Localized plastic strain in polycrystalline materials with hole and notches

Analyzed numerically are the localized strain of polycrystalline materials subjected to quasi-static loading. The objective is to study the peculiarities associated with the deformation process close to the stress concentrators such as holes, notches and interfaces of internal structure. Analytical results show that geometry and/or heterogeneous internal structure of material together with the action of maximum shear result in the development of a system of plastically deformed shear bands. Shears and rotations in the regions of strain localization are found to be higher than in other parts of the specimen while rotations are more sensitive to localization.     

The thermodynamics of gravitational radiation

I conjecture, and show for a large class of cases, that given a spacelike hypersurface on which is an arbitrary distribution of linearized gravitons and matter, the latter satisfying the positive energy condition, the expectation value of the number of events in which a graviton is absorbed by or scattered inelastically from the matter within any future time,t, is always less then the expectation value of the number of gravitons in the initial state (except for a set of initial configurations of measure zero). Consequences of this result are: (1) the impossibility of any system containing gravitational radiation reaching thermal equilibrium in a finite time, (2) the absence of an ultraviolet catastrophe for gravitational radiation, (3) the impossibility of measuring accurately the quantum state of the linearized gravitational field, and (4) the impossibility of constructing a gravitational wave laser.This essay received the second award from the Gravity Research Foundation for the year 1983 [Ed.].     

Superspace unmasked: The physical interpretation of supergravity theory

Supergravity admits a geometric formulation in terms of an expanded algebra of functions on ordinary spacetime. This formulation, called graded manifold theory, is equivalent to the usual superspace supergravity constructions but avoids the anticommuting coordinates of superspace. The geometry suggests an operational interpretation of supergravity theory in terms of measurements made by local observers in spacetime.     

Towards a theory of spacetime structure at very short distances

Renormalization-group arguments are summarized which suggest that at distances shorter than the Planck length the spacetime geometry should be asymptotically scale invariant. A new locally scale-invariant extension of general relativity is then proposed based on Weyl's conformally invariant geometry. It is shown that if the theory contains a Higgs phase, then it reduces to Einstein's theory in the limit of large distances. Finally, the theory incorporates the non-linear sigma model, which suggests a new approach to the calculation of non-perturbative, short-distance effects in quantum gravity.     

A fixed point for quantum gravity

Using the 1/N expansion a fixed point of the renormalization group is found for quantized gravitational theories which is non-trivial in all dimensions, d, including four. Using the fixed point it is shown how Einstein's theory can be renormalized for 3<d<4. In four dimensions the pure Einstein theory does not exist, but the R + C_μναβ² theory does. It is shown how gravitational theories whose quantum lagrangians are scale invariant may be renormalized such that the scale invariance is broken only by the choice of the critical renormalization group trajectory. A comparison is made with the renormalization of four-fermion and Yukawa theories in 4?? dimensions which suggests that quantum gravity might exist in four dimensions even if those theories do not.     

Formation of ice-like water structure on the surface of an antifreeze protein

Antifreeze proteins (AFPs) are found in different species from polar, alpine, and subarctic regions where they serve to inhibit ice crystal growth by adsorption to ice surfaces. Computational methods have the power to investigate the antifreeze mechanism in atomic detail. Molecular dynamics simulations of water under different conditions have been carried out to test our water model for simulations of biological macromolecules in extreme conditions: very low temperatures (200 K) and at the ice/liquid water interface. We show that the flexible F3C water model reproduces properties of water in the solid phase (ice I(h)), the supercooled liquid phase, and at the ice/liquid water interface. Additionally, the hydration of the type III AFP from ocean pout was studied as a function of temperature. Hydration waters on the ice-binding surface of the AFP were less distorted and more tetrahedral than elsewhere on the surface. More ice-like hydrating water structures formed on the ice-binding surface of the protein such that it created an ice-like structure in water within its first hydration layer but not beyond, suggesting that this portion of the protein has high affinity for ice surfaces.     

Entanglement of superpositions

Given a bipartite quantum state (in arbitrary dimension) and a decomposition of it as a superposition of two others, we find bounds on the entanglement of the superposition state in terms of the entanglement of the states being superposed. In the case that the two states being superposed are biorthogonal, the answer is simple, and, for example, the entanglement of the superposition cannot be more than one ebit more than the average of the entanglement of the two states being superposed. However, for more general states, the situation is very different.     

Extending Dualities to Trialities Deepens the Foundations of Dynamics

Dualities are often supposed to be foundational, but they may come into conflict with a strong form of background independence, which is the principle that the dynamical equations of a theory not depend on arbitrary, fixed, non-dynamical structures. This is because a hidden fixed structures is needed to define the duality transformation. Examples include a fixed, absolute notion of time, a fixed non-dynamical background geometry, or the metric of Hilbert space. We show that this conflict can be eliminated by extending a duality to a triality. This renders that fixed structure dynamical, while unifying it with the dual variables. To illustrate this, we study matrix models with a cubic action, which have a natural triality symmetry. We show how breaking this triality symmetry by imposing different compactifications, which are expansions around fixed classical solutions, yields particle mechanics, string theory and Chern-Simons theory. These result from compactifying, respectively, one, two and three dimensions. This may explain the origin of Born’s duality between position and momenta operators in quantum theory, as well as some of the the dualities of string theory.     

Fluorescence anisotropy of indole molecules under two-photon excitation in the spectral range of 485–510 nm

Decay of polarized fluorescence in indole dissolved in propylene glycol under two-photon excitation by femtosecond laser pulses in the wavelength range of 485–510 nm has been studied. It is shown that under the experimental conditions used the fluorescence decay signal can be well described by a single excited state lifetime τ_f and a single rotation diffusion time τ_rot. By processing the data obtained, the times τ_f and τ_rot as well as anisotropy parameter r ₀ characterizing the symmetry of two-photon excitation of indole molecules have been determined. Decreasing of the anisotropy parameter r₀ down to zero under two-photon excitation energy higher than 5.1 eV has been observed. Interpretation of the obtained results have been done on the basis of ab initio quantum-mechanical computations. A model of energy relaxation under the condition of twophoton excitation of indole in a polar solvent has been discussed.