2,681 research outputs found

### Towards a more balanced understanding of motor control systems

Roberts's book provides a reasonably thorough guide to the physiology and biomechanics of balance, unfortunately the discussion of the neural and cognitive aspects of motor control is less satisfactory. We propose that Roberts's statement of the problem of balance control should be extended to include control of non-equilibrium states, and we discuss sensorimotor calibration and integration in the context of maturation of the organism.Peer reviewe

### (Broken) Gauge Symmetries and Constraints in Regge Calculus

We will examine the issue of diffeomorphism symmetry in simplicial models of
(quantum) gravity, in particular for Regge calculus. We find that for a
solution with curvature there do not exist exact gauge symmetries on the
discrete level. Furthermore we derive a canonical formulation that exactly
matches the dynamics and hence symmetries of the covariant picture. In this
canonical formulation broken symmetries lead to the replacements of constraints
by so--called pseudo constraints. These considerations should be taken into
account in attempts to connect spin foam models, based on the Regge action,
with canonical loop quantum gravity, which aims at implementing proper
constraints. We will argue that the long standing problem of finding a
consistent constraint algebra for discretized gravity theories is equivalent to
the problem of finding an action with exact diffeomorphism symmetries. Finally
we will analyze different limits in which the pseudo constraints might turn
into proper constraints. This could be helpful to infer alternative
discretization schemes in which the symmetries are not broken.Comment: 32 pages, 15 figure

### Effects of heavy bosonic excitations on QED vacuum

We discuss the contribution of axion-like excitations (ALE) to the vacuum
birrefringence in the limit $m_A \gtrsim \omega$, where $m_A$ is the mass of
the excitation and $\omega$ the energy of test photons interacting with an
external (intense) magnetic field. The relevance of this term with respect to
the QED contribution depends on the ratio $g_A/m_A$ and, from present bounds on
the mass and the coupling constant $g_A$, we find that in the present low
energy regime, it ranges from $10^{-14}$ to $10^2$ suggesting an interesting
alternative to explore.Comment: References adde and section II rewritten. MPLA in pres

### Photon propagation in a cold axion background with and without magnetic field

A cold relic axion condensate resulting from vacuum misalignment in the early
universe oscillates with a frequency m, where m is the axion mass. We determine
the properties of photons propagating in a simplified version of such a
background where the sinusoidal variation is replaced by a square wave profile.
We prove that previous results that indicated that charged particles moving
fast in such a background radiate, originally derived assuming that all momenta
involved were much larger than m, hold for long wavelengths too. We also
analyze in detail how the introduction of a magnetic field changes the
properties of photon propagation in such a medium. We briefly comment on
possible astrophysical implications of these results.Comment: 17 pages, 4 figures, revised version includes an extended discussion
on physical implication

### From the discrete to the continuous - towards a cylindrically consistent dynamics

Discrete models usually represent approximations to continuum physics.
Cylindrical consistency provides a framework in which discretizations mirror
exactly the continuum limit. Being a standard tool for the kinematics of loop
quantum gravity we propose a coarse graining procedure that aims at
constructing a cylindrically consistent dynamics in the form of transition
amplitudes and Hamilton's principal functions. The coarse graining procedure,
which is motivated by tensor network renormalization methods, provides a
systematic approximation scheme towards this end. A crucial role in this coarse
graining scheme is played by embedding maps that allow the interpretation of
discrete boundary data as continuum configurations. These embedding maps should
be selected according to the dynamics of the system, as a choice of embedding
maps will determine a truncation of the renormalization flow.Comment: 22 page

### Optical probes of the quantum vacuum: The photon polarization tensor in external fields

The photon polarization tensor is the central building block of an effective
theory description of photon propagation in the quantum vacuum. It accounts for
the vacuum fluctuations of the underlying theory, and in the presence of
external electromagnetic fields, gives rise to such striking phenomena as
vacuum birefringence and dichroism. Standard approximations of the polarization
tensor are often restricted to on-the-light-cone dynamics in homogeneous
electromagnetic fields, and are limited to certain momentum regimes only. We
devise two different strategies to go beyond these limitations: First, we aim
at obtaining novel analytical insights into the photon polarization tensor for
homogeneous fields, while retaining its full momentum dependence. Second, we
employ wordline numerical methods to surpass the constant-field limit.Comment: 13 pages, 4 figures; typo in Eq. (5) corrected (matches journal
version

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