33,052 research outputs found
In an expanding universe, what doesn't expand?
The expansion of the universe is often viewed as a uniform stretching of
space that would affect compact objects, atoms and stars, as well as the
separation of galaxies. One usually hears that bound systems do not take part
in the general expansion, but a much more subtle question is whether bound
systems expand partially. In this paper, a very definitive answer is given for
a very simple system: a classical "atom" bound by electrical attraction. With a
mathemical description appropriate for undergraduate physics majors, we show
that this bound system either completely follows the cosmological expansion, or
-- after initial transients -- completely ignores it. This "all or nothing"
behavior can be understood with techniques of junior-level mechanics. Lastly,
the simple description is shown to be a justifiable approximation of the
relativistically correct formulation of the problem.Comment: 8 pages, 9 eps figure
Flow properties of a series of experimental thermoplastic polymides
The softening temperature to degradation temperature range of the polymers was about 440 to 650 K. All of the polymers retained small amounts of solvent as indicated by an increase in T(sub g) as the polymers were dried. The flow properties showed that all three polymers had very high apparent viscosities and would require high pressures and/or high temperatures and/or long times to obtain adequate flow in prepregging and molding. Although none was intended for such application, two of the polymers were combined with carbon fibers by solution prepregging. The prepregs were molded into laminates at temperatures and times, the selection of which was guided by the results from the flow measurements. These laminates had room temperature short beam shear strength similar to that of carbon fiber laminates with a thermosetting polyimide matrix. However, the strength had considerable scatter, and given the difficult processing, these polymides probably would not be suitable for continuous fiber composites
The Creation and Propagation of Radiation: Fields Inside and Outside of Sources
We present a new algorithm for computing the electromagnetic fields of
currents inside and outside of finite current sources, for arbitrary time
variations in the currents. Unexpectedly, we find that our solutions for these
fields are free of the concepts of differential calculus, in that our solutions
only involve the currents and their time integrals, and do not involve the time
derivatives of the currents. As examples, we give the solutions for two
configurations of current: a planar solenoid and a rotating spherical shell
carrying a uniform charge density. For slow time variations in the currents, we
show that our general solutions reduce to the standard expressions for the
fields in classic magnetic dipole radiation. In the limit of extremely fast
turn-on of the currents, we show that for our general solutions the amount of
energy radiated is exactly equal to the magnetic energy stored in the static
fields a long time after current creation. We give three associated problem
statements which can be used in courses at the undergraduate level, and one
problem statement suitable for courses at the graduate level. These problems
are of physical interest because: (1) they show that current systems of finite
extent can radiate even during time intervals when the currents are constant;
(2) they explicitly display transit time delays across a source associated with
its finite dimensions; and (3) they allow students to see directly the origin
of the reaction forces for time-varying systemsComment: 25 pages, 5 figure
Opposite Arrows of Time Can Reconcile Relativity and Nonlocality
We present a quantum model for the motion of N point particles, implying
nonlocal (i.e., superluminal) influences of external fields on the
trajectories, that is nonetheless fully relativistic. In contrast to other
models that have been proposed, this one involves no additional space-time
structure as would be provided by a (possibly dynamical) foliation of
space-time. This is achieved through the interplay of opposite microcausal and
macrocausal (i.e., thermodynamic) arrows of time.Comment: 12 pages, 4 figures; v5: section headlines adde
A New Concept for Controlled Lifting Entry Flight Experiments
Feasibility of trajectory guidance and control concept for lifting configuration with roll modulatio
Entropic issues in contemporary cosmology
Penrose [1] has emphasized how the initial big bang singularity requires a
special low entropy state. We address how recent brane cosmological schemes
address this problem and whether they offer any apparent resolution. Pushing
the start time back to or utilizing maximally symmetric AdS spaces
simply exacerbates or transfers the problem.
Because the entropy of de Sitter space is , using the
present acceleration of the universe as a low energy )
inflationary stage, as in cyclic ekpyrotic models, produces a gravitational
heat death after one cycle. Only higher energy driven inflation, together with
a suitable, quantum gravity holography style, restriction on {\em ab initio}
degrees of freedom, gives a suitable low entropy initial state. We question the
suggestion that a high energy inflationary stage could be naturally reentered
by Poincare recurrence within a finite causal region of an accelerating
universe.
We further give a heuristic argument that so-called eternal inflation is not
consistent with the 2nd law of thermodynamics within a causal patch.Comment: brief discussion on Poincare recurrence include
Piezoelectric copolymer hydrophones for ultrasonic field characterization
Hydrophones to be used in the characterization of medical ultrasonic transducers have
been fabricated using a new polyvinylidene fluoride/trifluoroethylene (VF2/VF3)
copolymer. The copolymer has an advantage over VF2 in that it does not require
prestretching before poling. Thin copolymer films can be cast from solution and then
poled using the corona discharge method. As there is a need for small‐diameter
hydrophones to provide good spatial resolution in measuring highly focused ultrasonic
beams, hydrophones with diameter as small as 0.1 mm have been made. Both
needle‐type and line hydrophones have been tested and their performance reported. In
the case of line hydrophones, the output signal is proportional to the line integral of the
acoustic pressure and a computer tomographic technique has been used to reconstruct the
beam profiles
Entanglement and the Thermodynamic Arrow of Time
We discuss quantum entanglement in the context of the thermodynamic arrow of
time. We review the role of correlations in entropy-decreasing events and prove
that the occurrence of a transformation between two thermodynamic states
constitutes a new type of entanglement witness, one not defined as a separating
plane in state space between separable and entangled states, but as a physical
process dependent on the local initial properties of the states. Extending work
by Partovi, we consider a general entangled multipartite system that allows
large reversals of the thermodynamic arrow of time. We describe a hierarchy of
arrows that arises from the different correlations allowed in a quantum state
and examine these features in the context of Maxwell's Demon. We examine in
detail the case of three qubits, and also propose some simple experimental
demonstrations possible with small numbers of qubits.Comment: 10 pages with 9 figure
Revision of the chewing louse genus Formicaphagus (Phthiraptera: Philopteridae) from Neotropical Antbirds and Gnateaters (Aves: Passeriformes)
Journal ArticleExamination of specimens of the 15 recognized species of Formicaphagus resulted in 6 new synonymies: F. laemostictus, F. latifrons, and F. peruvianus are junior synonyms of F. picturatus; F. huilae of F. angustifrons; and F. bolivianus and F. thoracicus of F. minutus. Two new species are described from material collected by the junior author in Peru: F. arnoldi (type host Conopophaga ardesiaca) and F. donpetersi (type host Conopophaga peruviana); these are the first louse species described from hosts in the family Conopophagidae, the gnateaters
Exchange Fluctuation Theorem for correlated quantum systems
We extend the Exchange Fluctuation Theorem for energy exchange between
thermal quantum systems beyond the assumption of molecular chaos, and describe
the non-equilibrium exchange dynamics of correlated quantum states. The
relation quantifies how the tendency for systems to equilibrate is modified in
high-correlation environments. Our results elucidate the role of measurement
disturbance for such scenarios. We show a simple application by finding a
semi-classical maximum work theorem in the presence of correlations.Comment: Lots of new material added, a figure, and a new author, 13 pages, 1
figure, comments welcom
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