Boltzmann Brains actually have a use?

[00:00:00]Boltzmann brains aren't just cosmic horror stories. They actually have a use. Named after the father of statistical mechanics, Ludwig Boltzmann, a Boltzmann brain is a fully conscious human brain formed spontaneously in the vacuum of space and complete with fake memories of a normal life on Earth. To a Boltzmann brain, one moment it would be cooking lasagna, and the next it would be freezing to death in the dark, empty void of outer space. Fortunately, the probability of a Boltzmann brain forming is incredibly low. You'd have to wait around 10 to the 10 to the 40 years, a time so long there's no meaningful comparison.

[00:00:39]So, why do we bother talking about them?

[00:00:41]Well, the probability of a Boltzmann brain forming may be low, but it's not zero. And in certain models of cosmology, it may be more likely for a Boltzmann brain to form than regular humans like us. To explain why our universe looked much more ordered than expected, Boltzmann hypothesized that it formed from an unfathomably rare statistical fluctuation that put everything into an extremely low entropy state. Like a shattered wine glass spontaneously repairing itself, but way more impressive. But critics pointed out that it's far more likely to have a small fluctuation than a large one. The math tells you it's way more probable that your brain, with your exact but fake memories, fluctuates into existence rather than a whole orderly universe.

[00:01:30]So, on Boltzmann's account, we should believe we're isolated brains with fake memories rather than normal humans living in a low entropy universe. This is obviously ridiculous, and so Boltzmann's cosmology can't be correct.

[00:01:44]And this is still the use of Boltzmann brains today. In the field of inflationary cosmology, inflation causes space-time to expand at an exponential rate, forming a a of bubble universes separated by the rapidly expanding space in between. In certain models, despite their rarity, Boltzmann brains can end up outnumbering normal observers like ourselves in stable orbits around nice energy-giving suns.

[00:02:11]If they do, this is a clear indication that the model must be wrong. Otherwise, we should expect to have been born as Boltzmann brains with fake memories. But then again, all this assumes that we are in fact normal observers.

[00:02:26]And who knows if that's really the case.