As you say, the version of string theory with an infinity of universes is an elegant concept.  However, when you say ". . . its most fundamental assumption, the existence of a supersymmerty relation between bosons and fermions, has never even come close to matching experimental observation," one could infer that there is little likelihood that SUSY will ever be shown to be a good theory.
This may change soon.  Wikipedia says "Experimentalists have not yet found any superpartners for known particles, either because they are too massive to be created in our current particle accelerators, or because they may not exist at all.   By the year 2007, the Large Hadron Collider at CERN should be ready for use, producing collisions at sufficiently high energies to detect the superpartners many theorists hope to see."
So maybe, in a couple of years, there WILL be experimental observation supporting SUSY.
I agree that the posts by Hal Finney and Wei Dai are well said and inspirational.  Thanks,
----- Original Message -----
From: Stephen Paul King
Sent: Monday, February 13, 2006 7:29 PM
Subject: Re: Multiverse concepts in string theory

Dear Wei and Friends,

    I have been following this thread with some interest (Hal initial post was wonderful, BTW!) and echo the comments of Wei here, but I would offer a note of caution: we must be very careful that the elevation of string theory (SUSY) to almost dogmatic "Sacred Cow" status does not bode well for many of us, particularly those that have found that its most fundamental assumption, the existence of a supersymmerty relation between bosons and fermions, has never even come close to matching experimental observation.
    Maybe, just maybe, SUSY is a good theory or maybe it is just a very elegant bit of pure mathematics. Remember, just because a mathematical theory can be shown to be self-consistent and elegant, there is no requirement that that theory have anything to do with the physical world we experience.
    I find that the choices presented by Weinberg and the Intelligent Design advocates are not the only possibilities. Consider that we still do not have a consistent and faithful model of observers within our physics and thus can not even start to coherently consider what the notion of "comprehensibility" means in the context of physics. ;-)



----- Original Message -----
From: Wei Dai
Sent: Monday, February 13, 2006 7:06 PM
Subject: Re: Multiverse concepts in string theory

Hal wrote:
> I also get the impression that Susskind's attempts to bring "disreputable"
> multiverse models into "holy" string theory is more likely to kill
> string theory than to rehabilitate multiverses.  Perhaps I am getting a
> biased view by only reading this one blog, which opposes string theory,
> but it seems that more and more people are saying that the emperor has
> no clothes.  If string theory needs a multiverse then it is even less
> likely to ever be able to make physical predictions, and its prospects
> are even worse than had been thought.  A lot of people seem to be piling
> on and saying that it is time for physics to explore alternative ideas.
> The hostile NY Times book review is just one example.

String theory isn't going to be killed until there's a replacement available, and any replacement is likely to face the same issue of describing a large collection of universes of which only a small subset can support life. So I wouldn't be concerned about more effort being devoted to looking for alternatives to string theory. In the mean time, the multiverse meme continues to spread. Take the review of Susskind's book in American Scientist ( for example:

In the end, however, good though this book is, I was left feeling that the argument was not carried to its logical conclusion. Despite his justified scorn for intelligent design, Susskind retains a hint of this worldview in his own attitude. It was Galileo who said that the book of Nature is written in mathematics, and almost all physicists subscribe to this view. When we contemplate the power and simplicity of constructions like general relativity, there is a temptation to carry intelligent design to an extreme in which God wrote the equations, from which all else follows. Frequently this perspective is quite explicit, as with Einstein (recall Bohr's admonition, "Stop telling God what to do!"). The landscape picture derails this thinking to some extent, but Susskind just transfers the quasi-religious awe to string theory, whose mathematical results he repeatedly describes as "miraculous."

But if life on Earth is a random accident in a universe where only chance yielded laws of physics suitable for life, why stop there? Perhaps string theory itself is nothing special and only part of a wider spectrum of possible prescriptions for reality. If the search for a unique and inevitable explanation of Nature has proved illusory at every step, is it really plausible that suddenly string theory can make everything right at the last? Reading Susskind's book should make you doubt that possibility, in which case we may have reached the end of the search for underlying simplicity that has driven physics since the beginning. A comment made by Steven Weinberg in his 1977 book The First Three Minutes sums things up well: "The more the universe seems comprehensible, the more it also seems pointless." Pointless to look for meaning in our existence in the universe, and also (according to Susskind) pointless to look for meaning in physics. To a physicist, this is a pretty depressing conclusion, but there is some consolation: The beauty we perceive in the laws of physics perhaps tells us as much about the human aesthetic response as it does about any fundamental design of the universe. In short, physics is a human creative art on the same level as painting and music, and that is reason enough to be proud of what the subject has achieved.

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