635 research outputs found
Neural dynamics of illusory tactile pulling sensations
Directional tactile pulling sensations are integral to everyday life, but their neural mechanisms remain unknown. Prior accounts hold that primary somatosensory (SI) activity is sufficient to generate pulling sensations, with alternative proposals suggesting that amodal frontal or parietal regions may be critical. We combined high-density EEG with asymmetric vibration, which creates an illusory pulling sensation, thereby unconfounding pulling sensations from unrelated sensorimotor processes. Oddballs that created opposite direction pulls to common stimuli were compared to the same oddballs after neutral common stimuli (symmetric vibration) and to neutral oddballs. We found evidence against the sensory-frontal N140 and in favor of the midline P200 tracking the emergence of pulling sensations, specifically contralateral parietal lobe activity 264-320ms, centered on the intraparietal sulcus. This suggests that SI is not sufficient to generate pulling sensations, which instead depend on the parietal association cortex, and may reflect the extraction of orientation information and related spatial processing
Covariant Equilibrium Statistical Mechanics
A manifest covariant equilibrium statistical mechanics is constructed
starting with a 8N dimensional extended phase space which is reduced to the 6N
physical degrees of freedom using the Poincare-invariant constrained
Hamiltonian dynamics describing the micro-dynamics of the system. The reduction
of the extended phase space is initiated forcing the particles on energy shell
and fixing their individual time coordinates with help of invariant time
constraints. The Liouville equation and the equilibrium condition are
formulated in respect to the scalar global evolution parameter which is
introduced by the time fixation conditions. The applicability of the developed
approach is shown for both, the perfect gas as well as the real gas. As a
simple application the canonical partition integral of the monatomic perfect
gas is calculated and compared with other approaches. Furthermore,
thermodynamical quantities are derived. All considerations are shrinked on the
classical Boltzmann gas composed of massive particles and hence quantum effects
are discarded.Comment: 22 pages, 1 figur
Energy in Generic Higher Curvature Gravity Theories
We define and compute the energy of higher curvature gravity theories in
arbitrary dimensions. Generically, these theories admit constant curvature
vacua (even in the absence of an explicit cosmological constant), and
asymptotically constant curvature solutions with non-trivial energy properties.
For concreteness, we study quadratic curvature models in detail. Among them,
the one whose action is the square of the traceless Ricci tensor always has
zero energy, unlike conformal (Weyl) gravity. We also study the string-inspired
Einstein-Gauss-Bonnet model and show that both its flat and Anti-de-Sitter
vacua are stable.Comment: 18 pages, typos corrected, one footnote added, to appear in Phys.
Rev.
Electromagnetic self-forces and generalized Killing fields
Building upon previous results in scalar field theory, a formalism is
developed that uses generalized Killing fields to understand the behavior of
extended charges interacting with their own electromagnetic fields. New notions
of effective linear and angular momenta are identified, and their evolution
equations are derived exactly in arbitrary (but fixed) curved spacetimes. A
slightly modified form of the Detweiler-Whiting axiom that a charge's motion
should only be influenced by the so-called "regular" component of its
self-field is shown to follow very easily. It is exact in some interesting
cases, and approximate in most others. Explicit equations describing the
center-of-mass motion, spin angular momentum, and changes in mass of a small
charge are also derived in a particular limit. The chosen approximations --
although standard -- incorporate dipole and spin forces that do not appear in
the traditional Abraham-Lorentz-Dirac or Dewitt-Brehme equations. They have,
however, been previously identified in the test body limit.Comment: 20 pages, minor typos correcte
Comparing scalar-tensor gravity and f(R)-gravity in the Newtonian limit
Recently, a strong debate has been pursued about the Newtonian limit (i.e.
small velocity and weak field) of fourth order gravity models. According to
some authors, the Newtonian limit of -gravity is equivalent to the one of
Brans-Dicke gravity with , so that the PPN parameters of these
models turn out to be ill defined. In this paper, we carefully discuss this
point considering that fourth order gravity models are dynamically equivalent
to the O'Hanlon Lagrangian. This is a special case of scalar-tensor gravity
characterized only by self-interaction potential and that, in the Newtonian
limit, this implies a non-standard behavior that cannot be compared with the
usual PPN limit of General Relativity.
The result turns out to be completely different from the one of Brans-Dicke
theory and in particular suggests that it is misleading to consider the PPN
parameters of this theory with in order to characterize the
homologous quantities of -gravity. Finally the solutions at Newtonian
level, obtained in the Jordan frame for a -gravity, reinterpreted as a
scalar-tensor theory, are linked to those in the Einstein frame.Comment: 9 page
Locality hypothesis and the speed of light
The locality hypothesis is generally considered necessary for the study of
the kinematics of non-inertial systems in special relativity. In this paper we
discuss this hypothesis, showing the necessity of an improvement, in order to
get a more clear understanding of the various concepts involved, like
coordinate velocity and standard velocity of light. Concrete examples are
shown, where these concepts are discussed.Comment: 23 page
An axiomatic approach to electromagnetic and gravitational radiation reaction of particles in curved spacetime
The problem of determining the electromagnetic and gravitational
``self-force'' on a particle in a curved spacetime is investigated using an
axiomatic approach. In the electromagnetic case, our key postulate is a
``comparison axiom'', which states that whenever two particles of the same
charge have the same magnitude of acceleration, the difference in their
self-force is given by the ordinary Lorentz force of the difference in their
(suitably compared) electromagnetic fields. We thereby derive an expression for
the electromagnetic self-force which agrees with that of DeWitt and Brehme as
corrected by Hobbs. Despite several important differences, our analysis of the
gravitational self-force proceeds in close parallel with the electromagnetic
case. In the gravitational case, our final expression for the (reduced order)
equations of motion shows that the deviation from geodesic motion arises
entirely from a ``tail term'', in agreement with recent results of Mino et al.
Throughout the paper, we take the view that ``point particles'' do not make
sense as fundamental objects, but that ``point particle equations of motion''
do make sense as means of encoding information about the motion of an extended
body in the limit where not only the size but also the charge and mass of the
body go to zero at a suitable rate. Plausibility arguments for the validity of
our comparison axiom are given by considering the limiting behavior of the
self-force on extended bodies.Comment: 37 pages, LaTeX with style package RevTeX 3.
On the motion of a classical charged particle
We show that the Lorentz-Dirac equation is not an unavoidable consequence of
energy-momentum conservation for a point charge. What follows solely from
conservation laws is a less restrictive equation already obtained by Honig and
Szamosi. The latter is not properly an equation of motion because, as it
contains an extra scalar variable, it does not determine the future evolution
of the charge. We show that a supplementary constitutive relation can be added
so that the motion is determined and free from the troubles that are customary
in Lorentz-Dirac equation, i. e. preacceleration and runaways
Comparative analysis of policy-mixes of research and innovation policies in Central and Eastern European countries
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