From Superluminal Velocity To Time Machines?

Various experiments have shown superluminal group and signal velocities recently. Experiments were essentials carried out with microwave tunnelling, with frustrated total internal reflection, and with gain-assisted anomalous dispersion. According to text books a superluminal signal velocity violates Einstein causality implying that cause and effect can be changed and time machines known from science fiction could be constructed. This naive analysis, however, assumes a signal to be a point in the time dimension neglecting its finite duration. A signal is not presented by a point nor by its front, but by its total length. On the other hand a signal energy is finite thus its frequency band is limited, the latter is a fundamental physical property in consequence of field quantization with quantum $h \nu$. All superluminal experiments have been carried out with rather narrow frequency bands. The narrow band width is a condition sine qua non to avoid pulse reshaping of the signal due to the dispersion relation of the tunnelling barrier or of the excited gas, respectively. In consequence of the narrow frequency band width the time duration of the signal is long so that causality is preserved. However, superluminal signal velocity shortens the otherwise luminal time span between cause and effect.Comment: 5 pages, 3 figure

Synchrotron Tomography for the Study of Void Formation in Internal Tin Nb3_{3}Sn Superconductors

Synchrotron absorption tomography has been applied for the study of voids formed during the thermal treatment of internal tin Nb$_{3}$Sn strands. Possible void formation mechanisms and in particular the effect of Sn phase transformations and melting are discussed based on a quantitative void description. Sn melting changes mainly the shape and volume of the individual voids but does not increase the total void volume in the strand

Resonance-like Goss-Haenchen Shift induced by nano-metal films

The influence of nano-metal films on the Goos-Haenchen shift (GHS) is investigated. The films deposited at the total reflecting surface of a perspex prism/air have a sheet resistance varying between Z = 25 and 3 000 Ohm. A resonance-like enhancement of the shift and of the absorption is found for TE polarized waves, when the sheet resistance approaches the value of the vacuum impedance. For TM waves the influence of the metal films on the GHS is comparatively weak. The experiments are carried out with microwaves. Keywords: Goos-Haenchen shift; nano-metallic films, microwaves PACS: 42.25.Bs, 42.25.Gy, 42.50.-p, 73.40.GkComment: 6 pages, 4 figure

Universal tunneling time for all fields

Tunneling is an important physical process. The observation that particles surmount a high mountain in spite of the fact that they don't have the necessary energy cannot be explained by classical physics. However, this so called tunneling became allowed by quantum mechanics. Experimental tunneling studies with different photonic barriers from microwave frequencies up to ultraviolet frequencies pointed towards a universal tunneling time (Haibel,Esposito). Experiments and calculations have shown that the tunneling time of opaque photonic barriers (optical mirrors, e.g.) equals approximately the reciprocal frequency of the corresponding electromagnetic wave. The tunneling process is described by virtual photons. Virtual particles like photons or electrons are not observable. However, from the theoretical point of view, they represent necessary intermediate states between observable real states. In the case of tunneling there is a virtual particle between the incident and the transmitted particle. Tunneling modes have a purely imaginary wave number. They represent solutions of the Schroedinger equation and of the classical Helmholtz equation. Recent experimental and theoretical data of electron and sound tunneling confirmed the conjecture that the tunneling process is characterized by a universal tunneling time independent of the kind of field. Tunneling proceeds at a time of the order of the reciprocal frequency of the wave.Comment: 7 pages latex, 3 figure

Nonlocal reflection by photonic barriers

The time behaviour of microwaves undergoing partial reflection by photonic barriers was measured in the time and in the frequency domain. It was observed that unlike the duration of partial reflection by dielectric layers, the measured reflection duration of barriers is independent of their length. The experimental results point to a nonlocal behaviour of evanescent modes at least over a distance of some ten wavelengths. Evanescent modes correspond to photonic tunnelling in quantum mechanics.Comment: 8 pages, 5 figure

