Tuesday, August 01, 2006

Why we don't remember the future, and other consequences of the 2nd law

Cosmic Variance is hurting my head again. Coincidentally, I've been listening to the IOT episode on the 2nd law of thermodynamics (excellent) and just the other day I tried to explain time's arrow to my 7 yo ...
Boltzmann’s Anthropic Brain | Cosmic Variance

... Suddenly, a thermodynamics problem became a puzzle for cosmology: why did the early universe have such a low entropy? Over and over, physicists have proposed one or another argument for why a low-entropy initial condition is somehow “natural” at early times. Of course, the definition of “early” is “low-entropy”! That is, given a change in entropy from one end of time to the other, we would always define the direction of lower entropy to be the past, and higher entropy to be the future. (Another fascinating but separate issue — 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 the lower-entropy direction.) ...
After this the essay gets much harder. Bayes Theorem makes an appearance, though it is not labelled. Crossing Bayes with the antrhopic principle yields yet more disturbing implications. Now if only CV would toss the Fermi Paradox into the mix ...

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