On the verge of <Emphasis Type="Italic">Umdeutung</Emphasis> in Minnesota: Van Vleck and the correspondence principle. Part one

In October 1924, The Physical Review, a relatively minor journal at the time, published a remarkable two-part paper by John H. Van Vleck, working in virtual isolation at the University of Minnesota. Using Bohr’s correspondence principle and Einstein’s quantum theory of radiation along with advanced techniques from classical mechanics, Van Vleck showed that quantum formulae for emission, absorption, and dispersion of radiation merge with their classical counterparts in the limit of high quantum numbers. For modern readers Van Vleck’s paper is much easier to follow than the famous paper by Kramers and Heisenberg on dispersion theory, which covers similar terrain and is widely credited to have led directly to Heisenberg’s Umdeutung paper. This makes Van Vleck’s paper extremely valuable for the reconstruction of the genesis of matrix mechanics. It also makes it tempting to ask why Van Vleck did not take the next step and develop matrix mechanics himself. This paper was written as part of a joint project in the history of quantum physics of the Max Planck Institut für Wissenschaftsgeschichte and the Fritz-Haber-Institut in Berlin.     

On the Association for Foundations of Science,Language and Cognition,AFOS

Varia

On the Association for Foundations of Science, Language and Cognition, AFOSSome facts about AFOS     

FGF-2/FGFR信号转导途径与肿瘤的关系及在乳腺癌发生中的作用

碱性成纤维细胞生长因子(FGF-2)是一种能调节细胞分裂、增殖、迁移及分化的多效生长因子. 研究表明, FGF-2在肿瘤生长和新血管生成中起重要的作用. FGF-2诱导的生物学效应很大程度上是依赖于TPK受体(酪氨酸激酶受体)和胞内二级信使分子(mitogen activted protein kinase)等蛋白分子组成的功能性信号转导网络来实现的, 该网络中不同信号转导通路的活化状态对应着各种不同的生物学效应. 本文将阐述在肿瘤及乳腺癌发生过程中FGF-2诱导的信号转导途径的作用机理方面的研究进展.     

Progressive field-state collapse and quantum non-demolition photon counting

The irreversible evolution of a microscopic system under measurement is a central feature of quantum theory. From an initial state generally exhibiting quantum uncertainty in the measured observable, the system is projected into a state in which this observable becomes precisely known. Its value is random, with a probability determined by the initial system's state. The evolution induced by measurement (known as 'state collapse') can be progressive, accumulating the effects of elementary state changes. Here we report the observation of such a step-by-step collapse by non-destructively measuring the photon number of a field stored in a cavity. Atoms behaving as microscopic clocks cross the cavity successively. By measuring the light-induced alterations of the clock rate, information is progressively extracted, until the initially uncertain photon number converges to an integer. The suppression of the photon number spread is demonstrated by correlations between repeated measurements. The procedure illustrates all the postulates of quantum measurement (state collapse, statistical results and repeatability) and should facilitate studies of non-classical fields trapped in cavities.     

Oxysterols direct immune cell migration via EBI2

Epstein-Barr virus-induced gene 2 (EBI2, also known as GPR183) is a G-protein-coupled receptor that is required for humoral immune responses; polymorphisms in the receptor have been associated with inflammatory autoimmune diseases. The natural ligand for EBI2 has been unknown. Here we describe the identification of 7α,25-dihydroxycholesterol (also called 7α,25-OHC or 5-cholesten-3β,7α,25-triol) as a potent and selective agonist of EBI2. Functional activation of human EBI2 by 7α,25-OHC and closely related oxysterols was verified by monitoring second messenger readouts and saturable, high-affinity radioligand binding. Furthermore, we find that 7α,25-OHC and closely related oxysterols act as chemoattractants for immune cells expressing EBI2 by directing cell migration in vitro and in vivo. A critical enzyme required for the generation of 7α,25-OHC is cholesterol 25-hydroxylase (CH25H). Similar to EBI2 receptor knockout mice, mice deficient in CH25H fail to position activated B cells within the spleen to the outer follicle and mount a reduced plasma cell response after an immune challenge. This demonstrates that CH25H generates EBI2 biological activity in vivo and indicates that the EBI2-oxysterol signalling pathway has an important role in the adaptive immune response.     

