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Roger Elliott 《Physica A》2007,384(1):44-52
This is the content of a general talk on the history of magnetism given as the Saha Memorial Lecture, January 2007. 相似文献
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Shape resonances, their definition, their nature, their use and supposedly almost miraculous capabilities, have been a rather controversial topic in the scientific literature for quite a long time. There was an upsurge of papers and heated discussions at conferences about these issues more than 10 years ago, the main point of which was whether the shape resonance position in energy above a core level ionization threshold in molecules, either isolated or adsorbed on surfaces, bears some information on bond lengths, with two opposite schools of thought, one maintaining that such connection, if existent at all, is rather loose, the other one ready to use some linear dependence of the shape resonance energy upon bond lengths to actually derive unknown bond distances in adsorbed molecules (the so-called ‘bond-lengths-with-a-ruler’ party). The latest technical and instrumental developments have given rise to another wave of shape resonance papers in the last 3 years. Criteria to assign shape resonances have been reconsidered under the light of new experimental results, and some molecules which were categorized as textbook examples for shape resonances have been shown not to exhibit such continuum phenomena at all. Therefore it is time for a historical survey which hopefully will straighten up some misconceptions still floating around in the literature. 相似文献
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Pankaj S Joshi 《Pramana》2000,55(4):529-544
An outstanding problem in gravitation theory and relativistic astrophysics today is to understand the final outcome of an
endless gravitational collapse. Such a continual collapse would take place when stars more massive than few times the mass
of the sun collapse under their own gravity on exhausting their nuclear fuel. According to the general theory of relativity,
this results either in a black hole, or a naked singularity — which can communicate with far away observers in the universe.
While black holes are (almost) being detected and are increasingly used to model high energy astrophysical phenomena, naked
singularities have turned into a topic of active discussion, aimed at understanding their structure and implications. Recent
developments here are reviewed, indicating future directions. 相似文献
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N.E. Frankel 《Physics letters. A》1982,90(6):323-326
A study of self-gravitating newtonian systems suggests that the universe may have been once in a highly anisotropic planar configuration. In particular, this study leads naturally to the expression for the characteristic mass of the universe, M≈ (?c/Gm2p)2mp 相似文献
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J. Villain A. Fort 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,17(1):69-83
Magnetic tunneling of a large spin subject to the Hamiltonian is investigated by elementary methods for weak fields H. In zero field (H=0) the tunnel frequency in the ground state is found to be equal to [0pt] multiplied by a quantity whose variation with s is slower than exponential. This result coincides with that of path integral methods [#!schi!#]. For the values of the longitudinal
field which allow tunneling, the tunnel frequency is shown to vanish when H
y
=0 for certain “diabolic" values of [0pt] , in qualitative agreement with experiments by Wernsdorfer and Sessoli. The particular case H
z
=0 was already obtained by Garg by means of path integrals. The diabolic values of are in agreement with numerical results but, as already noticed by Wernsdorfer and Sessoli, they disagree with the experimental
ones. This may be an effect of higher order anisotropy terms, which is briefly addressed in the conclusion.
