18,817 research outputs found
The Most Unforgettable Character I\u27ve Met
(First Place, Essay Division, 1946, Butler University Literary Contest)
The most unforgettable character I\u27ve met was my stepfather. In general stepfathers aren\u27t very popular with their stepchildren, but he was an exception in this respect as he was in so many others
Observation of the Final Boundary Condition: Extragalactic Background Radiation and the Time symmetry of the Universe
This paper examines an observable consequence for the diffuse extragalactic
background radiation (EGBR) of the hypothesis that if closed, our universe
possesses time symmetric boundary conditions. For simplicity, attention is
focused on optical wavelengths. The universe is modeled as closed Friedmann-
Roberston-Walker. It is shown that, over a wide range of frequencies,
electromagnetic radiation can propagate largely unabsorbed from the present
epoch into the recollapsing phase, confirming and demonstrating the generality
of results of Davies and Twamley. As a consequence, time symmetric boundary
conditions imply that the optical EGBR is at least twice that due to the
galaxies on our past light cone, and possibly considerably more. It is
therefore possible to test experimentally the notion that if our universe is
closed, it may be in a certain sense time symmetric. The lower bound on the
"excess" EGBR in a time symmetric universe is consistent with present
observations. Nevertheless, better observations and modelling may soon rule it
out entirely. In addition, many physical complications arise in attempting to
reconcile a transparent future light cone with time symmetric boundary
conditions, thereby providing further arguments against the possibility that
our universe is time symmetric.Comment: latex 2.09, 30 pages, 1 figure. Figure added, errors corrected.
Slight revision of published versio
The impact of FE / HE mergers : interim findings
"This paper outlines the results of an analysis of the curriculum offer and student numbers at former further education colleges who have transferred to the higher education sector following mergers with higher education institutions. It also includes some brief background
information on the situation with regard to the FE asset base in the new institutions" -- [page 2]
Path planning for simple wheeled robots : sub-Riemannian and elastic curves on SE(2)
This paper presents a motion planning method for a simple wheeled robot in two cases: (i) where translational and rotational speeds are arbitrary and (ii) where the robot is constrained to move forwards at unit speed. The motions are generated by formulating a constrained optimal control problem on the Special Euclidean group SE(2). An application of Pontryagin’s maximum principle for arbitrary speeds yields an optimal Hamiltonian which is completely integrable in terms of Jacobi elliptic functions. In the unit speed case, the rotational velocity is described in terms of elliptic integrals and the expression for the position reduced to quadratures. Reachable sets are defined in the arbitrary speed case and a numerical plot of the time-limited reachable sets presented for the unit speed case. The resulting analytical functions for the position and orientation of the robot can be parametrically optimised to match prescribed target states within the reachable sets. The method is shown to be easily adapted to obstacle avoidance for static obstacles in a known environment
A CFD based procedure for airspace integration of small unmanned aircraft within congested areas
Future integration of small unmanned aircraft within an urban airspace requires an a posteriori understanding of the
building-induced aerodynamics which could negatively impact on vehicle performance. Moving away from generalised
building formations, we model the centre of the city of Glasgow using Star-CCMþ, a commercial CFD package. After
establishing a critical turbulent kinetic energy for our vehicle, we analyse the CFD results to determine how best to
operate a small unmanned aircraft within this environment. As discovered in a previous study, the spatial distribution of
turbulence increases with altitude. It was recommended then that UAVs operate at the minimal allowable altitude within
a congested area. As the flow characteristics in an environment are similar, regardless of inlet velocity, we can determine
areas within a city which will have consistently low or high values of turbulent kinetic energy. As the distribution of
turbulence is dependent on prevailing wind directions, some directions are more favourable than others, even if the wind
speed is unchanging. Moving forward we should aim to gather more information about integrated aircraft and how they
respond to turbulence in a congested area
Consistent probabilities in loop quantum cosmology
A fundamental issue for any quantum cosmological theory is to specify how
probabilities can be assigned to various quantum events or sequences of events
such as the occurrence of singularities or bounces. In previous work, we have
demonstrated how this issue can be successfully addressed within the consistent
histories approach to quantum theory for Wheeler-DeWitt-quantized cosmological
models. In this work, we generalize that analysis to the exactly solvable loop
quantization of a spatially flat, homogeneous and isotropic cosmology sourced
with a massless, minimally coupled scalar field known as sLQC. We provide an
explicit, rigorous and complete decoherent histories formulation for this model
and compute the probabilities for the occurrence of a quantum bounce vs. a
singularity. Using the scalar field as an emergent internal time, we show for
generic states that the probability for a singularity to occur in this model is
zero, and that of a bounce is unity, complementing earlier studies of the
expectation values of the volume and matter density in this theory. We also
show from the consistent histories point of view that all states in this model,
whether quantum or classical, achieve arbitrarily large volume in the limit of
infinite `past' or `future' scalar `time', in the sense that the wave function
evaluated at any arbitrary fixed value of the volume vanishes in that limit.
Finally, we briefly discuss certain misconceptions concerning the utility of
the consistent histories approach in these models.Comment: 22 pages, 3 figures. Matches published versio
- …