482 research outputs found
Modified two-potential approach to tunneling problems
One-body quantum tunneling to continuum is treated via the two-potential
approach, dividing the tunneling potential into external and internal parts. We
show that corrections to this approach can be minimized by taking the
separation radius inside the interval determined by simple expressions. The
resulting two-potential approach reproduces the resonance energy and its width,
both for narrow and wide resonances. We also demonstrate that, without losing
its accuracy, the two-potential approach can be modified to a form resembling
the R-matrix theory, yet without any uncertainties of the latter related to the
choice of the matching radius.Comment: 7 two-column pages, 3 figures, extra-explanation added, Phys. Rev. A,
in pres
Gauged WZW models for space-time groups and gravitational actions
In this paper we investigate gauged Wess-Zumino-Witten models for space-time
groups as gravitational theories, following the trend of recent work by
Anabalon, Willison and Zanelli. We discuss the field equations in any dimension
and study in detail the simplest case of two space-time dimensions and gauge
group SO(2,1). For this model we study black hole solutions and we calculate
their mass and entropy which resulted in a null value for both.Comment: 26 pages, no figure
Real Time Relativity: exploration learning of special relativity
Real Time Relativity is a computer program that lets students fly at
relativistic speeds though a simulated world populated with planets, clocks,
and buildings. The counterintuitive and spectacular optical effects of
relativity are prominent, while systematic exploration of the simulation allows
the user to discover relativistic effects such as length contraction and the
relativity of simultaneity. We report on the physics and technology
underpinning the simulation, and our experience using it for teaching special
relativity to first year university students
Energy Extraction From Gravitational Collapse to Static Black Holes
The mass--energy formula of black holes implies that up to 50% of the energy
can be extracted from a static black hole. Such a result is reexamined using
the recently established analytic formulas for the collapse of a shell and
expression for the irreducible mass of a static black hole. It is shown that
the efficiency of energy extraction process during the formation of the black
hole is linked in an essential way to the gravitational binding energy, the
formation of the horizon and the reduction of the kinetic energy of implosion.
Here a maximum efficiency of 50% in the extraction of the mass energy is shown
to be generally attainable in the collapse of a spherically symmetric shell:
surprisingly this result holds as well in the two limiting cases of the
Schwarzschild and extreme Reissner-Nordstr\"{o}m space-times. Moreover, the
analytic expression recently found for the implosion of a spherical shell onto
an already formed black hole leads to a new exact analytic expression for the
energy extraction which results in an efficiency strictly less than 100% for
any physical implementable process. There appears to be no incompatibility
between General Relativity and Thermodynamics at this classical level.Comment: 7 pages, 2 figures, to appear on Int. Journ. Mod. Phys.
B2FH, the Cosmic Microwave Background and Cosmology
In this talk I shall start by describing how we set about and carried out the
work which led to the publication of B2FH in 1957. I then shall try and relate
this work and the circumstances that surrounded it to the larger problem of the
origin and formation of the universe. Here it is necessary to look back at the
way that ideas developed and how in many situations astronomers went astray. Of
course this is a personal view, though I very strongly believe that if he were
still here, it is the approach that Fred Hoyle would take.
I start by describing the problems originally encountered by Gamow and his
associates in trying to decide where the helium was made. This leads me to a
modern discussion of the origin of 2D, 3He, 4He and 7Li, originally described
by B2FH as due to the x-process. While it is generally argued, following Gamow,
Alpher, and Herman, that these isotopes were synthesized in a big bang I shall
show that it is equally likely that these isotopes were made in active galactic
nuclei, as was the cosmic microwave background (CMB), in a cyclic universe
model. The key piece of observational evidence is that the amount of energy
carried by the CMB, namely about 4.5 x 10-13 erg cm-3Comment: 6 pages, conference; July 24, 2008 replacement - corrected typo in
titl
Student experiences of virtual reality - a case study in learning special relativity
We present a study of student learning through the use of virtual reality. A
software package is used to introduce concepts of special relativity to
students in a game-like environment where users experience the effects of
travelling at near light speeds. From this new perspective, space and time are
significantly different to that experienced in everyday life. The study
explores how students have worked with this environment and how these students
have used this experience in their study of special relativity. A mixed method
approach has been taken to evaluate the outcomes of separate implementations of
the package at two universities. Students found the simulation to be a positive
learning experience and described the subject area as being less abstract after
its use. Also, students were more capable of correctly answering concept
questions relating to special relativity, and a small but measurable
improvement was observed in the final exam
Decay process accelerated by tunneling in its very early stage
We examine a fast decay process that arises in the transition period between
the Gaussian and exponential decay processes in quantum decay systems. It is
usually expected that the decay is decelerated by a confinement potential
barrier. However, we find a case where the decay in the transition period is
accelerated by tunneling through a confinement potential barrier. We show that
the acceleration gives rise to an appreciable effect on the time evolution of
the nonescape probability of the decay system.Comment: 4 pages, 6 figures; accepted for publication in Phys. Rev.
Majorana and the quasi-stationary states in Nuclear Physics
A complete theoretical model describing artificial disintegration of nuclei
by bombardment with alpha-particles, developed by Majorana as early as in 1930,
is discussed in detail alongside the basic experimental evidences that
motivated it. By following the quantum dynamics of a state resulting from the
superposition of a discrete state with a continuum one, whose interaction is
described by a given potential term, Majorana obtained (among the other
predictions) the explicit expression for the integrated cross section of the
nuclear process, which is the direct measurable quantity of interest in the
experiments. Though this is the first application of the concept of
quasi-stationary states to a Nuclear Physics problem, it seems also that the
unpublished Majorana's work anticipates by several years the related seminal
paper by Fano on Atomic Physics.Comment: latex, amsart, 13 page
- …