Mods: I am not sure if this is a Physics question or more appropriate for Cosmology. I read a short discussion (on another forum) about the Boltzmann Brain paradox. I did a little further reading on the web but most explanations were a bit too deep (read: over my head). I wonder if someone could offer a reasonably simple layman's explanation? My understanding from what I have read is that a universe should tend to exist in a state of high entropy, however the universe we observe has a relatively low entropy and arose from a time of maximal low entropy. This could occur as a statistical fluctuation - that is that over time, an eternal universe would most of the time be in a state of high entropy but fluctuations within that state could give rise to local regions of low entropy. However, the most likely of such fluctuations would overwhelmingly be smaller than the observed universe - that is, it is more likely that we would see a single galaxy, a solar system or even more likely a single brain. The ordered universe we do observe is exceedingly unlikely. If that's the idea, I don't quite see why it's a paradox? Surely any possible state must occur at some time no matter how rare. Why is it strange (paradoxical) to be in that particular state? I also read about the anthropic principle, and although I read about the strong and weak forms of this and the various takes on that, I generally understood it to say that "the universe is in the ordered state that we observe because we exist as observers to see this order". Or put another way, although it is unlikely to be in this state, the fact that we are here to observe is evidence that it has occurred and is real. The idea of the Boltzmann Brain Paradox appears to say "the universe in the ordered state we observe is highly unlikely, and thus is most likely not real". All of which boils down to "the universe appears of a form that is most unlikely, but that in itself is evidence that is real. Or not real." Clearly I have misunderstood one or the other (or both) of these ideas. Can anyone (simply) set me straight?