Boltzmann brain
The Boltzmann brain thought experiment suggests that it might be more likely for a single brain to spontaneously form in a void, complete with a memory of having existed in our universe, rather than for the entire universe to come about in the manner cosmologists think it actually did. Physicists use the Boltzmann brain thought experiment as a reductio ad absurdum argument for evaluating competing scientific theories. In contrast to brain in a vat thought experiments which are about perception and thought, Boltzmann brains are used in cosmology to test our assumptions about thermodynamics and the development of the universe. 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. The scenario initially involved only a single brain with false memories, but physicist Sean Carroll pointed out that, in a fluctuating universe, the scenario works just as well with entire bodies, even entire galaxies.The idea is named after the physicist Ludwig Boltzmann (1844–1906), who, in 1896, published a theory that tried to account for the fact that the universe is not as chaotic as the budding field of thermodynamics seemed to predict.