ethan@utastro.UUCP (Ethan Vishniac) (05/17/84)
[Eat this header] There is a recurring thread which runs through Mr. Gwyn's arguments on cosmology to which I take strong exception. I am *not* referring to his favorite unified theory (although I am not sure how it is supposed to relate to the current experimental and theoretical work to particle physics). What bothers me are his comments about symmetry and simplicity in a physical theory. Any one of a number of quotes from him would serve to illustrate my point. Let me confine myself to one. ( I don't think Mr. Gwyn would regard this one as unfair or taken out of context.) >The closed, bounded, perfect-cosmological-principle cosmology is >actually simpler in any measurable sense of which I am aware than >the expanding "big bang" universe. Now, Mr. Gwyn is clearly referring to simplicity in the sense that a mathematician would i.e. devoid of complications, maximizing the internal symmetries of the universe. This has nothing to do with simplicity in the sense of being the most economical explanation of the facts. The fundamental laws of physics may have many symmetries, none of which are realized exactly, or even approximately, in any real situation. Given that the laws of physics are rotationally invariant do we conclude that ferromagnets do not exist? As for translational invariance, the same laws of physics apply everywhere, but if you think that translates to translational invariance of our environment, then I invite you to test this by casting yourself off the nearest roof. I could go on, but I think that you all must see my point. A successful model of the universe should account, as simply as possible, for our observations. I have discussed elsewhere what these are. Mr. Gwyn seems to be willing to take only one of them seriously, the Hubble law. I think this is completely unjustified, but even on this one point his comments leave me unsatisfied. I have been left with the impression that he regards any ad hoc explanation as "simple" if it allows him to continue with his prejudices intact. I would prefer to base any explanation of this (or any other) observation on the laws of physics as we know them, or at least some recognizable expansion of them. I cannot say if the E-S theory falls into this category or not (anybody out there wish to comment?), but I haven't heard Mr. Gwyn say that the E-S theory can explain the Hubble law in any natural way. One final comment, the most symmetric possible metric (i.e. structure of space-time) is the DeSitter space mentioned by Mr. Gwyn. It has the interesting property of satisfying the perfect cosmological principal (homogeneity in space and time). For this reason it was used as the basis for the steady-state theory of Bondi, Gold, and Hoyle. It does indeed account for the Hubble flow, *because* it is an expanding universe. Particles following geodesics (i.e. not being influenced by nongravitational forces) diverge exponentially from one another at late times. The theory has been discarded within the astrophysical community because of its complete failure to address the other observational points I have mentioned. Even before these points became generally accepted it was already necessary to allow matter to be spontaneously produced from the vacuum in order to make the model logically consistent. "Just another Cosmic Cowboy" Ethan Vishniac {ut-sally,ut-ngp,kpno}!utastro!ethan Department of Astronomy University of Texas Austin, Texas 78712
gwyn@brl-vgr.UUCP (05/17/84)
There is no way I am going to convey the meaning of simplicity etc. in a reasonable number of words, but let me at least give a practical guide to comparing two competing theories of the fundamental laws of physics, in order to convey something of what I have in mind. Theory A consists of: (1) acceptance of several common mathematical ideas; (2) postulation of seven fundamental properties (charm, etc.?); (3) adoption of four principles of quantization, renormalization, and so forth; (4) acceptance of a Lagrangian with three independent coupling constants, of fourth order in the fundamental entities; (5) a 20-dimensional space; (6) a preferred frame of reference; (7) one principle of variation. Theory B consists of: (1) acceptance of several common mathematical ideas; (2) postulation of one fundamental property; (3) adoption of two operational principles; (4) acceptance of a second-order Largrangian with no added constants; (5) a 4-dimensional space; (6) no preferred reference frame; (7) one principle of variation. Then I would certainly claim that Theory B is the "simpler" theory, since it has considerably fewer things put into it a priori. This of course does not mean that Theory B is guaranteed to describe reality better than Theory A, although if it is comparable in accuracy one should be strongly inclined to prefer it over Theory B. (Note: Theories A & B are meant to remind one of certain real candidates, but the detailed counts of the basic enities, assumptions, etc. do not necessarily correctly reflect those of any actual theories.) There is even a quantitative method of assigning a measure to the degree to which a field theory constrains its basic entities (the basic idea is the asymptotic degrees of freedom of the higher- order Taylor coefficients of the fields). It is amusing that the measure number for the Einstein-Straus-Kaufman unified field theory (which I do NOT support) is precisely "42". Now we know what the ultimate question was (Life, the Universe, and Everything). Einstein preferred a "stronger" theory to a "weaker" (i.e. less asymptotic freedom) in the absence of any other criterion for choosing between competing theories. I prefer the "weaker" on the intuitive grounds that it is less of a special-case theory... Of course we assume that the theories in question agree with reality sufficiently well; otherwise the choice is trivial.
gwyn@brl-vgr.ARPA (Doug Gwyn ) (05/17/84)
Under the particular formulation of the Einstein-Schr"odinger theory that is most similar in form to General Relativity, and using the traditional (but not unquestioned!) identification of the entities in the formalism with aspects of the actual physical world, then it is indeed the case that a particularly symmetric special solution of the field equations is a deSitter universe. This means that the extended theory is certainly capable of "predicting" the Hubble effect. Whether this is the only reasonable cosmology one can have for this theory is another matter. I am not very familiar with the "diverging geodesic" argument Ethan has stated but I do know that geodesics do not mean quite the same thing in a pure affine theory. I would like to look into this further. I do not relate the theory to current developments in cosmology since in those cases where General Relativity is expected to be sufficiently valid (low energy density, medium scale of distance, and so forth) the extended theory should give comparable predictions if one uses the same methodology for the non-General Relativistic considerations (quantization or what have you). However, it is not the aim of the E-S theory to apply only in a limited problem domain with other methods imposed outside that domain. It is supposed to be complete (although I will certainly admit that the formulation of the theory does not guarantee that all relevant factors have been included; I consider that an open question although most theorists would disagree).