According to quantum mechanics, space is anything but empty. It is full of particles constantly flitting in and out of existence thanks to quantum fluctuations in energy. In principle, these fluctuations could give rise to collections of particles such as a hydrogen atom, a microwave oven, or even a living, thinking brain. The catch? The Universe would have to be infinitely big and exist for an infinite period of time for this to happen. Dubbed ‘Boltzmann brains’ after the 19th-Century Austrian physicist Ludwig Boltzmann, the idea goes like this: though the chance of such an event occurring is minute, it isn’t zero. Therefore, it is theoretically possible that, in an infinitely big universe, over an infinite period of time, such a thing could occur. There could be Boltzmann brains on planets, they could be floating in space. They could be anywhere.
According to some calculations, it is more likely that there are Boltzmann brains existing in the Universe than beings that evolved gradually though natural selection. The trouble is nobody has seen one. For some, the theory is something of an embarrassment, and debate still rages among cosmologists of how exactly to rid physics of this so-called ‘Boltzmann brain problem’ once and for all.