http://arxiv.org/abs/0707.2593 he argues that if you believe QM is "universally applicable" (whatever special thing that means for him) then you should believe in manyworld branching of reality. I don't, and I don't even consider it empirical science--so I guess I have to accept that QM is NOT "universally applicable". IMHO the interesting work nowadays is on the foundational reality that gives rise to QM as a useful appearance or approximation----not philosophical discussion of what QM means or how to interpret it. but Tegmark is a highly visible figure, articulating a recognized viewpoint, so we should know about his article: Many lives in many worlds Max Tegmark (MIT) Nature, 448, 23 (2007) Nature version with better graphics at http://www.nature.com/nature/journal/v448/n7149/full/448023a.html Everett bio and other links at http://space.mit.edu/home/tegmark/quantum.html (Submitted on 18 Jul 2007) "I argue that accepting quantum mechanics to be universally true means that you should also believe in parallel universes. I give my assessment of Everett's theory as it celebrates its 50th anniversary." ========== we are getting off into speculative candy for the imagination. If, in contrast to Tegmark's article, you instead want stuff about the cosmos that leads to calculating numbers and is eventually testable against observation then check out Bojowald's modeling of the big bounce. Tegmark's takes the questionable (in my view naive) step of attibuting PHYSICAL REALITY to the quantum wavefunction. (as if Nature or God had a hilbert space and there was an absolutely real state in it) Instead I think QM is a theory of information and each observer is equipped with a kind of calculator to store his information about the world, as a wavefunction if he likes that method, and evolve that information. I think it is a bad move to attribute a single absolute reality to some grandiose wavefunction---that is only today's best form in which to represent and handle physical information. there is no one absolute TIME either. In the schrodinger picture a wavefunction evolves in time----if you make one absolute you probably want to make the other one absolute as well. It is a naive idea---both are observer-dependent. ========== Tegmark says something rather misleading about FALSIFIABILITY. this is, I think, the most serious fault in the article (which is mostly on the light side). he says the falsifiable prediction that manyworld thinking makes is that YOU WILL NEVER SEE THE WAVEFUNCTION COLLAPSE. If you ever see a wavefunction collapse, Tegmark suggests, then many-think is WRONG. But I don't believe that a wavefunction is a physically real thing---I think it is an excellent way for me to represent some information about the fundamental degrees of freedom describing space time matter, from which QM arises. I don't expect to SEE a wavefunction, much less see one collapse. So how can I perform an experiment which I can reasonably expect could falsify many-think? maybe someone will describe for us a practical experiment that really could falsify many-think.. If there was a God, he would probably have something better than a Hilbert space to represent his information about the world. He would not have a complexvalue wave-clunkshun to represent the state of his universe. It's not a physically real thing so nobody has to get excited about it collapsing. Hugh Everett, it seems, thought it was real, worried about it collapsing, made up manyworldthink in 1957, just 50 years ago, and subsequently left academia. Tegmark's article is celebrating Everett's thesis of 50 years ago. have to look him up in Wikipedia and see what he did when he abandoned the manyworlds idea in 1959----optimization, nuclear weapons, computer consulting, programming, start-up companies, Wikipedia indicates considerable success outside theoretical physics. It was talking with Niels Bohr in 1959 that made him drop manyworlds and go in other directions. Here's an idea. Let's all celebrate in 2009. :-) ======================== another paper that came out today, don't know if it is of interest to anyone: http://arxiv.org/abs/0707.2748 Theory of gravitation theories: a no-progress report Authors: Thomas P Sotiriou, Valerio Faraoni, Stefano Liberati 21 pages (Submitted on 18 Jul 2007) "Already in the 1970s there where attempts to present a set of ground rules, sometimes referred to as a theory of gravitation theories, which theories of gravity should satisfy in order to be considered viable in principle and, therefore, interesting enough to deserve further investigation. From this perspective, an alternative title of the present paper could be 'why are we still unable to write a guide on how to propose viable alternatives to general relativity?'. Attempting to answer this question, it is argued here that earlier efforts to turn qualitative statements, such as the Einstein Equivalence Principle, into quantitative ones, such as the metric postulates, stand on rather shaky grounds -- probably contrary to popular belief -- as they appear to depend strongly on particular representations of the theory. This includes ambiguities in the identification of matter and gravitational fields, dependence of frequently used definitions, such as those of the stress-energy tensor or classical vacuum, on the choice of variables, etc. Various examples are discussed and possible approaches to this problem are pointed out. In the course of this study, several common misconceptions related to the various forms of the Equivalence Principle, the use of conformal frames and equivalence between theories are clarified."