16,644 research outputs found
Inverse Source Identification based on Acoustic Particle Velocity Measurements
A general applicable acoustic source identification method is the inverse frequency response function technique (IFRF). In the standard IFRF method acoustic pressures measured on a grid in the nearfield of the acoustic source are used. To relate the measured field pressures to the normal velocities on the surface of the source, a transfer matrix is calculated with a boundary element method. The resulting system of equations is ill-conditioned and can only be solved by applying regularization techniques. In this paper, it is described how the nearfield particle velocities can be used instead of pressures to reconstruct the original source vibrations. By means of a simulated experiment, a comparison is made between pressure based and velocity based IFRF
The Small-Is-Very-Small Principle
The central result of this paper is the small-is-very-small principle for
restricted sequential theories. The principle says roughly that whenever the
given theory shows that a property has a small witness, i.e. a witness in every
definable cut, then it shows that the property has a very small witness: i.e. a
witness below a given standard number.
We draw various consequences from the central result. For example (in rough
formulations): (i) Every restricted, recursively enumerable sequential theory
has a finitely axiomatized extension that is conservative w.r.t. formulas of
complexity . (ii) Every sequential model has, for any , an extension
that is elementary for formulas of complexity , in which the
intersection of all definable cuts is the natural numbers. (iii) We have
reflection for -sentences with sufficiently small witness in any
consistent restricted theory . (iv) Suppose is recursively enumerable
and sequential. Suppose further that every recursively enumerable and
sequential that locally inteprets , globally interprets . Then,
is mutually globally interpretable with a finitely axiomatized sequential
theory.
The paper contains some careful groundwork developing partial satisfaction
predicates in sequential theories for the complexity measure depth of
quantifier alternations
Tolman mass, generalized surface gravity, and entropy bounds
In any static spacetime the quasi-local Tolman mass contained within a volume
can be reduced to a Gauss-like surface integral involving the flux of a
suitably defined generalized surface gravity. By introducing some basic
thermodynamics and invoking the Unruh effect one can then develop elementary
bounds on the quasi-local entropy that are very similar in spirit to the
holographic bound, and closely related to entanglement entropy.Comment: V1: 4 pages. Uses revtex4-1; V2: Three references added; V3: Some
notational changes for clarity; introductory paragraph rewritten; no physics
changes. This version accepted for publication in Physical Review Letter
Gravitational Thermodynamics of Causal Diamonds in (A)dS
The static patch of de Sitter spacetime and the Rindler wedge of Minkowski
spacetime are causal diamonds admitting a true Killing field, and they behave
as thermodynamic equilibrium states under gravitational perturbations. We
explore the extension of this gravitational thermodynamics to all causal
diamonds in maximally symmetric spacetimes. Although such diamonds generally
admit only a conformal Killing vector, that seems in all respects to be
sufficient. We establish a Smarr formula for such diamonds and a "first law"
for variations to nearby solutions. The latter relates the variations of the
bounding area, spatial volume of the maximal slice, cosmological constant, and
matter Hamiltonian. The total Hamiltonian is the generator of evolution along
the conformal Killing vector that preserves the diamond. To interpret the first
law as a thermodynamic relation, it appears necessary to attribute a negative
temperature to the diamond, as has been previously suggested for the special
case of the static patch of de Sitter spacetime. With quantum corrections
included, for small diamonds we recover the "entanglement equilibrium" result
that the generalized entropy is stationary at the maximally symmetric vacuum at
fixed volume, and we reformulate this as the stationarity of free conformal
energy with the volume not fixed.Comment: v3: 64 pages, 6 appendices, 8 figures; matches published versio
An analytical decomposition protocol for optimal implementation of two-qubit entangling gates
This paper addresses the question how to implement a desired two-qubit gate U
using a given tunable two-qubit entangling interaction H_int. We present a
general method which is based on the K_1 A K_2 decomposition of unitary
matrices in SU(4) to calculate analytically the smallest number of two-qubit
gates U_int [based on H_int] and single-qubit rotations, and the explicit
sequence of these operations that are required to implement U. We illustrate
our protocol by calculating the implementation of (1) the transformation from
standard basis to Bell basis, (2) the CNOT gate, and (3) the quantum Fourier
transform for two kinds of interaction - Heisenberg exchange interaction and
quantum inductive coupling - and discuss the relevance of our results for
solid-state qubits.Comment: 16 pages, published versio
Vision based motion control for a humanoid head
This paper describes the design of a motion control algorithm for a humanoid robotic head, which consists of a neck with four degrees of freedom and two eyes (a stereo pair system) that tilt on a common axis and rotate sideways freely. The kinematic and dynamic properties of the head are analyzed and modeled using screw theory. The motion control algorithm is designed to receive, as an input, the output of a vision processing algorithm and to exploit the redundancy of the system for the realization of the movements. This algorithm is designed to enable the head to focus on and to follow a target, showing human-like motions. The performance of the control algorithm has been tested in a simulated environment and, then, experimentally applied to the real humanoid head
Hints towards the Emergent Nature of Gravity
A possible way out of the conundrum of quantum gravity is the proposal that
general relativity (GR) is not a fundamental theory but emerges from an
underlying microscopic description. Despite recent interest in the emergent
gravity program within the physics as well as the philosophy community, an
assessment of the theoretical evidence for this idea is lacking at the moment.
We intend to fill this gap in the literature by discussing the main arguments
in favour of the hypothesis that the metric field and its dynamics are
emergent. First, we distinguish between microstructure inspired from GR, such
as through quantization or discretization, and microstructure that is not
directly motivated from GR, such as strings, quantum bits or condensed matter
fields. The emergent gravity approach can then be defined as the view that the
metric field and its dynamics are derivable from the latter type of
microstructure. Subsequently, we assess in how far the following properties of
(semi-classical) GR are suggestive of underlying microstructure: (1) the
metric's universal coupling to matter fields, (2) perturbative
non-renormalizability, (3) black hole thermodynamics, and (4) the holographic
principle. In the conclusion we formalize the general structure of the
plausibility arguments put forward.Comment: 36 pages, v2: minor additions, references added. Journal version in
Studies in History and Philosophy of Modern Physic
Motion control of the Twente humanoid head
In this work, we present the design and the realization of the motion control algorithm implemented in the Twente hu- manoid head, a seven degrees of freedom (dof) robotic sys- tem. The aim of the project is to have a humanoid head that can serve as a research platform for human-machine interac- tion purposes. The head should not only be able to percieve its environment and track objects, but also be able to move in a human-like way, i.e. to reproduce the motions of human beings and to mime the human expressions
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