13,321 research outputs found
Time and dark matter from the conformal symmetries of Euclidean space
The quotient of the conformal group of Euclidean 4-space by its Weyl subgroup
results in a geometry possessing many of the properties of relativistic phase
space, including both a natural symplectic form and non-degenerate Killing
metric. We show that the general solution posesses orthogonal Lagrangian
submanifolds, with the induced metric and the spin connection on the
submanifolds necessarily Lorentzian, despite the Euclidean starting pont. By
examining the structure equations of the biconformal space in an orthonormal
frame adapted to its phase space properties, we also find that two new tensor
fields exist in this geometry, not present in Riemannian geometry. The first is
a combination of the Weyl vector with the scale factor on the metric, and
determines the timelike directions on the submanifolds. The second comes from
the components of the spin connection, symmetric with respect to the new
metric. Though this field comes from the spin connection it transforms
homogeneously. Finally, we show that in the absence of conformal curvature or
sources, the configuration space has geometric terms equivalent to a perfect
fluid and a cosmological constant.Comment: 26 pages, no figures. Appreciable introductory material added.
Results substantially strengthened and explained. New results concerning dark
matter and dark energy candidates added to this versio
Yang-Mills gravity in biconformal space
We write a gravity theory with Yang-Mills type action using the biconformal
gauging of the conformal group. We show that the resulting biconformal
Yang-Mills gravity theories describe 4-dim, scale-invariant general relativity
in the case of slowly changing fields. In addition, we systematically extend
arbitrary 4-dim Yang-Mills theories to biconformal space, providing a new arena
for studying flat space Yang-Mills theories. By applying the biconformal
extension to a 4-dim pure Yang-Mills theory with conformal symmetry, we
establish a 1-1, onto mapping between a set of gravitational gauge theories and
4-dim, flat space gauge theories.Comment: 27 pages; paper emphasis shifted to focus on gravity; references
adde
The existence of time
Of those gauge theories of gravity known to be equivalent to general
relativity, only the biconformal gauging introduces new structures - the
quotient of the conformal group of any pseudo-Euclidean space by its Weyl
subgroup always has natural symplectic and metric structures. Using this metric
and symplectic form, we show that there exist canonically conjugate,
orthogonal, metric submanifolds if and only if the original gauged space is
Euclidean or signature 0. In the Euclidean cases, the resultant configuration
space must be Lorentzian. Therefore, in this context, time may be viewed as a
derived property of general relativity.Comment: 21 pages (Reduced to clarify and focus on central argument; some
calculations condensed; typos corrected
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Effect of elevated CO2 and high temperature on seed-set and grain quality of rice
Hybrid vigour may help overcome the negative effects of climate change in rice. A popular rice hybrid (IR75217H), a heat-tolerant check (N22), and a mega-variety (IR64) were tested for tolerance of seed-set and grain quality to high-temperature stress at anthesis at ambient and elevated [CO2]. Under an ambient air temperature of 29 °C (tissue temperature 28.3 °C), elevated [CO2] increased vegetative and reproductive growth, including seed yield in all three genotypes. Seed-set was reduced by high temperature in all three genotypes, with the hybrid and IR64 equally affected and twice as sensitive as the tolerant cultivar N22. No interaction occurred between temperature and [CO2] for seed-set. The hybrid had significantly more anthesed spikelets at all temperatures than IR64 and at 29 °C this resulted in a large yield advantage. At 35 °C (tissue temperature 32.9 °C) the hybrid had a higher seed yield than IR64 due to the higher spikelet number, but at 38 °C (tissue temperature 34–35 °C) there was no yield advantage. Grain gel consistency in the hybrid and IR64 was reduced by high temperatures only at elevated [CO2], while the percentage of broken grains increased from 10% at 29 °C to 35% at 38 °C in the hybrid. It is concluded that seed-set of hybrids is susceptible to short episodes of high temperature during anthesis, but that at intermediate tissue temperatures of 32.9 °C higher spikelet number (yield potential) of the hybrid can compensate to some extent. If the heat tolerance from N22 or other tolerant donors could be transferred into hybrids, yield could be maintained under the higher temperatures predicted with climate change
Opposite Thermodynamic Arrows of Time
A model in which two weakly coupled systems maintain opposite running
thermodynamic arrows of time is exhibited. Each experiences its own retarded
electromagnetic interaction and can be seen by the other. The possibility of
