The Anthropic Principle: Good Physics or Not??

In a recent post, we talked about L. Susskind's ideas about the huge number of vacuum states (environments for physics and possible universes) implied by our current understanding of string theory.

According to these ideas, we exist in a "pocket universe" with a tiny (but non-zero) effective cosmological constant. Without the cosmological constant having a value in a small range around a tiny number, our variety of intelligent observers could not have evolved to a state like the present. The tiny non-zero value of the effective cosmological constant is hard to understand using traditional particle physics arguments.

Was this an act of "God" or a natural and possible outcome of the "laws of nature". According to Susskind's ideas, our universe is the way it is because a) it is one of a huge number of possible string theory vacua , and b) "we live where we can". This is called an "anthropic explanation".

Physics would then "give up" the job of trying to explain why our universe is the unique prediction of the "laws of nature", by finding some ultimate theory which has within it logically the prediction that our universe could not be other than it is.

Andre Linde, in his book Inflation and Quantum Cosmology (p. 152) remarks: "One should note, that until very recently, the general attitude of physicists to the Anthropic Principle was rather skeptical, to say the least. It was believed that the weak, strong, and electromagnetic interactions are the same in all parts of our universe, that the fundamental constants of Nature are universal and it is meaningless to discuss the possibilty of life in a universe of a different type."

"... if many exponentially large domains with the low-energy physics of our type do exist in the universe described by a given theory, then it is quite natural that we live in one of such domains rather than in a domain where life of our type is impossible".

L. Smolin

L. Susskind

All this is preamble to calling your attention to a good back and forth argument about the nature of good physics and the possible place for the "anthropic principle" type of argument. The Edge has posted a multipart exchange of views between Lee Smolin (anti anthropic prin.) and Leonard Susskind (pro anthropic prin.).