A theory of electromagnetism with uniquely defined potential and covariant conserved spin

A theory of electromagnetism is proposed that is based on the Fermi Lagrangian, which is symmetric under electromagnetic spin rotation. Its features are: - the four-potential is unambiguously determined by the inhomogeneous wave equation and boundary conditions at infinity. - the Lorenz condition and minimal coupling then follow from charge conservation. - electromagnetic spin is conserved and spin operators can be defined without sacrificing covariance. - quantisation is fully covariant, without redefining the metric. - all experimental predictions are the same as in the standard theory. This result proves that electromagnetic gauge invariance is redundant as a fundamental principle of physics.Comment: 4 pages, 1 figure, 20 equation

HORTICULTURAL RESEARCH IN THE NETHERLANDS: CHANGES AND CHALLENGES FOR 2010

Major shifts in world economy, society and technology will cause dramatic changes in Dutch horticulture and in the attitude of the government towards research. The horticulture industry will change from a production-driven to a customer-driven strategy while developing market-oriented product chains. More than ever, knowledge becomes a critical factor in competition. In contrast with the past, the horticulture industry will protect knowledge to increase its profitability. The government will choose to focus on basic sciences, leaving applied research mainly to the industry. This is a major shift since the government used to be responsible for most research. Complete institutional research programmes are no longer state-financed; instead, a system of financing research programmes by competitive bids is being developed. The government is also restructuring the infrastructure of agricultural research. Breakthroughs in science and technology will have strong effects on how the industry will develop. The more important ones are described. In the future, research institutes will act in a competitive environment with great uncertainties. The knowledge market used to be dominated by governments, but will be dominated by internationally active private enterprises. Both management and researchers will take more customer-oriented attitudes. Research institutes will need to develop strategies to survive under these circumstances. Some possibilities are discussed. It is concluded that horticultural research will change. Instead of focusing on plant production, it will include many more disciplines and multi-disciplinary collaboration in agreement to the information flow in product chains. The horticulture industry will have to decide whether separate horticultural stations for applied research should remain and what kind of work must be done in the future.foresight, research policy, strategy development, horticulture, Netherlands, Research and Development/Tech Change/Emerging Technologies,

More on Geometric Morphisms between Realizability Toposes

Geometric morphisms between realizability toposes are studied in terms of morphisms between partial combinatory algebras (pcas). The morphisms inducing geometric morphisms (the {\em computationally dense\/} ones) are seen to be the ones whose `lifts' to a kind of completion have right adjoints. We characterize topos inclusions corresponding to a general form of relative computability. We characterize pcas whose realizability topos admits a geometric morphism to the effective topos.Comment: 20 page

The Sierpinski Object in the Scott Realizability Topos

We study the Sierpinski object $\Sigma$ in the realizability topos based on Scott's graph model of the $\lambda$-calculus. Our starting observation is that the object of realizers in this topos is the exponential $\Sigma ^N$, where $N$ is the natural numbers object. We define order-discrete objects by orthogonality to $\Sigma$. We show that the order-discrete objects form a reflective subcategory of the topos, and that many fundamental objects in higher-type arithmetic are order-discrete. Building on work by Lietz, we give some new results regarding the internal logic of the topos. Then we consider $\Sigma$ as a dominance; we explicitly construct the lift functor and characterize $\Sigma$-subobjects. Contrary to our expectations the dominance $\Sigma$ is not closed under unions. In the last section we build a model for homotopy theory, where the order-discrete objects are exactly those objects which only have constant paths

Ab initio determination of exchange integrals and Neel temperature in the chain cuprates

We report ab initio quantum chemical cluster calculations of the chain (J_a) and the largest interchain (J_b) Heisenberg exchange of the chain cuprates Ca_2CuO_3 and Sr_2CuO_3. We find that J_a is comparable to the in-plane J in layered cuprates and J_a/J_b ~250-400. Using recent theory we obtain close agreement with experiment for the staggered moments and the critical temperatures. This implies that T_N does not depend on the third parameter J_c << J_b, and cannot be calculated using spin-wave theory. We propose an explanation for this interms of a 1D->2D cross-over.Comment: ps, 19 pages. To appear in Chemical Physics Letter

Shade for creatures of the stream

Stream-dwelling animals are sensitive to rising temperatures and fluctuations in water flow. alterra researchers are collecting data on this to help efforts to ward off the effects of climate change. an exploration of the Rovertse ley stream with a fishing net

Density Distribution of a Bose-Einstein Condensate of Photons in a Dye-Filled Microcavity

The achievement of Bose-Einstein condensation of photons (phBEC) in a dye-filled microcavity has led to a renewed interest in the density distribution of the ideal Bose gas in a two-dimensional harmonic oscillator. We present measurements of the radial profile of photons inside the microcavity below and above the critical point for phBEC with a good signal-to-noise ratio. We obtain a good agreement with theoretical profiles obtained using exact summation of eigenstates.Comment: 5 pages, 4 figure