opposite-arrow systems at stellar distances is explored and a relation to dark
matter suggested.Comment: To appear in Phys. Rev. Let
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Evidence for a climate signal in trends of global crop yield variability over the past 50 years
Low variability of crop production from year to year is desirable for many reasons, including reduced income risk and stability of supplies. Therefore, it is important to understand the nature of yield variability, whether it is changing through time, and how it varies between crops and regions. Previous studies have shown that national crop yield variability has changed in the past, with the direction and magnitude dependent on crop type and location. Whilst such studies acknowledge the importance of climate variability in determining yield variability, it has been assumed that its magnitude and its effect on crop production have not changed through time and, hence, that changes to yield variability have been due to non-climatic factors. We address this assumption by jointly examining yield and climate variability for three major crops (rice, wheat and maize) over the past 50 years. National yield time series and growing season temperature and precipitation were de-trended and related using multiple linear regression. Yield variability changed significantly in half of the crop–country combinations examined. For several crop–country combinations, changes in yield variability were related to changes in climate variability
Field Theory as Free Fall
It is shown that the classical field equations pertaining to gravity coupled
to other bosonic fields are equivalent to a single geodesic equation,
describing the free fall of a point particle in superspace. Some implications
for quantum gravity are discussed.Comment: 18 pages, plain late
Combining general relativity and quantum theory: points of conflict and contact
The issues related to bringing together the principles of general relativity
and quantum theory are discussed. After briefly summarising the points of
conflict between the two formalisms I focus on four specific themes in which
some contact has been established in the past between GR and quantum field
theory: (i) The role of planck length in the microstructure of spacetime (ii)
The role of quantum effects in cosmology and origin of the universe (iii) The
thermodynamics of spacetimes with horizons and especially the concept of
entropy related to spacetime geometry (iv) The problem of the cosmological
constant.Comment: Invited Talk at "The Early Universe and Cosmological Observations: a
Critical Review", UCT, Cape Town, 23-25 July,2001; to appear in
Class.Quan.Gra
NMR implementation of Quantum Delayed-Choice Experiment
We report the first experimental demonstration of quantum delayed-choice
experiment via nuclear magnetic resonance techniques. An ensemble of molecules
each with two spin-1/2 nuclei are used as target and the ancilla qubits to
perform the quantum circuit corresponding the delayed-choice setup. As expected
in theory, our experiments clearly demonstrate the continuous morphing of the
target qubit between particle-like and wave-like behaviors. The experimental
visibility of the interference patterns shows good agreement with the theory.Comment: Revised text, more figures adde
Corticosterone Regulates Both Naturally Occurring and Cocaine‐Induced Dopamine Signaling by Selectively Decreasing Dopamine Uptake
Stressful and aversive events promote maladaptive reward‐seeking behaviors such as drug addiction by acting, in part, on the mesolimbic dopamine system. Using animal models, data from our laboratory and others show that stress and cocaine can interact to produce a synergistic effect on reward circuitry. This effect is also observed when the stress hormone corticosterone is administered directly into the nucleus accumbens (NAc), indicating that glucocorticoids act locally in dopamine terminal regions to enhance cocaine\u27s effects on dopamine signaling. However, prior studies in behaving animals have not provided mechanistic insight. Using fast‐scan cyclic voltammetry, we examined the effect of systemic corticosterone on spontaneous dopamine release events (transients) in the NAc core and shell in behaving rats. A physiologically relevant systemic injection of corticosterone (2 mg/kg i.p.) induced an increase in dopamine transient amplitude and duration (both voltammetric measures sensitive to decreases in dopamine clearance), but had no effect on the frequency of transient release events. This effect was compounded by cocaine (2.5 mg/kg i.p.). However, a second experiment indicated that the same injection of corticosterone had no detectable effect on the dopaminergic encoding of a palatable natural reward (saccharin). Taken together, these results suggest that corticosterone interferes with naturally occurring dopamine uptake locally, and this effect is a critical determinant of dopamine concentration specifically in situations in which the dopamine transporter is pharmacologically blocked by cocaine
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