A charged rotating cylindrical shell

We give an example of a spacetime having an infinite thin rotating cylindrical shell constituted by a charged perfect fluid as a source. As the interior of the shell the Bonnor--Melvin universe is considered, while its exterior is represented by Datta--Raychaudhuri spacetime. We discuss the energy conditions and we show that our spacetime contains closed timelike curves. Trajectories of charged test particles both inside and outside the cylinder are also examined. Expression for the angular velocity of a circular motion inside the cylinder is given.Comment: 14 pages, 4 figures, minor corrections, to appear in Gen.Rel.Gra

Stability of Closed Timelike Curves in Goedel Universe

We study, in some detail, the linear stability of closed timelike curves in the Goedel metric. We show that these curves are stable. We present a simple extension (deformation) of the Goedel metric that contains a class of closed timelike curves similar to the ones associated to the original Goedel metric. This extension correspond to the addition of matter whose energy-momentum tensor is analyzed. We find the conditions to have matter that satisfies the usual energy conditions. We study the stability of closed timelike curves in the presence of usual matter as well as in the presence of exotic matter (matter that does satisfy the above mentioned conditions). We find that the closed timelike curves in Goedel universe with or whithout the inclusion of regular or exotic matter are also stable under linear perturbations. We also find a sort of structural stability.Comment: 12 pages, 11 figures, RevTex, several typos corrected. GRG, in pres

Stationary Cylindrical Anisotropic Fluid

We present the whole set of equations with regularity and matching conditions required for the description of physically meaningful stationary cylindrically symmmetric distributions of matter, smoothly matched to Lewis vacuum spacetime. A specific example is given. The electric and magnetic parts of the Weyl tensor are calculated, and it is shown that purely electric solutions are necessarily static. Then, it is shown that no conformally flat stationary cylindrical fluid exits, satisfying regularity and matching conditions.Comment: 17 pages Latex. To appear in Gen.Rel.Gra

Detection of mycobacteria in the environment of the Moravian Karst (Bull Rock Cave and the relevant water catchment area): the impact of water sediment, earthworm castings and bat guano

The presence of mycobacteria was studied in Bull Rock Cave (“Byci skala”) and the water catchment area of Jedovnice Brook (“Jedovnicky potok”) using direct microscopy after Ziehl-Neelsen (ZN) staining, culture examination and molecular techniques. Mycobacteria were detected in 47.1% of a total of 68 samples. The mycobacterial genes hsp65 and dnaA were detected and sequenced in 37 (74.0%) out of the 50 cave environmental samples and in 10 (55.6%) out of the 18 samples of water catchment sediments. Nine species of slowly growing mycobacteria (M. terrae, M. arupense, M. gordonae, M. lentiflavum, M. parascrofulaceum, M. parmense, M. saskatchewanense, M. simiae and M. xenopi) and two subsp. (M. avium subsp. avium and M. avium subsp. hominissuis) were detected. Fourteen species of rapidly growing mycobacteria (M. chelonae, M. chubuense, M. poriferae, M. flavescens, M. fortuitum, M. porcinum, M. rhodesiae, M. gilvum, M. goodii, M. peregrinum, M. mageritense, M. vanbaalenii, M. gadium and M. insubricum) were detected. The highest mycobacterial presence was documented by ZN staining and/or culture examinations in earthworm castings and bat guano (73.3% positivity out of the 15 samples) in the cave environment and in the water sediments collected under the outflow from the wastewater treatment plants (77.8% positivity out of nine samples). The highest total organic carbon (TOC) was detected in wooden material and earthworm castings with pH values between 5.0 and 7.7 in the cave environment and in water sediments collected under the outflow from the wastewater treatment plants with pH between 5.8 and 7.0. It could be concluded that the karst cave environment with its running surface water contaminated with different microorganisms or chemical substances creates favourable conditions not only for animals (especially earthworms) but also for mycobacteria. This fact is also demonstrated by the presence of these mycobacteria in the cave environment mainly in earthworm castings and bat guano

International Timescales with Optical Clocks (ITOC)

A new collaborative European project “International timescales with optical clocks” (ITOC) aims to tackle the key challenges that must be addressed prior to a redefinition of the SI second. A coordinated programme of comparisons will be carried out between European optical clocks developed in five different laboratories, enabling their performance levels to be validated at an unprecedented level of accuracy. Supporting work will be carried out to evaluate relativistic effects that influence the comparisons, including the gravitational redshift of the clock transition frequencies. A proof-of-principle experiment will also be performed to demonstrate that optical clocks could be used to make direct measurements of the Earth’s gravity potential with high temporal resolution

International timescales with optical clocks

International audienceA key prerequisite for a redefinition of the SI second based on optical atomic clocks is their integration intothe international timescales TAI and UTC. This requires a coordinated programme of clock comparisons to becarried out, to validate the uncertainty budgets of the optical clocks, to anchor their frequencies to the present definitionof the second, and to establish the leading contenders for a new definition. Such a comparison programmeis underway within the EMRP-funded project InternationalTimescales with Optical Clocks (ITOC). Illustrated inFig. 1, this involves four different types of measurementand optical clocks in five different laboratories.Locally, comparisons are being carried out betweenoptical clocks developed in individual laboratories, eitherby direct beat frequency comparison or by using femtosecondcombs to measure optical frequency ratios. To compareoptical clocks developed in different laboratories, twodifferent techniques are being explored, both of whichhave the potential to be applied on an intercontinentalscale. Two comparisons will be performed using transportableoptical clocks, and an improved two-way satellitetime and frequency transfer (TWSTFT) technique based onan increased chip rate is being investigated. In addition tothe direct optical clock comparisons, absolute frequencymeasurements of the optical clocks are also being performed. Several new measurements have already been completedand the current status of the clock comparison programme will be reported at the conference. New methodsdeveloped to analyze the self-consistency of the clock comparison data and to derive optimized values for the frequencyof each optical clock transition will also be described.To support the clock comparison programme, a complete evaluation is being made of all relativistic effects influencingtime and frequency comparisons at the 10-18 level of accuracy, including the gravitational redshifts ofthe clock transition frequencies. Significant progress has been made towards improved determination of the gravitypotential at the sites participating in the optical clock comparisons; gravity surveys have been carried out at alllocations and will feed into the computation of a revised European geoid model.Finally, an experiment is in preparation to demonstrate the future impact that optical atomic clocks could haveon the field of geodesy. This aims to measure with high temporal resolution the gravity potential difference betweentwo well-defined locations separated by a long baseline (~90 km) and a height difference of 1000 m