A Boltzmann Brain is a hypothetical minimally self-aware entity that pops into existence as a result of vacuum energy fluctuations. Vacuum energy fluctuations are those peaks and troughs in vacuum energy that produce virtual particles, little bits and bobs of matter and energy that literally appear from nothing and vanish just as quickly as a result of the fizzing of the quantum foam that makes up the substrate of the observed universe. In an infinite universe, it’s possible that complex structures could pop into existence in the same way, such as worlds and galaxies, perhaps even living, intelligent beings. These beings are the Boltzmann Brains.
Boltzmann Brains (named after Ludwig Boltzmann) were thought up to help explain why the universe that we observe around us is so organized, when the Second Law of Thermodynamics suggests that the level of entropy should be much higher. Boltzmann’s solution was to suggest that we’re just the product of a random fluctuation in a high entropy universe, and that the order we see around us is just a reflection of the temporary low-probability local conditions that spontaneously created us in the first place.
The brain-twisting bit comes when the probability of our world, containing billions of self-aware entities, is compared with the probability of just one self-aware mind popping out of the ether. Given that it is statistically extremely unlikely that we just appeared out of nothing, the probability of the existence of lone Boltzmann Brains is much higher. In a universe like the one we observe around us, there should be large numbers of these spontaneous intelligences floating around.
One rather disturbing possibility of the Boltzmann Brains hypothesis is that there’s nothing to stop such spontaneous creations from coming into being with a full set of false memories and an imagined history. Though it’s very unlikely, we could all be Boltzmann Brains, having fizzed into existence just a femto-second ago, and we’d never know it.
There are some other, really odd implications, one being that, if we’re not Boltzmann Brains, then in order for us to remain “typical observers” of the universe, the universe needs to self-destruct in about 20 billion years (probably through another vacuum energy fluctuation birthing a baby universe that then inflates and overwrites this one). If it doesn’t, the continued spontaneous creation of Boltzmann Brains means that they will eventually outnumber us, making us atypical observers. Models suggest that Boltzmann Brains would observe the universe in a radically different way to us, which seems okay until you think about the fact that there’s considerable evidence that our observations really do create reality. If Boltzmann Brains eventually usurp the position of “typical observer” from us, we might find reality being re-written around us.
I can feel the plates in my skull re-aligning. Gah.