23,662 research outputs found
Observations of nonlinear run-up patterns on plane and rhythmic beach morphology
Application of non-linear forecasting and bispectral analysis to video observations of run-up over cuspate topography shows that these alongshore patterns in the morphology are accompanied by changes to the fundamental behaviour of the run-up timeseries. Nonlinear forecasting indicates that at beach cusp horns, the behaviour of swash flow is more predictable and global (meaning that characteristics of individual swash events are well represented by the behaviour of the timeseries as a whole). Conversely, at beach cusp bays, the behaviour of swash flow is less predictable and more local (meaning that the characteristics of individual swash events are best represented by the behaviour of a small fraction of the timeseries). Bispectral analysis indicates that there is a nonlinear transfer of energy from the incident wave frequency f to infragravity frequency ~f/2 which only occurs in the bay, suggesting that the local behaviour is caused by interactions between successive swash cycles which are magnified by channelling caused by the beach cusp geometry. The local behaviour and the bispectral signatures are not present in offshore measurements, and are not present in runup timeseries collected when the beach was planar. These results provide evidence that interactions between successive run-ups are a fundamental characteristic of beach cusp bays. Ultimately, these interactions could lead to the growth of an infragravity wave with an alongshore wavelength forced by the presence of beach cusps
Quantum Breathing Mode for Electrons with 1/r^2 Interaction
We discuss a collective "breathing" mode of electrons with inverse-square-law
interactions in a two-dimensional quantum dot and a perpendicular magnetic
field.Comment: 1 page, Revtex, submitted to "Comment" Section of Phys. Rev. Let
Planck-Length Phenomenology
This author's recent proposal of interferometric tests of
Planck-scale-related properties of space-time is here revisited from a strictly
phenomenological viewpoint. The results announced previously are rederived
using elementary dimensional considerations. The dimensional analysis is then
extended to the other two classes of experiments (observations of neutral kaons
at particle accelerators and observations of the gamma rays we detect from
distant astrophysical sources) which have been recently considered as
opportunities to explore "foamy" properties of space-time. The emerging picture
suggests that there is an objective and intuitive way to connect the
sensitivities of these three experiments with the Planck length. While in
previous studies the emphasis was always on some quantum-gravity scenario and
the analysis was always primarily aimed at showing that the chosen scenario
would leave a trace in a certain class of doable experiments, the analysis here
reported takes as starting point the experiments and, by relating in a direct
quantitative way the sensitivities to the Planck length, provides a
model-independent description of the status of Planck-length phenomenology.Comment: Paper awarded an ``honorable mention'' in the Annual Competition of
the Gravity Research Foundation for the year 2000 (LaTex, 7 pages, no
figures
Technical Change and Industrial Dynamics as Evolutionary Processes
This work prepared for B. Hall and N. Rosenberg (eds.) Handbook of Innovation, Elsevier (2010), lays out the basic premises of this research and review and integrate much of what has been learned on the processes of technological evolution, their main features and their effects on the evolution of industries. First, we map and integrate the various pieces of evidence concerning the nature and structure of technological knowledge the sources of novel opportunities, the dynamics through which they are tapped and the revealed outcomes in terms of advances in production techniques and product characteristics. Explicit recognition of the evolutionary manners through which technological change proceed has also profound implications for the way economists theorize about and analyze a number of topics central to the discipline. One is the theory of the firm in industries where technological and organizational innovation is important. Indeed a large literature has grown up on this topic, addressing the nature of the technological and organizational capabilities which business firms embody and the ways they evolve over time. Another domain concerns the nature of competition in such industries, wherein innovation and diffusion affect growth and survival probabilities of heterogeneous firms, and, relatedly, the determinants of industrial structure. The processes of knowledge accumulation and diffusion involve winners and losers, changing distributions of competitive abilities across different firms, and, with that, changing industrial structures. Both the sector-specific characteristics of technologies and their degrees of maturity over their life cycles influence the patterns of industrial organization ? including of course size distributions, degrees of concentration, relative importance of incumbents and entrants, etc. This is the second set of topics which we address. Finally, in the conclusions, we briefly flag some fundamental aspects of economic growth and development as an innovation driven evolutionary process.Innovation, Technological paradigms, Technological regimes and trajectories, Evolution, Learning, Capability-based theories of the firm, Selection, Industrial dynamics, Emergent properties, Endogenous growth
Comment on "Modifying the variational principle in the action integral functional derivation of time-dependent density functional theory" by Jochen Schirmer [arXiv:1010.4223]
In a paper recently published in Phys. Rev. A [arXiv:1010.4223], Schirmer has
criticized an earlier work of mine [arXiv:0803.2727], as well as the
foundations of time-dependent density functional theory. In Ref.[2], I showed
that the so-called "causality paradox" - i.e., the failure of the
exchange-correlation potential derived from the Runge-Gross time-dependent
variational principle to satisfy causality requirements - can be solved by a
careful reformulation of that variational principle. Fortunately, the criticism
presented in Ref.[1] is based on elementary misunderstandings of the nature of
functionals, gauge transformations, and the time-dependent variational
principle. In this Comment I wish to point out and clear these
misunderstandings.Comment: 4 pages. Accepted for publication in Phys. Rev.
