## Monday, March 10, 2008

### String Vacuum Project 2008

Keith Dienes, Gordon Kane, and Stuart Raby organize a kick-off meeting of the String Vacuum Project in Arizona, in April 2008. Click their names to see details.

They have their own explanations why it is important and I fully agree with them, except for the implicit assumption of the "statistics" sub-project that one should adopt the "democratic" measure on the space of vacua. But let me say one more thing:

String phenomenology has become a rather extensive field and many experts may have become over-specialized. I think that some people are focused on heterotic vacua and others are focused on type IIB flux vacua or type IIA braneworlds.

I think that a kind of "thermal" exchange between the groups is desirable not only to share their insights but also to evaluate the relative merit of the different corners of the landscape and to find and appreciate new dualities and relationships if they exist.

This project to classify the compactifications could be, in some sense, analogous to the mathematicians' project to classify all finite groups. It took several decades but it was eventually completed. An "atlas" of string compactifications could require a similar or greater effort.

Defending the inevitability of the anthropic reasoning

Bert Schellekens has written down a web page that defends the anthropic reasoning as a necessary feature of a realistic theory. Sorry, the page will be displayed as an XML source in Internet Explorer: try Firefox here.

The picture that summarizes his opinions is the following:

In words, if you have a theory of any kind that would lead to a unique prediction of physical phenomena, it is very likely that its prediction for low-energy parameters would be incompatible with complexity, life, and intelligence. Once you use this prediction as evidence in Bayesian inference, the probability that an explanation based on a unique vacuum is correct plummets.

The statement in the previous paragraph might very well be true and I can imagine that the proponents of the anthropic principle will be proven correct in the future when they say that it is pure wishful thinking for someone to argue that the "intelligent" Universe around us should be described by a unique solution to some underlying equations. Intelligence is rare and apparently independent of the predictions of a unique theory so it is unlikely that they overlap.

Schellekens also presents a far-fetched loophole that sounds as a science fiction: the equations determining a unique solution are actually secretly equivalent to some equations that try to maximize the amount of intelligent life in the Universe, so there is nothing unexpected if the unique vacuum agrees with one of the rare intelligent regions. ;-)

This is actually a very intriguing idea that I have been trying to make more quantitative many times. How do you measure that our Universe is intelligent? For example, are universes with a high Kolmogorov complexity those that you want to get?

Not really. The Kolmogorov complexity is the size of an exe file that is capable to produce a given pattern, information, or a sequence of bits. In other words, the Kolmogorov complexity is the amount of information after a maximum compression.

Is our Universe exceptionally rich or exceptionally poor in Kolmogorov complexity? I think it is neither. Much of the useful information in our Universe is redundant - for example, books are printed in many copies - but it is not true that everything in our Universe including its history (that depended on random outcomes of quantum events) can be generated from a tiny amount of "bits" to start with.

But it is very tempting to try to find a quantity that could express how much the Universe is able to print and refine books and reproduce, mutate, and improve animals or corporations. If this quantity had a simple enough definition, it could actually be equivalent to a refined version of a Hartle-Hawking prior probability distribution on the landscape. ;-)

This sounds as a real miracle but I tend to agree that if someone believes that the intelligent life only occurs in a small portion of the low-energy parameter space, a belief like the miracle above (or a belief that the laws of mathematics are simply lucky and put a unique solution to an intelligent region) is kind of necessary to avoid the anthropic reasoning completely. Do I believe it? I don't know and it is a good idea to be open-minded about open questions. But it is probably not the possibility I would bet the ranch on. I find it more likely that the intelligent life is simply not as rare as the anthropic people would like to believe.