As a commenter here noted last night, and other commenters have discussed in the last posting, Steven Weinberg has just put on the arXiv an article entitled Living in the Multiverse. In it, he correctly points out that theoretical physics was immensely successful during the twentieth century as it adopted a fundamental paradigm of exploiting symmetries and quantum mechanical consistency conditions, using these to develop extremely powerful and predictive theories. Initial hopes for superstring theory were that it would lead to further progress along similar lines, but these have not worked out at all.
Faced with the failure of superstring theory to provide any new predictions based on a useful new symmetry principle or consistency condition, instead of drawing the obvious conclusion that it’s just a wrong idea about how to get beyond the standard model, Weinberg instead proposes to dump the lessons of the success of twentieth century physics:
Now we may be at a new turning point, a radical change in what we accept as a legitimate foundation for a physical theory. The current excitement is of course a consequence of the discovery of a vast number of solutions of string theory, beginning in 2000 with the work of Bousso and Polchinski.
What Weinberg sees as “excitement” is what some others have characterized as “depression and desperation”. His “radical change in what we accept as a legitimate foundation for a physical theory” seems to be to give up on the idea of a fundamental theory that predicts things and instead adopt the “anthropic reasoning” paradigm of how to do physics. Weinberg goes through various examples of his own recent work of this kind, announcing that the probability of seeing a vacuum energy of the observed value is 15.6% (this seems to me to violate my high school physics teacher’s dictum about not quoting results to insignificant figures, but I’m not sure how you’d put error bars on that kind of number anyway). He also quotes approvingly recent anthropic work of Arkani-Hamed, Dimopoulos and Kachru, as well as that of his colleague Jacques Distler. All he has to say about the underlying string theory motivation for all this is that “it wouldn’t hurt in this work if we knew what string theory is.”
In his final comments he acknowledges that this new vision of fundamental physics is not as solidly based as the theory of evolution. Describing the strength of his belief in it, he says “I have just enough confidence about the multiverse to bet the lives of both Andrei Linde and Martin Rees’s dog.” One can’t be sure exactly what that means without knowing how he personally feels about Andrei Linde, or cruelty to innocent dogs.
Weinberg’s article is based on a talk given at a symposium in September at Cambridge on the topic “Expectations of a Final Theory”. I haven’t been able to find out anything else about this symposium, and would be interested to hear any other information about it that anyone else has. The article will be published in a Cambridge University Press volume Universe or Multiverse?, edited by Bernard Carr (the president of the Society for Psychical Research), about which I’ve posted earlier here.
I’m curious whether this Cambridge symposium was one of the infinite number of such things funded by the Templeton Foundation. Next week the Vatican will be sponsoring a Templeton-funded conference held in the Vatican City on the topic of Infinity in Science, Philosophy and Theology. It will feature a talk by Juan Maldacena on “Infinity as Simplification”, and is part of a larger Vatican/Templeton project called Science, Theology and the Ontological Quest. This project is designed to promote the vision of scientific research outlined by Pope John Paul II in two encyclical letters, including the rule that scientific research must be “grounded in the ‘fear of God’ whose transcendent sovereignty and provident love in the governance of the world reason must recognize.”
Update: Lubos Motl has some comments on the Weinberg article. This is one topic on which we seem to be in agreement.