(This article was originally posted on blogger.com on Nov 4, 2005)
Today, Steven Weinberg posted an article at the arXiv entitled ”Living in the Multiverse” (which is based on a talk he gave at a symposium in September at Cambridge on the topic “Expectations of a Final Theory”).
Weinberg is by any standard an exceptional physicist. He has made numerous contributions to particle physics and also cosmology. One of his controversial – but in many ways also successful – predictions was the prediction in 1987 of the size of the cosmological constant assuming that galaxies should have formed. If the cosmological constant is too large, galaxies and stars cannot have formed since then space would expand and dilute too quickly, before any kind of galaxy can be created by the gravitational pull. On the other hand, if the cosmological constant is too negative, the Universe would simply collapse – that is, approach the Big Crunch too early. Later experiments showed that Weinberg’s rough estimate was correct. Weinberg’s calculation of the cosmological constant is a typical application of the “Anthropic Principle”: the constants of nature (and everything else, I guess) should be such that galaxies and eventually life could form.
As a side remark, we could ask the following question: What did we actually learn from Weinberg’s successful calculation of the cosmological constant? Basically only that nature is the way it is, since it is the way it is. I would have found it much more interesting if Weinberg’s estimate was completely off from what later experiments confirmed. Then we would have known, that our understanding of galaxy formation – and maybe even particle physics – was fundamentally flawed.
In the talk mentioned above, Weinberg presents some novel ideas to support the anthropic principle. Weinberg argues that revolutions in physics not only answer questions in new ways (as for example by principles of symmetry as in the general theory of relativity or in quantum mechanics), but they even change our view of which questions are important and well-defined and which are not (as for example the one in quantum mechanics as: “what path did the electron follow?”).
Weinberg argues that the anthropic principle and the “landscape” of string theory seem to be implying something along these lines. As an example, he says:
The larger the number of possible values of physical parameters provided by the string landscape, the more string theory legitimates anthropic reasoning as a new basis for physical theories: Any scientist who study nature must live in a part of the landscape where physical parameters take values suitable for the appearance of life and its evolution into scientists.
The last statement is kind of trivial. The first one does not seem to be substantiated in any way: that the number of solutions is large (~10^500) does not imply, that there is no scientific principles which determine the “correct” solutions (and here I don’t count the anthropic principle as scientific). I find his comment about the hierarchy problem even more problematic:
If the electroweak symmetry breaking scale is anthropically fixed, then we can give up the decades long search for a natural solution of the hierarchy problem. This is a very attractive prospect, because none of the “natural” solutions that have been proposed, such as technicolor or low energy supersymmetry, were ever free of difficulties.
How can we ever know, that this scale is “anthropically” fixed? By realizing, that we could not solve the hierarchy problem in 30 years? In 120 years? Sorry, but to me, this kind of reasoning sounds much like “Intelligent Design”. Just because there is something – like the immune system, blood clotting or the relative masses of ‘fundamental’ particles – which we have not fully understood yet, does not imply that we should resort to aliens, a God (which I don’t think Weinberg would) or our own existence as beings for an “explanation”.
In conclusion, Weinberg seems to be suggesting that asking for an explanation of the values of the free parameters of the Standard Model is equally meaningless as asking how to describe the path followed by an elementary particle.