observe this very unlikely universe because the unlikely conditions are necessary for us to be here.