The Sciences

Cosmology and Philosophy at La Pietra

Cosmic VarianceBy Sean CarrollJul 26, 2012 9:32 AM


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I've traded off my reasons for not blogging much of late. Last week and before it was The Particle at the End of the Universe (in stores November 13!), but that's now been handed in and I can kick back and catch up on my martini-drinking. Except that instead of doing that, I instantly hopped on a plane for Europe, where I'm now participating in a workshop on philosophy and cosmology. Not that you should feel sorry for me -- the workshop is being held at the La Pietra conference center, a beautiful facility owned by NYU in Florence. I'm not sure why NYU owns a conference center in Florence; it could have been a targeted purchase, but it could easily have just been a gift. (Caltech for a while owned an abandoned gold mine. Universities get all sorts of crazy gifts.) But at least temporarily, martinis have been put aside for Chianti and limoncello.

And work, of course. This is my favorite kind of workshop: less than twenty people, gathered around a table, with no fixed agenda, talking about issues of mutual interest as they come up. This group has both scientists and philosophers, although probably more of the latter. So far each day has featured a scientist -- Joel Primack, me, Brian Greene, Scott Aaronson -- giving some very general remarks, while everyone else takes turns whacking them with (metaphorical) sticks. My own talk started at 11 a.m. and didn't finish until 5:30 p.m., with breaks for lunch and coffee. So it's exhausting both intellectually and physically, but very rewarding to have the chance to dig very deeply into difficult issues. My talk was about -- you guessed it -- the arrow of time. Most people in the room are already familiar with the basic story that time's arrow is (at least mostly) a consequence of the increase of entropy over time, and that our current universe has low entropy, but the entropy was even much lower in the past, and that last fact demands cosmological explanation. The central question concerned what would count as an "explanation." One idea might be that whenever you have a simple true statement that has enormously powerful consequence for many different facts about the universe, that statement would qualify as a "law of nature." Often we think of such laws as patterns that apply individually to each subset of the universe, but one could argue that the statement "the early universe was in a low-entropy state of the kind that would naturally evolve into the universe we see" should also count as a law. If you buy into that, then one shouldn't necessarily look for a deeper explanation of that fact; maybe that's just how the universe is. I think that's an allowed position, but certainly not the most attractive from the point of view of most cosmologists. We tend to look at the low entropy of the early universe as a clue in our search for a better theory than the one we currently have. From that perspective, what would count as having explained the low-entropy state? My argument was the following: imagine you have a certain theory of everything, consisting of a space of states (Hilbert space or whatever) and a dynamical evolution law. And imagine that, for almost all solutions to the equations of motion, some certain fact X about the system turn out to be associated with another fact Y almost all the time. (Fact X = our entropy is low, fact Y = it was even lower in the past.) Then, if we do indeed observe that fact X and fact Y are both true, then fact Y is "explained" by this theory. You might still want to explain fact X, but that's a harder question that I didn't get into. It's enough of a challenge to explain the lower-entropy past even if you accept as given the current low-entropy state. (And of course we're imagining that we have well-defined theories with this feature, which at the moment we really don't, although there are some rough ideas.) I won't claim that I convinced anyone who didn't already agree, but I do think that there is more mutual understanding now than when we began. And the Chianti has been quite good.

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