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Ahmad Shariati

Publications and source records attributed to Ahmad Shariati.

At least 19 recordsLinked to original sources
arXiv2026

A Survey through Conformal Time

We revisit conformal time $η$ in a spatially flat Friedmann--Robertson--Walker universe and use a $1+1$-dimensional setting as a technically transparent pedagogical arena. Our purpose is to clarify the relation among cosmic time $t$, conformal time $η$, and the scale factor $a(t)$, and then to follow how this relation governs the geodesics of freely moving particles and the curvature of the corresponding manifold. The radiation-dominated, matter-dominated, and exact vacuum-only de Sitter cases are treated separately, because each of them produces a distinct conformal-time dependence and therefore a distinct geodesic structure. We then write the affine-parameter formalism in a form that is genuinely general for any spatially flat conformal metric, and we record the straightforward extension to the spatially flat $3+1$ case. The presentation remains elementary in spirit, but the notation, the curvature formulas, and the de Sitter interpretation are kept explicit.

gr-qc↗
arXiv2021

Modified Inertia as Nonconservative Newtonian Dynamics

Modified Newtonian dynamics by Milgrom is a paradigm for explaining the rotation curves of spiral galaxies and various other large scale structures. This paradigm includes several different theories. Here we present Milgrom's modified inertia (MI) theory in terms of a simple and tractable non-conservative Newtonian dynamics, which is useful in obtaining observable predictions of MI. It is found that: 1) Modified inertia theory is equivalent to a Newtonian theory, with a non-conservative gravitational field, and dark matter density; 2) The tidal force in the equivalent Newtonian dynamics is non-conservative, and its effect on a binary system in free fall in the gravitational field of a spheroid is addressed. We also discuss attempts to restore conservation in MI.

gr-qc↗
arXiv2016

Decoherence in quantum systems in a static gravitational field

A small quantum system is studied which is a superposition of states localized in different positions in a static gravitational field. The time evolution of the correlation between different positions is investigated, and it is seen that there are two time scales for such an evolution (decoherence). Both time scales are inversely proportional to the red shift difference between the two points. These time scales correspond to decoherences which are linear and quadratic, respectively, in time.

quant-ph↗
arXiv2014

Closedness of orbits in a space with SU(2) Poisson structure

The closedness of orbits of central forces is addressed in a three dimensional space in which the Poisson bracket among the coordinates is that of the SU(2) Lie algebra. In particular it is shown that among problems with spherically symmetric potential energies, it is only the Kepler problem for which all of the bounded orbits are closed. In analogy with the case of the ordinary space, a conserved vector (apart from the angular momentum) is explicitly constructed, which is responsible for the orbits being closed. This is the analog of the Laplace-Runge-Lenz vector. The algebra of the constants of the motion is also worked out.

physics.class-ph↗
arXiv2014

On the relation between the spin and the magnetic moment of the proton

In the context of the quark model of hadrons the spin and the magnetic moment of proton can not be taken proportional. This is in contradiction with the widely used relation between these two properties of the proton. This apparent difficulty is addressed by the most elementary notions of the relevant physics. In particular it is emphasized that the widely used relation is only valid in the lowest orders of perturbation, in which transitions between different baryons do not occur. For other processes where such transitions do occur, such as inelastic scattering off the protons, the quark model relation for the magnetic moment is used to give an estimation for the amplitude of transition between states with different total spins.

hep-ph↗
arXiv2014

Is there a map between Galilean relativity and special relativity?

Mandanici has provided a map which he claims to be a two way map between Galilean relativity and special relativity. We argue that this map is simply a curvilinear coordinate system on a subset of the two-dimensional Minkowski space-time, and is not a two way map between 1+1 dimensional Galilean relativity and 1+1 dimensional special relativity.

gr-qc↗
arXiv2013

Entropy as a measure of diffusion

The time variation of entropy, as an alternative to the variance, is proposed as a measure of the diffusion rate. It is shown that for linear and time-translationally invariant systems having a large-time limit for the density, at large times the entropy tends exponentially to a constant. For systems with no stationary density, at large times the entropy is logarithmic with a coefficient specifying the speed of the diffusion. As an example, the large time behaviors of the entropy and the variance are compared for various types of fractional-derivative diffusions.

