2,745 research outputs found
Transcription and the Pitch Angle of DNA
The question of the value of the pitch angle of DNA is visited from the
perspective of a geometrical analysis of transcription. It is suggested that
for transcription to be possible, the pitch angle of B-DNA must be smaller than
the angle of zero-twist. At the zero-twist angle the double helix is maximally
rotated and its strain-twist coupling vanishes. A numerical estimate of the
pitch angle for B-DNA based on differential geometry is compared with numbers
obtained from existing empirical data. The crystallographic studies shows that
the pitch angle is approximately 38 deg., less than the corresponding
zero-twist angle of 41.8 deg., which is consistent with the suggested principle
for transcription.Comment: 7 pages, 3 figures; v2: minor modifications; v3: major modifications
compared to v2. Added discussion about transcription, and reference
Complementarity and Scientific Rationality
Bohr's interpretation of quantum mechanics has been criticized as incoherent
and opportunistic, and based on doubtful philosophical premises. If so Bohr's
influence, in the pre-war period of 1927-1939, is the harder to explain, and
the acceptance of his approach to quantum mechanics over de Broglie's had no
reasonable foundation. But Bohr's interpretation changed little from the time
of its first appearance, and stood independent of any philosophical
presuppositions. The principle of complementarity is itself best read as a
conjecture of unusually wide scope, on the nature and future course of
explanations in the sciences (and not only the physical sciences). If it must
be judged a failure today, it is not because of any internal inconsistency.Comment: 29 page
Transient effects on electron spin observation
In an earlier publication we addressed the problem of splitting an electron beam in the Stern-Gerlach experiment. In contrast to arguments put forward in the early days of quantum theory, we concluded that there are no issues of principle preventing the observation of electron spin during free flight. In that paper, however, we considered only a sudden switch off of the separating magnetic field. In this work we consider the possible effects of finite switching times at the beginning and the end of the interaction period. We consider a model where the coupling between the electron and the field is time dependent. As a result of the time dependence, the field also acquires an electric component, but this seems to cause no significant change of our conclusions. On the other hand, the smooth change of the interaction enforces the same longitudinal velocity on the electron both at the beginning and end of the interaction period because of conservation laws; this effect was missing in our earlier calculations. As the electrons are supposed to travel as a beam, this feature helps by restoring the beam quality after the interaction
A simple description of the states and in
A sixth-order quadrupole boson Hamiltonian is used to describe 26 states
and 67 states which have been recently identified in .
Two closed expressions are alternatively used for energy levels. One
corresponds to a semi-classical approach while the other one represents the
exact eigenvalue of the model Hamiltonian. The semi-classical expression
involves four parameters, while the exact eigenvalue is determined by five
parameters. In each of the two descriptions a least square fit procedure is
adopted.
Both expressions provide a surprisingly good agreement with the experimental
data.Comment: 9 pages, 5 figure
Objective properties from subjective quantum states: Environment as a witness
We study the emergence of objective properties in open quantum systems. In
our analysis, the environment is promoted from a passive role of reservoir
selectively destroying quantum coherence, to an active role of amplifier
selectively proliferating information about the system. We show that only
preferred pointer states of the system can leave a redundant and therefore
easily detectable imprint on the environment. Observers who--as it is almost
always the case--discover the state of the system indirectly (by probing a
fraction of its environment) will find out only about the corresponding pointer
observable. Many observers can act in this fashion independently and without
perturbing the system: they will agree about the state of the system. In this
operational sense, preferred pointer states exist objectively.Comment: 5 pages, 1 figure, extensive changes, presentation improve
From finite nuclei to the nuclear liquid drop: leptodermous expansion based on the self-consistent mean-field theory
The parameters of the nuclear liquid drop model, such as the volume, surface,
symmetry, and curvature constants, as well as bulk radii, are extracted from
the non-relativistic and relativistic energy density functionals used in
microscopic calculations for finite nuclei. The microscopic liquid drop energy,
obtained self-consistently for a large sample of finite, spherical nuclei, has
been expanded in terms of powers of A^{-1/3} (or inverse nuclear radius) and
the isospin excess (or neutron-to-proton asymmetry). In order to perform a
reliable extrapolation in the inverse radius, the calculations have been
carried out for nuclei with huge numbers of nucleons, of the order of 10^6. The
Coulomb interaction has been ignored to be able to approach nuclei of arbitrary
sizes and to avoid radial instabilities characteristic of systems with very
large atomic numbers. The main contribution to the fluctuating part of the
binding energy has been removed using the Green's function method to calculate
the shell correction. The limitations of applying the leptodermous expansion to
actual nuclei are discussed. While the leading terms in the macroscopic energy
expansion can be extracted very precisely, the higher-order, isospin-dependent
terms are prone to large uncertainties due to finite-size effects.Comment: 13 pages revtex4, 7 eps figures, submitted to Phys. Rev.
Reply to Comment by Galapon on 'Almost-periodic time observables for bound quantum systems'
In a recent paper [1] (also at http://lanl.arxiv.org/abs/0803.3721), I made
several critical remarks on a 'Hermitian time operator' proposed by Galapon [2]
(also at http://lanl.arxiv.org/abs/quant-ph/0111061).
Galapon has correctly pointed out that remarks pertaining to 'denseness' of
the commutator domain are wrong [3]. However, the other remarks still apply,
and it is further noted that a given quantum system can be a member of this
domain only at a set of times of total measure zero.Comment: 3 page
A new puzzle for random interaction
We continue a series of numerical experiments on many-body systems with
random two-body interactions, by examining correlations in ratios in excitation
energies of yrast = 0, 2, 4, 6, 8 states. Previous studies, limited only to
= 0,2,4 states, had shown strong correlations in boson systems but not
fermion systems. By including states and considering different
scatter plots, strong and realistic correlations appear in both boson and
fermion systems. Such correlations are a challenge to explanations of random
interactions.Comment: 4 pages, 4 figure
Glauber-model analysis of total reaction cross sections for Ne, Mg, Si, and S isotopes with Skyrme-Hartree-Fock densities
A systematic analysis is made on the total reaction cross sections for Ne,
Mg, Si, and S isotopes. The high-energy nucleus-nucleus collision is described
based on the Glauber model. Using the Skyrme-Hartree-Fock method in the
three-dimensional grid-space representation, we determine the nuclear density
distribution for a wide range of nuclei self-consistently without assuming any
spatial symmetry. The calculated total reaction cross sections consistently
agree with the recent cross section data on NeC collision at
240\,MeV, which makes it possible to discuss the radius and deformation of
the isotopes. The total reaction cross sections for MgC, SiC
and SC cases are predicted for future measurements. We also find that
the high-energy cross section data for O, Ne, and Mg isotopes on a C
target at around 1000\,MeV can not be reproduced consistently with the
corresponding data at 240\,MeV.Comment: 10 pages, 14 figure
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