The New York Times that is.
In a reprise of John Tierney's NYT interview with Nick (we're in a simulation) Bostrum Dennis Overbye writes about a topic featured in some of my favorite physics blogs - Boltzmann's brain ...
Big Brain Theory: Have Cosmologists Lost Theirs? - New York Times
...cosmologists try to square the predictions of their cherished theories with their convictions that we and the universe are real. The basic problem is that across the eons of time, the standard theories suggest, the universe can recur over and over again in an endless cycle of big bangs, but it’s hard for nature to make a whole universe. It’s much easier to make fragments of one, like planets, yourself maybe in a spacesuit or even — in the most absurd and troubling example — a naked brain floating in space. Nature tends to do what is easiest, from the standpoint of energy and probability. And so these fragments — in particular the brains — would appear far more frequently than real full-fledged universes, or than us. Or they might be us.
CV fills in some more background
... The point about Boltzmann’s Brains is not that they are a fascinating prediction of an exciting new picture of the multiverse. On the contrary, the point is that they constitute a reductio ad absurdum that is meant to show the silliness of a certain kind of cosmology — one in which the low-entropy universe we see is a statistical fluctuation around an equilibrium state of maximal entropy...
In other words, the silliness of the naked flying space brain demonstrates that we don't understand entropy and the arrow of time.
CV uses the same Bayesian logic used by Bostrum to point out that if our current model of entropy were correct then it would be overwhelmingly likely that you are a Boltzmann's Brain and I don't exist...
... In the set of all such fluctuations, some brains would be embedded in universes like ours, but an enormously larger number would be all by themselves. This theory, therefore, predicts that a typical conscious observer is overwhelmingly likely to be such a brain...
CV concludes today's essay with a, to me, tantalizing comment ...
...So what are we to conclude? That our observed universe is not a statistical fluctuation around a thermal equilibrium state. That’s very important to know, but doesn’t pin down the truth. If the universe is eternal, and has a maximum value for its entropy, then we it would (almost always) be in thermal equilibrium. Therefore, either it’s not eternal, or there is no state of maximum entropy. I personally believe the latter, but there’s plenty of work to be done before we have any of this pinned down..
That last link is to an 207 CV article on how it all began.
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