On the formation of voids in internal tin Nb3_{3}Sn superconductors

In this article we describe three void growth mechanisms in Nb$_{3}$Sn strands of the internal tin design on the basis of combined synchrotron micro-tomography and x-ray diffraction measurements during in-situ heating cycles. Initially void growth is driven by a reduction of void surface area by void agglomeration. The main void volume increase is caused by density changes during the formation of Cu3Sn in the strand. Subsequent transformation of Cu-Sn intermetallics into the lower density a-bronze reduces the void volume again. Long lasting temperature ramps and isothermal holding steps can neither reduce the void volume nor improve the chemical strand homogeneity prior to the superconducting A15 phase nucleation and growth

Reflectionless Tunnelling of Light in Gradient Optics

We analyse the optical (or microwave) tunnelling properties of electromagnetic waves passing through thin films presenting a specific index profile providing a cut-off frequency, when they are used below this frequency. We show that contrary to the usual case of a square index profile, where tunnelling is accompanied with a strong attenuation of the wave due to reflection, such films present the possibility of a reflectionless tunnelling, where the incoming intensity is totally transmitted

results from the German Spondyloarthritis Inception Cohort

Background Functional status and spinal mobility in patients with axial spondyloarthritis (axSpA) are known to be determined both by disease activity and by structural damage in the spine. The impact of structural damage in the sacroiliac joints (SIJ) on physical function and spinal mobility in axSpA has not been studied so far. The objective of the study was to analyze the impact of radiographic sacroiliitis on functional status and spinal mobility in patients with axSpA. Methods In total, 210 patients with axSpA were included in the analysis. Radiographs of SIJ obtained at baseline and after 2 years of follow up were scored by two trained readers according to the modified New York criteria grading system (grade 0–4). The mean of two readers’ scores for each joint and a sum score for both SIJ were calculated for each patient giving a sacroiliitis sum score between 0 and 8. The Bath Ankylosing Spondylitis Functional Index (BASFI) and Bath Ankylosing Spondylitis Metrology Index (BASMI) at baseline and after 2 years were used as outcome measures. Results Longitudinal mixed model analysis adjusted for structural damage in the spine (modified Stoke Ankylosing Spondylitis Spine Score - mSASSS), disease activity (Bath Ankylosing Spondylitis Disease Activity Index - BASDAI and C-reactive protein level) and gender, revealed an independent association of the sacroiliitis sum score with the BASFI: b = 0.10 (95% CI 0.01–0.19) and the BASMI: b = 0.12 (95% CI 0.03–0.21), respectively, indicating that change by one radiographic sacroiliitis grade in one joint is associated with BASFI/BASMI worsening by 0.10/0.12 points, respectively, independently of disease activity and structural damage in the spine. Conclusion Structural damage in the SIJ might have an impact on functional status and spinal mobility in axSpA independently of spinal structural damage and disease activity. Trial registration ClinicalTrials.gov, NCT01277419. Registered on 14 January 2011

Negative phase time for Scattering at Quantum Wells: A Microwave Analogy Experiment

If a quantum mechanical particle is scattered by a potential well, the wave function of the particle can propagate with negative phase time. Due to the analogy of the Schr\"odinger and the Helmholtz equation this phenomenon is expected to be observable for electromagnetic wave propagation. Experimental data of electromagnetic wells realized by wave guides filled with different dielectrics confirm this conjecture now.Comment: 10 pages, 6 figure

Negative and Positive Lateral Shift of a Light Beam Reflected from a Grounded Slab

We consider the lateral shift of a light beam reflecting from a dielectric slab backed by a metal. It is found that the lateral shift of the reflected beam can be negative while the intensity of reflected beam is almost equal to the incident one under a certain condition. The explanation for the negativity of the lateral shift is given in terms of the interference of the reflected waves from the two interfaces. It is also shown that the lateral shift can be enhanced or suppressed under some other conditions. The numerical calculation on the lateral shift for a realistic Gaussian-shaped beam confirms our theoretical prediction.Comment: 8pages,4 figure