4,571 research outputs found
Uniformly Accelerated Mirrors. Part 1: Mean Fluxes
The Davies-Fulling model describes the scattering of a massless field by a
moving mirror in 1+1 dimensions. When the mirror travels under uniform
acceleration, one encounters severe problems which are due to the infinite blue
shift effects associated with the horizons. On one hand, the Bogoliubov
coefficients are ill-defined and the total energy emitted diverges. On the
other hand, the instantaneous mean flux vanishes. To obtained well-defined
expressions we introduce an alternative model based on an action principle. The
usefulness of this model is to allow to switch on and off the interaction at
asymptotically large times. By an appropriate choice of the switching function,
we obtain analytical expressions for the scattering amplitudes and the fluxes
emitted by the mirror. When the coupling is constant, we recover the vanishing
flux. However it is now followed by transients which inevitably become singular
when the switching off is performed at late time. Our analysis reveals that the
scattering amplitudes (and the Bogoliubov coefficients) should be seen as
distributions and not as mere functions. Moreover, our regularized amplitudes
can be put in a one to one correspondence with the transition amplitudes of an
accelerated detector, thereby unifying the physics of uniformly accelerated
systems. In a forthcoming article, we shall use our scattering amplitudes to
analyze the quantum correlations amongst emitted particles which are also
ill-defined in the Davies-Fulling model in the presence of horizons.Comment: 23 pages, 7 postscript figure
From Vacuum Fluctuations to Radiation: Accelerated Detectors and Black Holes
The vacuum fluctuations that induce the transitions and the thermalisation of
a uniformly accelerated two level atom are studied in detail. Their energy
content is revealed through the weak measurement formalism of Aharonov et al.
It is shown that each time the detector makes a transition it radiates a
Minkowski photon. The same analysis is then applied to the conversion of vacuum
fluctuations into real quanta in the context of black hole radiation. Initially
these fluctuations are located around the light like geodesic that shall
generate the horizon and carry zero total energy. However upon exiting from the
star they break up into two pieces one of which gradually acquires positive
energy and becomes a Hawking quantum, the other, its ''partner", ends up in the
singularity. As time goes by the vacuum fluctuations generating Hawking quanta
have exponentially large energy densities. This implies that back reaction
effects are large.Comment: definitive version, 39 pages and 5 figures available upon request
from S.M., ULB-TH 94/0
A simulation study of curved, descending, decelerating, landing approaches for transport aircraft
A system which is capable of controlling an aircraft automatically along a curved, descending, decelerating approach was described. A simulation study was conducted to determine the necessary modifications to the basic flight-proven control system. This basic system is presently being used to accomplish straight-in automatic landing approaches on a short-haul transport aircraft (B-737 terminal configured vehicle). This study shows that both 3 deg (normal) and 5 deg (steep) approaches could be accomplished with only minor modifications to the basic control system
Biochemical correlates of cardiac hypertrophy. IV. Observations on the cellular organization of growth during myocardial hypertrophy in the rat
The mechanisms by which the DNA content of the heart increases following acutely induced cardiac hypertrophy were investigated in mature Sprague-Dawley rats. Special attention was given to the cellular organization of the growth process. Autoradiographic studies provided conclusive evidence that the uptake of tritiated thymidine is completely limited to nonmuscular cellular elements, chiefly connective tissue cells. The frequency of labeled nuclei was increased by sixfold during hypertrophy. The thymidine pool was not appreciably different in the hypertrophied hearts. Connective tissue nuclei formed a larger proportion of the total nuclear population in hypertrophied hearts, and their distribution was less uniform than in the normal heart. Quantitative histologic studies also showed that the total number of left ventricular muscle cell nuclei did not increase during hypertrophy but rather may have decreased slightly. Both the concentration and the total amount of hydroxyproline increased in parallel with the proliferative changes in the connective tissue and provide further supportive evidence to the autoradiographic and histologic studies
Hawking Radiation from Feynman Diagrams
The aim of this letter is to clarify the relationships between Hawking
radiation and the scattering of light by matter falling into a black hole. To
this end we analyze the S-matrix elements of a model composed of a massive
infalling particle (described by a quantized field) and the radiation field.
These fields are coupled by current-current interactions and propagate in the
Schwarzschild geometry. As long as the photons energy is much smaller than the
mass of the infalling particle, one recovers Hawking radiation since our
S-matrix elements identically reproduce the Bogoliubov coefficients obtained by
treating the trajectory of the infalling particle classically. But after a
brief period, the energy of the `partners' of Hawking photons reaches this mass
and the production of thermal photons through these interactions stops. The
implications of this result are discussed.Comment: 12 pages, revtex, no figure
Radiation from a uniformly accelerating harmonic oscillator
We consider a radiation from a uniformly accelerating harmonic oscillator
whose minimal coupling to the scalar field changes suddenly. The exact time
evolutions of the quantum operators are given in terms of a classical solution
of a forced harmonic oscillator. After the jumping of the coupling constant
there occurs a fast absorption of energy into the oscillator, and then a slow
emission follows. Here the absorbed energy is independent of the acceleration
and proportional to the log of a high momentum cutoff of the field. The emitted
energy depends on the acceleration and also proportional to the log of the
cutoff. Especially, if the coupling is comparable to the natural frequency of
the detector () enormous energies are radiated away
from the oscillator.Comment: 26 pages, 1 eps figure, RevTeX, minor correction in grammar, add a
discussio
High Energy Break and Reflection Features in the Seyfert Galaxy MCG+8-11-11
We present the results from ASCA and OSSE simultaneous observations of the
Seyfert 1.5 galaxy MCG+8-11-11 performed in August-September 1995. The ASCA
observations indicate a modest flux increase (20%) in 3 days, possibly
correlated to a softening of the 0.6-9 keV spectrum. The spectrum is well
described by a hard power law (Gamma=1.64) absorbed by a column density
slightly larger than the Galactic value, with an iron line at 6.4 keV of EW=400
eV. The simultaneous OSSE data are characterized by a much softer power law
with photon index Gamma=3.0, strongly suggesting the presence of a spectral
break in the hard X/soft gamma-ray band. A joint fit to OSSE and ASCA data
clearly shows an exponential cut-off at about 300 keV, and strong reflection
component. MCG+8-11-11 features a spectral break in the underlying continuum
unambiguously. This, together with the inferred low compactness of this source,
favours thermal or quasi-thermal electron Comptonization in a structured Corona
as the leading process of high energy radiation production.Comment: 13 pages, + 4 figure.ps AAS LateX [11pt,aasms4]{article} To be
published in ApJ, Main Journa
Domain wall dynamics in expanding spaces
We study the effects on the dynamics of kinks due to expansions and
contractions of the space. We show that the propagation velocity of the kink
can be adiabatically tuned through slow expansions/contractions, while its
width is given as a function of the velocity. We also analyze the case of fast
expansions/contractions, where we are no longer on the adiabatic regime. In
this case the kink moves more slowly after an expansion-contraction cycle as a
consequence of loss of energy through radiation. All these effects are
numerically studied in the nonlinear Klein-Gordon equations (both for the
sine-Gordon and for the phi^4 potential), and they are also studied within the
framework of the collective coordinate evolution equations for the width and
the center of mass of the kink. These collective coordinate evolution equations
are obtained with a procedure that allows us to consider even the case of large
expansions/contractions.Comment: LaTeX, 18 pages, 2 figures, improved version to appear in Phys Rev
- …