Carroll: Boltzmann's Anthropic Brain—Noted
Sean Carroll: Boltzmann's Anthropic Brain https://www.discovermagazine.com/the-sciences/boltzmanns-anthropic-brain: ‘The process of "remembering" involves establishing correlations that inevitably increase the entropy, so the direction of time that we remember [and therefore label "the past"] is always... lower-entropy.... The real puzzle is... why are conditions at one end of time so dramatically different from those at the other? If we do not assume temporal asymmetry a priori, it is impossible in principle to answer this question by suggesting why a certain initial condition is "natural"—without temporal asymmetry, the same condition would be equally natural at late times...
...On the one hand, Boltzmann's fluctuations of entropy around equilibrium allow for the existence of dynamical regions, where the entropy is (just by chance) in the midst of evolving to or from a low-entropy minimum. And we could certainly live in one of those regions.... oltzmann's goal is perfectly reasonable: to describe a history of the universe on ultra-large scales that is on the one hand perfectly natural and not finely-tuned, and on the other features patches that look just like what we see. But, having taken a bite of the apple, we have no choice but to swallow....
The most basic problem has been colorfully labeled "Boltzmann's Brain" by Albrecht and Sorbo. Remember that the low-entropy fluctuations we are talking about are incredibly rare, and the lower the entropy goes, the rarer they are.... If we are explaining our low-entropy universe by appealing to the anthropic criterion that it must be possible for intelligent life to exist, quite a strong prediction follows: we should find ourselves in the minimum possible entropy fluctuation consistent with life's existence. And that minimum fluctuation would be "Boltzmann's Brain."
Out of the background thermal equilibrium, a fluctuation randomly appears that collects some degrees of freedom into the form of a conscious brain, with just enough sensory apparatus to look around and say "Hey! I exist!", before dissolving back into the equilibrated ooze. You might object that such a fluctuation is very rare, and indeed it is. But... the momentary decrease in entropy required to produce such a brain is fantastically less than that required to make our whole universe....
This is the general thrust of argument with which many anthropic claims run into trouble. Our observed universe has something like a hundred billion galaxies with something like a hundred billion stars each. That's an extremely expansive and profligate universe, if its features are constrained solely by the demand that we exist.... Anthropic arguments would be more persuasive if our universe was minimally constructed to allow for our existence; e.g. if the vacuum energy were small enough to allow for a single galaxy to arise out of a really rare density fluctuation. Instead we have a hundred billion such galaxies, not to count all of those outside our Hubble radius....
But, returning to Boltzmann, it gets worse, in an interesting and profound way.... Assuming that we got to this macrostate via some fluctuation out of thermal equilibrium, what kind of trajectory is likely to have gotten us here?... If we ask "What kind of early universe tends to naturally evolve into what we see?", the answer is the ordinary smooth and low-entropy Big Bang. But here we are asking "What do most of the states that could possibly evolve into our current universe look like?", and the answer there is a chaotic high-entropy mess. Of course, nobody in their right minds believes that we really did pop out of a chaotic mess into a finely-tuned state with false memories about the Big Bang....
Price's conclusion from all this (pdf) is that we should take seriously the Gold universe, in which there is a low-entropy future collapsing state that mirrors our low-entropy Big Bang in the past. It's an uncomfortable answer, as nobody knows any reason why there should be low-entropy boundary conditions in both the past and the future, which would involve an absurd amount of fine-tuning of our particular microstate at every instant of time. (Not to mention that the universe shows no sign of wanting to recollapse)....
Explaining the difference in entropy between the past and future is at least as fundamental, if not more so, as explaining the horizon and flatness problems with which cosmologists are so enamored. If we're going to presume to talk sensibly and scientifically about the entire history of the universe, we have to take Boltzmann's legacy seriously…
.#noted #2020-07-06
