The Sciences

Duff on Susskind

Cosmic VarianceBy cjohnsonDec 6, 2005 12:45 AM


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Physics World has a review* of Leonard Susskind's new book entitled "The Cosmic Landscape: String Theory and the Illusion of Intelligent Design". The review is by Michael Duff of Imperial College (London), who is well known in the field. Well, I have not read the book, so I'll simply point out the review, and not make comment directly on the book itself. A review of a review if you like. I've always liked Mike's sense of humour, and so it is nice to see it sneak into the article here and there, such as at the end of this key pair of paragraphs:

Susskind believes that it is more than dumb luck that the universe is so accommodating to human beings. "Can science explain the extraordinary fact that the universe appears to be uncannily, nay, spectacularly, well designed for our own existence?" he asks. But does this mean that the religious fundamentalists have won? Must we invoke the existence of a god to account for the gaps in our knowledge? Susskind's answer is "no" on both counts. As you might have guessed from the book's subtitle, he argues that while "the appearance of intelligent design is undeniable", science can nevertheless explain it all. Phew! Thank God for that.

Background (and my humble opinion) on the issue of the Landscape in string theory research can be found in an article I wrote here. (Tread carefully through the 172 (to date) comment bloodbath tagged on the end.) So it seems, according to those two paragraphs, that Lenny is being provocative with his book title while still holding on to the view that we don't need to go beyond science to answer several truly fundamental questions about our universe. I'm relieved, because I have great respect for Lenny, and so this is rather good news. Hmmm. We must not, however, forget that he puts forth various versions of the Anthropic Principle to answer the questions instead, and there are those (myself included) who question whether that (at least in its strongest form) is still doing science. I have not yet made up my own mind whether it sits well with me or not..... Let me tell you one reason why I am conflicted: I don't mind anthropic arguments when they are used to predict something important...This has been done in science before: I've given the example of Hoyle's amazing result here. But I do not like an anthropic argument to be used to "explain" something we already know. These seem to be two different things in my mind. I could be wrong. As I said in the other article, what some string theorists, led by Lenny (a very small group, despite what is often said - it is not the entire field) want to put forth is the idea that since the theory (as far as we currently understand it - key point here) seems to give us a vast number of solutions, rather than just one corresponding to our world, this should be regarded as a feature rather than a bug: This vast "Landscape" of solutions each corresponds to a different possible universe, and then we have to use the fact that we are here to ask the question "which one?" as the reason that we are here to ask the question "which one?". This bothers me a bit, since we already know that we are in this universe and have this solution.....and so this does not seem very predictive. It is not in the spirit of Hoyle's precisely predicting a previously unknown resonance of the carbon nucleus on the grounds that we would not be around if it did not exist, in order to allow star to generate heavy nuclei. Here is a long extract from the article talking about the seeds of the modern Anthropic applications to fundamental physics, in its form attributed to Weinberg:

Nevertheless, no less a person than Nobel laureate and arch-atheist Steven Weinberg believes that one particular constant of nature - Einstein's cosmological constant Λ - may be anthropically determined. The size of Λ has long been an enigma. Theoretically its most natural value would be unity in natural units, but anything bigger than 10^(-120) would be inconsistent with astronomical data - and a world record for the worst agreement between theory and experiment! So Weinberg set out to see if any bigger value would prevent life. The answer, it turned out, did not have anything to do with molecular chemistry or the stability of the solar system. Weinberg found that if Λ were just an order of magnitude bigger than 10^(-120), no galaxies, stars or planets would have formed. His anthropic arguments not only provided a limit on Λ, they also give some idea of its expected value. In 1992 he wrote, "Thus if such a cosmological constant is confirmed by observation, it will be reasonable to infer that our own existence plays an important role in explaining why the universe is the way it is." Even sceptics had to take notice, therefore, when recent astrophysical observations indicated that Λ is, in fact, non-zero and has just about the value Weinberg predicted.

Well, this is perhaps a little strong, but you see that it is somewhere in between the two anthropic practices; the Hoyle practice vs the Susskind practice, if we wish to give these things names....It is perhaps much closer to Hoyle, and a bit more swallowable....but I have my doubts still. Anyway, Duff ends (almost) by quoting Susskind:

Susskind concludes that questions such as "why is a certain constant of nature one number rather than another?" may well be answered by "somewhere in the megaverse the constant equals this number: somewhere else it is that number. We live in one tiny pocket where the value of the constant is consistent with our kind of life. That's it! That's all. There is no other answer to the question".

I'll end my review of the review by summarising my view so that I don't have to do it again in the comments. It will be ignored by readers and the members of the press anyway, as it is not sensational enough. (sigh): (1) I claim that this (see above quote paragraph) alone is not predictive. (2) I claim that some anthropic reasoning can be used to do science.....but it must predict, not postdict. See the Hoyle example I link to above. (3) We don't understand string theory at all well. So we don't know if there are the huge number of solutions that Susskind uses to motivate using Anthropic reasoning in string theory. This, in my opinion, is really premature.....but this is why the field is healthy. Nobody knows, so let's try several approaches. (4) In the context of string theory, this approach would be believable if we were to build in Anthropicity for one or at most, a few (discuss), undetermined constant(s) (like the cosmological constant Λ), and then actually predicted several new things we can go out and measure as a result. Maybe this will yet happen? Too early to tell. With (4) in mind, I can see why people are playing with the idea and exploring the program a little. I do not agree that because a few people are playing with such ideas (whether they have my view in mind or not) that the entire field of string theory is therefore doomed, as Peter Woit (see his blog) and others have claimed. That's a bit apocalyptic.... a rather dark view based on personal reasons, in my opinion. (But a view to be considered and brighter view is also based on personal reasons.......But personal reasons are irrelevant in science: the research alone will carry the day. We must let the research happen.) So this endeavour is part of the research program in string theory. There are other people carrying on trying to understand several other aspects of string theory. We're still doing science here. -cvj (*Thanks Count Iblis!)

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