cond-mat.stat-mech↗
arXiv2011

Statistical mechanics of free particles on space with Lie type noncommutativity

Effects of Lie type noncommutativity on thermodynamic properties of a system of free identical particles are investigated. A definition for finite volume of the configuration space is given, and the grandcanonical partition function in the thermodynamic limit is calculated. Two possible definitions for the pressure are discussed, which are equivalent when the noncommutativity vanishes. The thermodynamic observables are extracted from the partition function. Different limits are discussed where either the noncommutativity or the quantum effects are important. Finally specific cases are discussed where the group is SU(2) or SO(3), and the partition function of a nondegenerate gas is calculated.

hep-th↗
arXiv2008

Comment on "Learning to Build the Bomb"

According to Alisa Carrigan's opinion in Physics Today Dec 2007, to prevent proliferation of nuclear weapons, certain rules should be set to prevent the spread of a particular kind of knowledge. Here, I argue that a knowledge blockade does not work, and it is not a suitable means to establish a sustainable peace.

physics.soc-ph↗
arXiv2008

Quantum mechanics on space with SU(2) fuzziness

Quantum mechanics of models is considered which are constructed in spaces with Lie algebra type commutation relations between spatial coordinates. The case is specialized to that of the group SU(2), for which the formulation of the problem via the Euler parameterization is also presented. SU(2)-invariant systems are discussed, and the corresponding eigenvalue problem for the Hamiltonian is reduced to an ordinary differential equation, as it is the case with such models on commutative spaces.

hep-th↗
arXiv2008

Poisson structure on a space with linear SU(2) fuzziness

The Poisson structure is constructed for a model in which spatial coordinates of configuration space are noncommutative and satisfy the commutation relations of a Lie algebra. The case is specialized to that of the group SU(2), for which the counterpart of the angular momentum, as well as the Euler parameterization of the phase space are introduced. SU(2)-invariant classical systems are discussed, and it is observed that the path of particle can be obtained by the solution of a first-order equation, as the case with such models on commutative spaces. The examples of free particle, rotationally-invariant potentials, and specially the isotropic harmonic oscillator are investigated in more detail.

hep-th↗
arXiv2008

TeV-photon paradox and space with SU(2) fuzziness

The possibility is examined that a model based on space noncommutativity of linear type can explain why photons from distant sources with multi-TeV energies can reach earth. In particular within a model in which space coordinates satisfy the algebra of SU(2) Lie group, it is shown that there is the possibility that the pair production through the reaction of CMB and energetic photons would be forbidden kinematically.

astro-ph↗
arXiv2007

MOND, dark matter, and conservation of energy

The MOND equation $m \vec a μ(a) = \vec F$ could be transformed to the equivalent form $m \vec a = \vec{F'}$, where $\Vec{F'}$ is a transformed force. Using this transformation we argue that MOND could not avoid introducing dark matter, and introduces nonconservative terms to the equations of motion.

gr-qc↗
arXiv2006

Doubly Special Relativity: A New Relativity or Not?

Double Special Relativity theories are the relativistic theories in which the transformations between inertial observers are characterized by two observer-independent scales of the light speed and the Planck length. We study two main examples of these theories and want to show that these theories are not the new theories of relativity, but only are re-descriptions of Einstein's special relativity in the non-conventional coordinates.

gr-qc↗
arXiv2005

A tight constraint on some varying speed of light theories

We argue that if the velocity of electromagnetic waves in vacuum, \cem, is different from the limiting velocity for massive particles, \cst, then, a proton moving with velocity $\cem < v < \cst$ would radiate Cherenkov radiation. Because we have seen protons with energy $\sim 10^{19}$ eV coming from a distance of order more than 1 Mpc, we can put a constraint on $Δ:= (\cst - \cem)/\cst$: $Δ< 10^{-27}$.

gr-qc↗
arXiv2004

Comments on the Quantum Vacuum and the Light Acceleration

The recent observations on the far quasars absorption lines spectra and comparison of these lines with laboratory ones provide a framework for explantation of these observations by considering a varying fine structure constant, over the cosmological time-scale. Also, there seems to be an anomalous acceleration in the Pioneer spacecraft 10/11 about $ 10^{-10} {\rm m}/{\rm s^2}$. These matters lead Ranada to study the quantum vacuum to explain these problems by introducing a phenomenological model for the variation of $α$. In this manuscript we want to show that this model is not a quantum model; it is a classical model that is only in accordance with mentioned observations by adjusting some parameters and is not based on a fundamental physical intuition.

gr-qc↗