Received 17 May 2000 相似文献
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R. Thurstans 《Contemporary Physics》2013,54(4):243-245
When diatomic and triatomic molecules are exposed to subpicosecond laser pulses (focused intensities well in excess of 1014 W cm?2; wavelengths of the order 600 nm), they are highly ionized. The laser electric fields are so large that they compete with, and in some cases exceed, the internal fields binding the valence electrons. The result is that conventional perturbation theory is inapplicable and one has to resort to alternative approaches. A simple starting point is the field ionization, Coulomb explosion model. This model is partially successful in explaining the ionization and subsequent fragmentation of the heavy molecule, I2, exposed to short (sub-100 fs) laser pulses, but even here the fine details are not understood. The dynamics of the fragmentation of the lighter molecules such as N2 and CO are not understood at all; one complication is that the time scales of dissociation and stepwise ionization overlap. In the case of the simplest molecular ion, H+ 2, there has been some progress, leading to the tentative conclusion that bond softening and laser-induced bound states will be important ingredients of a successful theory of multielectron dissociative ionization. 相似文献
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Hod S 《Physical review letters》2008,100(12):121101
Spacetime singularities that arise in gravitational collapse are always hidden inside of black holes. This is the essence of the weak cosmic censorship conjecture. The hypothesis, put forward by Penrose 40 years ago, is still one of the most important open questions in general relativity. In this Letter, we reanalyze extreme situations which have been considered as counterexamples to the weak cosmic censorship conjecture. In particular, we consider the absorption of scalar particles with large angular momentum by a black hole. Ignoring back reaction effects may lead one to conclude that the incident wave may overspin the black hole, thereby exposing its inner singularity to distant observers. However, we show that when back reaction effects are properly taken into account, the stability of the black-hole event horizon is irrefutable. We therefore conclude that cosmic censorship is actually respected in this type of gedanken experiments. 相似文献
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Karl Popper 《Foundations of Physics》1982,12(10):971-976
The rational understanding of the quantum theory achieved by de Broglie is contrasted with the Copenhagen emphasis on the quantum postulate with its inherent irrationality (Niels Bohr, Ref. 1). 相似文献
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A second spectrum of frequencies was reported in early analytical work on the vibrations of Timoshenko beams. However, in subsequent finite element modelling this phenomenon was either ignored or not definitively classified and recorded. In fact, from a recent finite element analysis with a high precision element it was even concluded that there is no separate second spectrum of frequencies except for the special case of hinged-hinged beams and it was asserted that previous investigators had misinterpreted some frequencies thus introducing the notion of second frequencies. In this paper, a simple linear beam element with independent displacement fields and reduced integration to eliminate shear locking is used and enables one to detect the second spectrum accurately. Guidelines are provided which help to identify and classify the frequencies into two separate spectra. 相似文献
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To study the effect of the shape of molecules on properties of polar fluids, the second pressure virial coefficient,B(T), of quadrupolar hard homonuclear dumbells is investigated both numerically and analytically. Convergence of the expansion ofB(T) about the hard dumbell reference is examined and a simple approximate analytic formula is derived which provides a good estimate ofB(T) for temperatures higher than the Boyle temperature,T
B. BelowT
B the examined expansion methods break down. Independently of the calculations ofB(T), an approximate analytic expression of the average Boltzmann factor of the model considered has been also derived. 相似文献
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T. Padmanabhan 《General Relativity and Gravitation》2008,40(10):2031-2036
It is well known that one could determine the kinematics of gravity by using the Principle of Equivalence and local inertial frames. I describe how the dynamics of gravity can be similarly understood by suitable thought experiments in a local Rindler frame. This approach puts in proper context several unexplained features of gravity and describes the dynamics of spacetime in a broader setting than in Einstein’s theory. First Award in the 2008 Essay Competition of the Gravity Research Foundation. 相似文献
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V.V. Ilyushin E.A. Alekseev S.F. Dyubko I. Kleiner 《Journal of Molecular Spectroscopy》2003,220(2):170-186
We have fitted to within experimental accuracy a data set for acetic acid CH3COOH consisting of 2103 vt=0, 1111 vt=1 and 634 vt=2 microwave lines, using the so-called “rho axis method” and a model extended to include perturbation terms through eighth order. The previously published 2109 vt=0 and 409 vt=1 microwave lines have been extended by new measurements from Kharkov. The final fit requires only 62 parameters to achieve a unitless weighted standard deviation for the whole fit of 0.85 for a total of 3848 lines and includes transitions with J27 (15) and |Ka|13 (8) for the vt=2 A (E)-species, respectively. This result represents a significant improvement over our previous fit which was dominated by vt=0 transitions. A calculation of the linestrengths of torsion–rotation transitions is also provided. 相似文献