Lee Smolin gives a very nice over-view of his theory of "cosmological natural selection", and tears into the anthropic principle, in a recent pre-print:
Quite remarkably, it contains no math, and fairly little jargon; I'd imagine it'd be quite accessible to anyone who'd normally read a popular science book on astronomy. While it makes me want to tackle Smolin's more technical works, this is probably not going to happen, since even pedagogical papers on loop quantum gravity remind me of why I left fundamental theory for statistical physics in the first place --- I'm just not smart enough for the former. (Update, 27 August 2004: I should mention an even more recent Smolin pre-print, "An Invitation to Loop Quantum Gravity", hep-th/0408048.) Having said that, I do have three comments.
First, I think Smolin's absolutely right about the anthropic principle. Take, for instance, the example he gives in section 5.1.3. Fred Hoyle once reasoned that carbon is necessary for life, that carbon must have been formed by stellar nucleosynthesis, and that this reaction could only have proceeded if carbon nuclei had certain properties, which experimentalists then proceeded to show they did have. Smolin fairly schematizes this as follows. (1) X is necessary for life (or intelligence, etc.). (2) X is, as it happens, true. (3) If X is true, and the laws of physics are Y, then Z must also be true. (4) Therefore Z.
We see clearly that the prediction of Z in no way depends on step 1. The argument has the same force if step 1 is removed. To see this ask what we would do were Z found not to be true. Our only option would be to question either Y or the deduction from the presently known laws of physics to Z. We might conclude that the deduction was wrong, for example if we made a mistake in a calculation. If no such option worked, we might have to conclude that the laws of physics might have to be modified. But we would never question 1, because, while a true fact, it plays no role in the logic of the argument leading to the prediction for Z.Exactly so.
Second, which is a quibble, I think the idea of falsifiability, while still very valuable and basically right-headed, is somewhat more complicated than Smolin, following Popper, allows. This is especially true when statistical hypotheses are at issue. On this point, I recommend the work of Deborah Mayo, especially Error and the Growth of Experimental Knowledge. I don't think this upsets the points Smolin is making, however.
Thirdly, one of Smolin's statements, while correct, needs to be read with caution. Smolin imagines that there is a set of fundamental, dimensionless constants of physics, p, living in some space P, and that universes give birth to new universes, with a child's value of p differing slightly from that of its parent. (He has a particular mechanism in mind, involving quantum-gravitational effects smearing out the singularities of classical black holes.) For a given value of p, there will be a certain average number of descendants per universe, F(p). He then states his selection principle as follows (section 5.2):
If p is changed from the present value in any direction in P the first significant changes in F(p) encountered must be to decrease F(p).Notice that this does not (as I first thought it did) commit Smolin to the belief that the fitness function has only isolated local maxima. This principle is entirely compatible with having a continuous extended set in P where F(p) has a constant value, and that value is a local maximum. This is what evolutionary biologists would call a "neutral network", where by changing several parameter simultaneously we can keep the fitness constant. (Such neutral networks play an important role in evolutionary dynamics, or so my friends tell me.) In fact, Smolin's selection principle is very carefully worded to allow us to be on a neutral network, provided that's a local maximum.
(Via Milan Cirkovic, in e-mail.)
Posted by crshalizi at August 21, 2004 11:26 | permanent link