Ambiguities in order-theoretic formulations of thermodynamics
Since the 1909 work of Carath\'eodory, formulations of thermodynamics have
gained ground which highlight the role of the the binary relation of adiabatic
accessibility between equilibrium states. A feature of Carath\'eodory's system
is that the version therein of the second law contains an ambiguity about the
nature of irreversible adiabatic processes, making it weaker than the
traditional Kelvin-Planck statement of the law. This paper attempts first to
clarify the nature of this ambiguity, by defining the arrow of time in
thermodynamics by way of the Equilibrium Principle ("Minus First Law"). It then
argues that the ambiguity reappears in the important 1999 axiomatisation due to
Lieb and Yngvason.Comment: 13 pages, 1 figur
The use of video imagery to analyse groundwater and shoreline dynamics on a dissipative beach
Groundwater seepage is known to influence beach erosion and accretion processes. However, field measurements of the variation of the groundwater seepage line (GWSL) and the vertical elevation difference between the GWSL and the shoreline are limited. We developed a methodology to extract the temporal variability of the shoreline and the wet-dry boundary using video imagery, with the overarching aim to examine elevation differences between the wet-dry boundary and the shoreline position in relation to rainfall and wave characteristics, during a tidal cycle. The wet-dry boundary was detected from 10-minute time-averaged images collected at Ngaranui Beach, Raglan, New Zealand. An algorithm discriminated between the dry and wet cells using a threshold related to the maximum of the red, green and blue intensities in Hue-Saturation-Value. Field measurements showed this corresponded to the location where the watertable was within 2 cm of the beachface surface. Timestacks, time series of pixels extracted from cross-shore transects in the video imagery, were used to determine the location of the shoreline by manually digitizing the maximum run-up and minimum run-down location for each swash cycle, and averaging the result. In our test data set of 14 days covering a range of wave and rainfall conditions, we found 6 days when the elevation difference between the wet-dry boundary and the shoreline remained approximately constant during the tidal cycle. For these days, the wet-dry boundary corresponded to the upper limit of the swash zone. On the other 8 days, the wet-dry boundary and the shoreline decoupled with falling tide, leading to elevation differences of up to 2.5 m at low tide. Elevation differences between the GWSL and the shoreline at low-tide were particularly large when the cumulative rainfall in the preceding month was greater than 200 mm. This research shows that the wet-dry boundary (such as often used in video shoreline-finding algorithms) is related to groundwater seepage on low-sloped, medium to fine sand beaches such as Ngaranui Beach (mean grain size~0.27 mm, beach slope ~1:70) and may not be a good indicator of the position of the shoreline
The Holographic Interpretation of Hawking Radiation
Holography gives us a tool to view the Hawking effect from a new, classical
perspective. In the context of Randall-Sundrum braneworld models, we show that
the basic features of four-dimensional evaporating solutions are nicely
translated into classical five-dimensional language. This includes the dual
bulk description of particles tunneling through the horizon.Comment: 10 pages, 1 figure, Honorable Mention in the Gravity Research
Foundation Essay Competition 200
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