A tension-induced mechanotransduction pathway promotes epithelial morphogenesis

Mechanotransduction refers to the transformation of physical forces into chemical signals. It generally involves stretch-sensitive channels or conformational change of cytoskeleton-associated proteins. Mechanotransduction is crucial for the physiology of several organs and for cell migration. The extent to which mechanical inputs contribute to development, and how they do this, remains poorly defined. Here we show that a mechanotransduction pathway operates between the body-wall muscles of Caenorhabditis elegans and the epidermis. This pathway involves, in addition to a Rac GTPase, three signalling proteins found at the hemidesmosome: p21-activated kinase (PAK-1), the adaptor GIT-1 and its partner PIX-1. The phosphorylation of intermediate filaments is one output of this pathway. Tension exerted by adjacent muscles or externally exerted mechanical pressure maintains GIT-1 at hemidesmosomes and stimulates PAK-1 activity through PIX-1 and Rac. This pathway promotes the maturation of a hemidesmosome into a junction that can resist mechanical stress and contributes to coordinating the morphogenesis of epidermal and muscle tissues. Our findings suggest that the C. elegans hemidesmosome is not only an attachment structure, but also a mechanosensor that responds to tension by triggering signalling processes. We suggest that similar pathways could promote epithelial morphogenesis or wound healing in other organisms in which epithelial cells adhere to tension-generating contractile cells.     

Copy number variation and selection during reprogramming to pluripotency

The mechanisms underlying the low efficiency of reprogramming somatic cells into induced pluripotent stem (iPS) cells are poorly understood. There is a clear need to study whether the reprogramming process itself compromises genomic integrity and, through this, the efficiency of iPS cell establishment. Using a high-resolution single nucleotide polymorphism array, we compared copy number variations (CNVs) of different passages of human iPS cells with their fibroblast cell origins and with human embryonic stem (ES) cells. Here we show that significantly more CNVs are present in early-passage human iPS cells than intermediate passage human iPS cells, fibroblasts or human ES cells. Most CNVs are formed de novo and generate genetic mosaicism in early-passage human iPS cells. Most of these novel CNVs rendered the affected cells at a selective disadvantage. Remarkably, expansion of human iPS cells in culture selects rapidly against mutated cells, driving the lines towards a genetic state resembling human ES cells.     

The twins and the bucket: How Einstein made gravity rather than motion relative in general relativity

In publications in 1914 and 1918, Einstein claimed that his new theory of gravity in some sense relativizes the rotation of a body with respect to the distant stars (a stripped-down version of Newton's rotating bucket experiment) and the acceleration of the traveler with respect to the stay-at-home in the twin paradox. What he showed was that phenomena seen as inertial effects in a space-time coordinate system in which the non-accelerating body is at rest can be seen as a combination of inertial and gravitational effects in a (suitably chosen) space-time coordinate system in which the accelerating body is at rest. Two different relativity principles play a role in these accounts: (a) the relativity of non-uniform motion, in the weak sense that the laws of physics are the same in the two space-time coordinate systems involved; (b) what Einstein in 1920 called the relativity of the gravitational field, the notion that there is a unified inertio-gravitational field that splits differently into inertial and gravitational components in different coordinate systems. I provide a detailed reconstruction of Einstein's rather sketchy accounts of the twins and the bucket and examine the role of these two relativity principles. I argue that we can hold on to (b) but that (a) is either false or trivial.     

A mixture likelihood approach for generalized linear models

A mixture model approach is developed that simultaneously estimates the posterior membership probabilities of observations to a number of unobservable groups or latent classes, and the parameters of a generalized linear model which relates the observations, distributed according to some member of the exponential family, to a set of specified covariates within each Class. We demonstrate how this approach handles many of the existing latent class regression procedures as special cases, as well as a host of other parametric specifications in the exponential family heretofore not mentioned in the latent class literature. As such we generalize the McCullagh and Nelder approach to a latent class framework. The parameters are estimated using maximum likelihood, and an EM algorithm for estimation is provided. A Monte Carlo study of the performance of the algorithm for several distributions is provided, and the model is illustrated in two empirical applications.