The Alternative Models

A central, but elusive aim of nuclear research for almost a century has been the understanding of nuclear synthesis, particularly in the hands of astrophysicists. The dearth of experimental data to drive such enquiry has often resulted in the bending of synthesis theory to follow current thinking in cosmogony, rather than the other way around. This situation has not changed materially since the publication of two comprehensive reviews of the problem in 1950, after the decline of Lemaitre s, but before the advent of Gamow s big-bang theories. This period provided an opportunity to consider rival theories without prejudice. 

Two back-to-back reviews (Ter Haar, 1950 Alpher Hermann, 1950) compared the strengths and weaknesses of the models and reached opposite conclusions. Ter Haar concluded that the equilibrium theory offers a better solution how to account for the observed abundances of the chemical elements than the a — j3 — theory .  

Not surprisingly, Alpher and Herman argued for a number of indications that element formation probably took place in an early prestellar state of the universe, in which it is difficult to conceive of the existence for a sufficiently long time of the physical conditions required for an equilibrium among nuclei . They added Difficulties of the kind mentioned and in particular the freezing-in problem have led to the development of non-equilibrium theories 

Alpher s preference is clearly conditioned by cosmology rather than science, but in the event it led to the total elimination of the equilibrium model from subsequent enquiry. A complicating factor in the argument is the continuously changing evidence provided by astronomical observation, in both theories, the success is measured in terms of predicted nuclear abundances and invariably these predictions depend on the nature and characteristics of known types of star, assumed as the seat of nucleogenesis. As more powerful telescopes identify new types of heavenly body, new possibilities of nuclear synthesis open up and the model has to be reworked. This process continues for the a — /d — 7 model only. The equilibrium model was abandoned before the discovery of quasars and black holes, that obviously provide more attractive environments for nuclear synthesis. The only mechanism for the dispersal of freshly synthesized material is still assumed to be supernovae and this assumption could also stand reassessment. 

The main premise of both models is that a successful theory should predict the correct cosmic abundances of all nuclides. This idea, that abundances hold the clue to nucleogenesis probably stems from a proposal by Harkins (1931) in 1915 that the abundance of an element depends on two factors  

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Every alternating atom in the PDMS chain is the electronegative oxygen, which makes it a polar polymer. The authors observed that at low loadings of PDMS, PVC is destabilized. This is to be expected according to the alternative model for degradation and stabilization of PVC put forward by Naqvi. But for compositions with 50% or more PDMS, both polymers are stabilized. No possible explanation can be given for this reversal in thermal stability at this time. 

Starting from ( )-enolates, the corresponding argument predicts that transition state 10a is lower in energy than the alternative model which suffers from mutual steric hindrance of the substituent R1 and the methyl group in a gauche orientation thus, stereoconvergence results. 

This model is consistent with the results discussed in Section III in that it also postulates the Cu(II)/Cu(I) redox cycle. Otherwise, there are significant differences, such as the removal of H2O2 as in Eq. (115), or the formation of the hydroxyl radical, which would require justification. Since calculations were not presented to support the kinetic model, it is open to question as to how well the observations would be reproduced by reactions (112116) or, for that matter, by the alternative models. 

Alternative models. In complex high-dimensional data sets there will commonly be a number of very different-looking models all of which describe the data about equally well. Scanning the forest uncovers these alternative models. The alternative models can look different but be relatively trivial, based on correlated variables. Or they can point to multiple mechanisms. 

A conceptually simple approach that avoids the difficulties of model weighting is scenario analysis or the 1-at-a-time method, where the alternative models are analyzed separately and the resnlts are compared. In the example of the previous section, this might produce a conclusion of the type If model A is true then 0 people will get cancer if model B is trne then 200 people will get cancer. However, this... 

I therefore consider the alternative model to be perfectly reasonable as far as it goes, but assert that it does not go far enough. I further believe that its important claims can easily be integrated into the new... 

In the following, only the properties of the minimal model are discussed since the alternative model gives rise to very similar results. 

The alternative model would predict that the greater the age of a cell the greater is its probability of division and hence the semilog plot of a against age would constantly curve downwards. In practice it is exceedingly difficult to distinguish the two possibilities and there is considerable discussion as to which model is correct (Baserga, 1978). 

The alternative model to the dual-competent chemical engineer is the chemical engineer as a member of a multidisciplinary team, one who knows the vocabulary of the other team members but not their theories. Undoubtedly, this model may be necessary at times, but such a member would have little chance to become the dominant creative mind or project leader. For less investment, one gets less results. 

The alternative model D, proposed for (E )-alkenes, emphasizes the steric requirements imposed by 0s04 on the alkene and its allylic substituent as expected for a product-like transition state not influenced by hyperconjugative effects3. According to this model the less-demanding allylic hydrogen is close to the bulky osmium atom and the relatively compact oxygen atom occupies the inside position. 

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