200 research outputs found
Two-integral distribution functions for axisymmetric galaxies
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
Time as an Illusion
We review the idea, due to Einstein, Eddington, Hoyle and Ballard, that time
is a subjective label, whose primary purpose is to order events, perhaps in a
higher-dimensional universe. In this approach, all moments in time exist
simultaneously, but they are ordered to create the illusion of an unfolding
experience by some physical mechanism. This, in the language of relativity, may
be connected to a hypersurface in a world that extends beyond spacetime. Death
in such a scenario may be merely a phase change
On "New Massive" 4D Gravity
We construct a four-dimensional (4D) gauge theory that propagates, unitarily,
the five polarization modes of a massive spin-2 particle. These modes are
described by a "dual" graviton gauge potential and the Lagrangian is 4th-order
in derivatives. As the construction mimics that of 3D "new massive gravity", we
call this 4D model (linearized) "new massive dual gravity". We analyse its
massless limit, and discuss similarities to the Eddington-Schroedinger model.Comment: 17 pages, v2 : version published in JHE
Micro-Electro-Mechanical-Systems (MEMS) and Fluid Flows
The micromachining technology that emerged in the late 1980s can provide micron-sized sensors and actuators. These micro transducers are able to be integrated with signal conditioning and processing circuitry to form micro-electro-mechanical-systems (MEMS) that can perform real-time distributed control. This capability opens up a new territory for flow control research. On the other hand, surface effects dominate the fluid flowing through these miniature mechanical devices because of the large surface-to-volume ratio in micron-scale configurations. We need to reexamine the surface forces in the momentum equation. Owing to their smallness, gas flows experience large Knudsen numbers, and therefore boundary conditions need to be modified. Besides being an enabling technology, MEMS also provide many challenges for fundamental flow-science research
Time-Distance Helioseismology of Deep Meridional Circulation
A key component of solar interior dynamics is the meridional circulation
(MC), whose poleward component in the surface layers has been well observed.
Time-distance helioseismic studies of the deep structure of MC, however, have
yielded conflicting inferences. Here, following a summary of existing results
we show how a large center-to-limb systematics (CLS) in the measured travel
times of acoustic waves affect the inferences through an analysis of frequency
dependence of CLS, using data from the Helioseismic and Doppler Imager (HMI)
onboard Solar Dynamics Observatory (SDO). Our results point to the residual
systematics in travel times as a major cause of differing inferences on the
deep structure of MC.Comment: 6 pages, 3 figures, to appear in the Springer series Astrophysics and
Space Science Proceedings of "Dynamics of the Sun & Stars: Honoring the Life
& Work of Michael Thompson" (2020
f(R) Gravities, Killing Spinor Equations, "BPS" Domain Walls and Cosmology
We derive the condition on f(R) gravities that admit Killing spinor equations
and construct explicit such examples. The Killing spinor equations can be used
to reduce the fourth-order differential equations of motion to the first order
for both the domain wall and FLRW cosmological solutions. We obtain exact "BPS"
domain walls that describe the smooth Randall-Sundrum II, AdS wormholes and the
RG flow from IR to UV. We also obtain exact smooth cosmological solutions that
describe the evolution from an inflationary starting point with a larger
cosmological constant to an ever-expanding universe with a smaller cosmological
constant. In addition, We find exact smooth solutions of pre-big bang models,
bouncing or crunching universes. An important feature is that the scalar
curvature R of all these metrics is varying rather than a constant. Another
intriguing feature is that there are two different f(R) gravities that give
rise to the same "BPS" solution. We also study linearized f(R) gravities in
(A)dS vacua.Comment: 37 pages, discussion on gravity trapping in RSII modified, typos
corrected, further comments and references added; version to appear in JHE
Nonlocal observables and lightcone-averaging in relativistic thermodynamics
The unification of relativity and thermodynamics has been a subject of
considerable debate over the last 100 years. The reasons for this are twofold:
(i) Thermodynamic variables are nonlocal quantities and, thus, single out a
preferred class of hyperplanes in spacetime. (ii) There exist different,
seemingly equally plausible ways of defining heat and work in relativistic
systems. These ambiguities led, for example, to various proposals for the
Lorentz transformation law of temperature. Traditional 'isochronous'
formulations of relativistic thermodynamics are neither theoretically
satisfactory nor experimentally feasible. Here, we demonstrate how these
deficiencies can be resolved by defining thermodynamic quantities with respect
to the backward-lightcone of an observation event. This approach yields novel,
testable predictions and allows for a straightforward-extension of
thermodynamics to General Relativity. Our theoretical considerations are
illustrated through three-dimensional relativistic many-body simulations.Comment: typos in Eqs. (12) and (14) corrected, minor additions in the tex
The Reionization of the Universe by the First Stars and Quasars
The first light from stars and quasars ended the ``dark ages'' of the
universe and led to the reionization of hydrogen by redshift 7. Current
observations are at the threshold of probing this epoch. The study of
high-redshift sources is likely to attract major attention in observational and
theoretical cosmology over the next decade.Comment: 60 pages, including 21 figures; to be published in the 2001 Volume of
Annual Reviews of Astronomy and Astrophysics; A more extensive review, for
Physics Reports, is also available, with a different astro-ph number, or at
http://www.cita.utoronto.ca/~barkana/review.htm
Symmetry, Reference Frames, and Relational Quantities in Quantum Mechanics
We propose that observables in quantum theory are properly understood as representatives of symmetry-invariant quantities relating one system to another, the latter to be called a reference system. We provide a rigorous mathematical language to introduce and study quantum reference systems, showing that the orthodox "absolute" quantities are good representatives of observable relative quantities if the reference state is suitably localised. We use this relational formalism to critique the literature on the relationship between reference frames and superselection rules, settling a long-standing debate on the subject
The evolution of rotating stars
First, we review the main physical effects to be considered in the building
of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The
internal rotation law evolves as a result of contraction and expansion,
meridional circulation, diffusion processes and mass loss. In turn,
differential rotation and mixing exert a feedback on circulation and diffusion,
so that a consistent treatment is necessary.
We review recent results on the evolution of internal rotation and the
surface rotational velocities for stars on the Upper MS, for red giants,
supergiants and W-R stars. A fast rotation is enhancing the mass loss by
stellar winds and reciprocally high mass loss is removing a lot of angular
momentum. The problem of the ``break-up'' or -limit is critically
examined in connection with the origin of Be and LBV stars. The effects of
rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the
blue to red supergiant ratios, the formation of W-R stars, the chemical
abundances in massive stars as well as in red giants and AGB stars, are
reviewed in relation to recent observations for stars in the Galaxy and
Magellanic Clouds. The effects of rotation on the final stages and on the
chemical yields are examined, as well as the constraints placed by the periods
of pulsars. On the whole, this review points out that stellar evolution is not
only a function of mass M and metallicity Z, but of angular velocity
as well.Comment: 78 pages, 7 figures, review for Annual Review of Astronomy and
Astrophysics, vol. 38 (2000
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