15 research outputs found
Some aspects of dispersive horizons: lessons from surface waves
Hydrodynamic surface waves propagating on a moving background flow experience
an effective curved space-time. We discuss experiments with gravity waves and
capillary-gravity waves in which we study hydrodynamic black/white-hole
horizons and the possibility of penetrating across them. Such possibility of
penetration is due to the interaction with an additional "blue" horizon, which
results from the inclusion of surface tension in the low-frequency gravity-wave
theory. This interaction leads to a dispersive cusp beyond which both horizons
completely disappear. We speculate the appearance of high-frequency
"superluminal" corrections to be a universal characteristic of analogue gravity
systems, and discuss their relevance for the trans-Planckian problem. We also
discuss the role of Airy interference in hybridising the incoming waves with
the flowing background (the effective spacetime) and blurring the position of
the black/white-hole horizon.Comment: 29 pages. Lecture Notes for the IX SIGRAV School on "Analogue
Gravity", Como (Italy), May 201
Experimental demonstration of the supersonic-subsonic bifurcation in the circular jump: A hydrodynamic white hole
We provide an experimental demonstration that the circular hydraulic jump
represents a hydrodynamic white hole or gravitational fountain (the
time-reverse of a black hole) by measuring the angle of the Mach cone created
by an object in the "supersonic" inner flow region. We emphasise the general
character of this gravitational analogy by showing theoretically that the white
hole horizon constitutes a stationary and spatial saddle-node bifurcation
within dynamical-systems theory. We also demonstrate that the inner region has
a "superluminal" dispersion relation, i.e., that the group velocity of the
surface waves increases with frequency, and discuss some possible consequences
with respect to the robustness of Hawking radiation. Finally, we point out that
our experiment shows a concrete example of a possible "transplanckian
distortion" of black/white holes.Comment: 5 pages, 5 figures. New "transplanckian effect" described. Several
clarifications, additional figures and references. Published versio
Horizon effects for surface waves in wave channels and circular jumps
Surface waves in classical fluids experience a rich array of black/white hole
horizon effects. The dispersion relation depends on the characteristics of the
fluid (in our case, water and silicon oil) as well as on the fluid depth and
the wavelength regime. In some cases, it can be tuned to obtain a relativistic
regime plus high-frequency dispersive effects. We discuss two types of ongoing
analogue white-hole experiments: deep water waves propagating against a
counter-current in a wave channel and shallow waves on a circular hydraulic
jump.Comment: 4 pages, 2 figs. To appear in: Proceedings of the Spanish Relativity
Meeting (ERE2010
Observation of negative-frequency waves in a water tank: A classical analogue to the Hawking effect?
The conversion of positive-frequency waves into negative-frequency waves at
the event horizon is the mechanism at the heart of the Hawking radiation of
black holes. In black-hole analogues, horizons are formed for waves propagating
in a medium against the current when and where the flow exceeds the wave
velocity. We report on the first direct observation of negative-frequency waves
converted from positive-frequency waves in a moving medium. The measured degree
of mode conversion is significantly higher than expected from theory
LASSO: Large Adaptive optics Survey for Substellar Objects using the new SAPHIRA detector on Robo-AO
We report on initial results from the largest infrared AO direct imaging survey searching for wide orbit (≳ 100 AU) massive exoplanets and brown dwarfs as companions around young nearby stars using Robo-AO at the 2.1-m telescope on Kitt Peak, Arizona. The occurrence rates of these rare substellar companions are critical to furthering our understanding of the origin of planetary-mass companions on wide orbits. The observing efficiency of Robo-AO allows us to conduct a survey an order of magnitude larger than previously possible. We commissioned a low-noise high-speed SAPHIRA near-infrared camera to conduct this survey and report on its sensitivity, performance, and data reduction process
Large Adaptive Optics Survey for Substellar Objects around Young, Nearby, Low-mass Stars with Robo-AO
We present results from the Large Adaptive optics Survey for Substellar Objects, where the goal is to directly image new substellar companions ( 11.8), while only 23₋₆⁺¹¹% of the remaining 22 stars with no detected companion have significant accelerations. The significance of the acceleration decreases with increasing companion separation. These young accelerating low-mass stars with companions will eventually yield dynamical masses with future orbit monitoring
A Landau-de Gennes theory of nematic polymers
For a semi-flexible, nematic polymer, the Landau-de Gennes expansion for the free energy of the homogeneous melt is derived. The theory contains three parameters : the degree of polymerization, the inherent elasticity of the polymer chain, and the quadrupolar mean field between monomers. The dependence on chain length and chain flexibility of the transition temperature, the order parameter and the latent heat of the nematic-isotropic transition is discussed.On exprime l'énergie libre d'un polymère nématique semi-flexible sous forme d'un développement de Landau-de Gennes. Les paramètres qui interviennent sont le degré de polymérisation, l'élasticité propre de la chaine et l'interaction quadrupolaire moyenne entre monomères. On étudie l'influence de la longueur et de la flexibilité des chaînes sur la température, le paramètre d'ordre et la chaleur latente de la transition nématique-isotrope