9,327 research outputs found
Identifying cross country skiing techniques using power meters in ski poles
Power meters are becoming a widely used tool for measuring training and
racing effort in cycling, and are now spreading also to other sports. This
means that increasing volumes of data can be collected from athletes, with the
aim of helping coaches and athletes analyse and understanding training load,
racing efforts, technique etc. In this project, we have collaborated with
Skisens AB, a company producing handles for cross country ski poles equipped
with power meters. We have conducted a pilot study in the use of machine
learning techniques on data from Skisens poles to identify which "gear" a skier
is using (double poling or gears 2-4 in skating), based only on the sensor data
from the ski poles. The dataset for this pilot study contained labelled
time-series data from three individual skiers using four different gears
recorded in varied locations and varied terrain. We systematically evaluated a
number of machine learning techniques based on neural networks with best
results obtained by a LSTM network (accuracy of 95% correctly classified
strokes), when a subset of data from all three skiers was used for training. As
expected, accuracy dropped to 78% when the model was trained on data from only
two skiers and tested on the third. To achieve better generalisation to
individuals not appearing in the training set more data is required, which is
ongoing work.Comment: Presented at the Norwegian Artificial Intelligence Symposium 201
Flavor Neutrino Oscillations and Time-Energy Uncertainty Relation
We consider neutrino oscillations as non stationary phenomenon based on
Schrodinger evolution equation and mixed states of neutrinos with definite
flavors. We show that time-energy uncertainty relation plays a crucial role in
neutrino oscillations. We compare neutrino oscillations with
oscillations.Comment: A report at the 2nd Scandinavian Neutrino Workshop, SNOW 2006,
Stockholm, May 2-6, 200
The role of the lattice structure in determining the magnon-mediated interactions between charge carriers doped into a magnetically ordered background
We use two recently proposed methods to calculate exactly the spectrum of two
spin- charge carriers moving in a ferromagnetic background, at zero
temperature, for three types of models. By comparing the low-energy states in
both the one-carrier and the two-carrier sectors, we analyze whether complex
models with multiple sublattices can be accurately described by simpler
Hamiltonians, such as one-band models. We find that while this is possible in
the one-particle sector, the magnon-mediated interactions which are key to
properly describe the two-carrier states of the complex model are not
reproduced by the simpler models. We argue that this is true not just for
ferromagnetic, but also for antiferromagnetic backgrounds. Our results question
the ability of simple one-band models to accurately describe the low-energy
physics of cuprate layers.Comment: 15 pages, 10 figure
Coherent pion production by neutrinos on nuclei
The main part of coherent pion production by neutrinos on nuclei is
essentially determined by PCAC, provided that the leptonic momentum transferred
square Q^2 remains sufficiently small. We give the formulas for the charged and
neutral current cross sections, including also the small non-PCAC transverse
current contributions and taking into account the effect of the \mu^- mass. Our
results are compared with the experimental ones and other theoretical
treatments.Comment: 18 pages, 4 figure
Enhancement of Blackbody Friction due to the Finite Lifetime of Atomic Levels
The thermal friction force acting on an atom moving relative to a thermal
photon bath is known to be proportional to an integral over the imaginary part
of the frequency-dependent atomic (dipole) polarizability. Using a numerical
approach, we find that blackbody friction on atoms either in dilute
environments or in hot ovens is larger than previously thought by orders of
magnitude. This enhancement is due to far off-resonant driving of transitions
by low-frequency thermal radiation. At typical temperatures, the blackbody
radiation maximum lies far below the atomic transition wavelengths.
Surprisingly, due to the finite lifetime of atomic levels, which gives rise to
Lorentzian line profiles, far off-resonant excitation leads to the dominant
contribution to the blackbody friction.Comment: 4 pages; RevTe
The hbar Expansion in Quantum Field Theory
We show how expansions in powers of Planck's constant hbar = h/2\pi can give
new insights into perturbative and nonperturbative properties of quantum field
theories. Since hbar is a fundamental parameter, exact Lorentz invariance and
gauge invariance are maintained at each order of the expansion. The physics of
the hbar expansion depends on the scheme; i.e., different expansions are
obtained depending on which quantities (momenta, couplings and masses) are
assumed to be independent of hbar. We show that if the coupling and mass
parameters appearing in the Lagrangian density are taken to be independent of
hbar, then each loop in perturbation theory brings a factor of hbar. In the
case of quantum electrodynamics, this scheme implies that the classical charge
e, as well as the fine structure constant are linear in hbar. The connection
between the number of loops and factors of hbar is more subtle for bound states
since the binding energies and bound-state momenta themselves scale with hbar.
The hbar expansion allows one to identify equal-time relativistic bound states
in QED and QCD which are of lowest order in hbar and transform dynamically
under Lorentz boosts. The possibility to use retarded propagators at the Born
level gives valence-like wave-functions which implicitly describe the sea
constituents of the bound states normally present in its Fock state
representation.Comment: 8 pages, 1 figure. Version to be published in Phys. Rev.
Path integrals and wavepacket evolution for damped mechanical systems
Damped mechanical systems with various forms of damping are quantized using
the path integral formalism. In particular, we obtain the path integral kernel
for the linearly damped harmonic oscillator and a particle in a uniform
gravitational field with linearly or quadratically damped motion. In each case,
we study the evolution of Gaussian wavepackets and discuss the characteristic
features that help us distinguish between different types of damping. For
quadratic damping, we show that the action and equation of motion of such a
system has a connection with the zero dimensional version of a currently
popular scalar field theory. Furthermore we demonstrate that the equation of
motion (for quadratic damping) can be identified as a geodesic equation in a
fictitious two-dimensional space.Comment: 15 pages, 6 figure
Hardy's argument and successive spin-s measurements
We consider a hidden-variable theoretic description of successive
measurements of non commuting spin observables on a input spin-s state. In this
scenario, the hidden-variable theory leads to a Hardy-type argument that
quantum predictions violate it. We show that the maximum probability of success
of Hardy's argument in quantum theory is , which is more
than in the spatial case.Comment: 7 page
Non-relativistic electron-electron interaction in a Maxwell-Chern-Simons-Proca model endowed with a timelike Lorentz-violating background
A planar Maxwell-Chern-Simons-Proca model endowed with a Lorentz-violating
background is taken as framework to investigate the electron-electron
interaction. The Dirac sector is introduced exhibiting a Yukawa and a minimal
coupling with the scalar and the gauge fields, respectively. The the
electron-electron interaction is then exactly evaluated as the Fourier
transform of the Moller scattering amplitude (carried out in the
non-relativistic limit) for the case of a purely time-like background. The
interaction potential exhibits a totally screened behavior far from the origin
as consequence of massive character of the physical mediators. The total
interaction (scalar plus gauge potential) can always be attractive, revealing
that this model may lead to the formation of electron-electron bound states.Comment: 14 pages, 4 figures, style revtex. To appear in International Journal
Modern Physics
- …