794 research outputs found
Generalizing Planck's distribution by using the Carati-Galgani model of molecular collisions
Classical systems of coupled harmonic oscillators are studied using the
Carati-Galgani model. We investigate the consequences for Einstein's conjecture
by considering that the exchanges of energy, in molecular collisions, follows
the L\'evy type statistics. We develop a generalization of Planck's
distribution admitting that there are analogous relations in the equilibrium
quantum statistical mechanics of the relations found using the nonequilibrium
classical statistical mechanics approach. The generalization of Planck's law
based on the nonextensive statistical mechanics formalism is compatible with
the our analysis.Comment: 10 pages, 3 figure
On the definition of temperature using time--averages
This paper is a natural continuation of a previous one by the author, which
was concerned with the foundations of statistical thermodynamics far from
equilibrium. One of the problems left open in that paper was the correct
definition of temperature. In the literature, temperature is in general defined
through the mean kinetic energy of the particles of a given system. In this
paper, instead, temperature is defined "a la Caratheodory", the system being
coupled to a heat bath, and temperature being singled out as the ``right''
integrating factor of the exchanged heat. As a byproduct, the ``right''
expression for the entropy is also obtained. In particular, in the case of a
q-distributions the entropy turns out to be that of Tsallis, which we however
show to be additive, at variance with what is usually maintained
Chopping Time of the FPU alpha-Model
We study, both numerically and analytically, the time needed to
observe the breaking of an FPU \u3b1-chain in two or more pieces, starting
from an unbroken configuration at a given temperature. It is found
that such a \u201cchopping\u201d time is given by a formula that, at low temperatures, is of the Arrhenius-Kramers form, so that the chain does
not break up on an observable time-scale. The result explains why the
study of the FPU problem is meaningful also in the ill-posed case of
the \u3b1-model
Classical microscopic theory of dispersion, emission and absorption of light in dielectrics
This paper is a continuation of a recent one in which, apparently for the
first time, the existence of polaritons in ionic crystals was proven in a
microscopic electrodynamic theory. This was obtained through an explicit
computation of the dispersion curves. Here the main further contribution
consists in studying electric susceptibility, from which the spectrum can be
inferred. We show how susceptibility is obtained by the Green--Kubo methods of
Hamiltonian statistical mechanics, and give for it a concrete expression in
terms of time--correlation functions. As in the previous paper, here too we
work in a completely classical framework, in which the electrodynamic forces
acting on the charges are all taken into account, both the retarded forces and
the radiation reaction ones. So, in order to apply the methods of statistical
mechanics, the system has to be previously reduced to a Hamiltonian one. This
is made possible in virtue of two global properties of classical
electrodynamics, namely, the Wheeler--Feynman identity and the Ewald
resummation properties, the proofs of which were already given for ordered
system. The second contribution consists in formulating the theory in a
completely general way, so that in principle it applies also to disordered
systems such as glasses, or liquids or gases, provided the two general
properties mentioned above continue to hold. A first step in this direction is
made here by providing a completely general proof of the Wheeler--Feynman
identity, which is shown to be the counterpart of a general causality property
of classical electrodynamics. Finally it is shown how a line spectrum can
appear at all in classical systems, as a counterpart of suitable stability
properties of the motions, with a broadening due to a coexistence of
chaoticity
On the definition of temperature in FPU systems
It is usually assumed, in classical statistical mechanics, that the
temperature should coincide, apart from a suitable constant factor, with the
mean kinetic energy of the particles. We show that this is not the case for
\FPU systems, in conditions in which energy equipartition between the modes is
not attained. We find that the temperature should be rather identified with the
mean value of the energy of the low frequency modes.Comment: 12 pages, 4 Figure
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