Faster-than-Light Signals and Causality

The problems of causality are analyzed in terms of space-time models which admit the propagation of signals with superrelativistic velocities. It is shown that there is no violation in causality if the propagation of faster-than-light signals is described by general-covariant equations and occurs along invariant curves, as it is in some well-known models.     

Rigorous solution for transient propagation of electromagnetic waves through a medium: causality plus diffraction in time

A rigorous solution for transient propagation of electromagnetic waves through a medium has been found. Because of this rigorousness, the solution exhibits apparent consistency with Einsteinian causality. Thus, it is confirmed that faster-than-light or superluminal propagation of electromagnetic waves is not possible. Evanescent transmission gives rise to diffraction in time, which is the actual reason for deformation of group propagation.     

Further Evidence against the Possibility of Faster-than-Light Particles

Arguments are added to the growing number of arguments against the possibility of faster-than-light particles. Causal loops with faster-than-light particles are discussed in the framework of non-quantum special relativity. Thought experiments with steady currents of faster-than-light particles show that the reinterpretation principle is incompatible with the causality principle. If both principles are used consequently paradoxial situations arise. These are visualized utilizing Brehme diagrams. The final conclusion is that particles cannot travel on a macroscopic scale faster than light if space and time are homogeneous and if the relativity and causality principles are valid.     

Propagation and radiation characteristics of the circular electric, circular magnetic and hybrid waveguide modes

The field distributions and propagation constants of the circular electric, circular magnetic and hybrid modes of oversized waveguides are expressed, taking the effects of walls into account. These effects are described by wall functions which depend on surface impedances of the wall, and are determined for different types of guides. The near and far field patterns are derived in the case of real wall functions. It is shown that, for very oversized waveguides, the terms containing wall functions can be ignored in the calculations, and it results that the expressions of fields and propagation constants become independent of the types of waveguides. An application to corrugated waveguides for Electron Cyclotron Resonance Heating experiments shows the variations of the radiation characteristics versus geometric parameters of the corrugations and determines the ranges of interest for these parameters.     

Comments on Shimony’s “An Analysis of Stapp’s ‘A Bell-Type Theorem without Hidden Variables’ ”

The hidden-variable theorems of Bell and followers depend upon an assumption, namely the hidden-variable assumption, that conflicts with the precepts of quantum philosophy. Hence from an orthodox quantum perspective those theorems entail no faster-than-light transfer of information. They merely reinforce the ban on hidden variables. The need for some sort of faster-than-light information transfer can be shown by using counterfactuals instead of hidden variables. Shimony’s criticism of that argument fails to take into account the distinction between no-faster-than-light connection in one direction and that same condition in both directions. The argument can be cleanly formulated within the framework of a fixed past, open future interpretation of quantum theory, which neatly accommodates the critical assumptions that the experimenters are free to choose which experiments they will perform. The assumptions are compatible with the Tomonaga–Schwinger formulation of quantum field theory, and hence with orthodox quantum precepts, and with the relativistic requirement that no prediction pertaining to an outcome in one region can depend upon a free choice made in a region spacelike-separated from the first.     

Superluminosity,sound propagation,and causality

Different field theories may lead to identical equations of state. Depending on the field theory, from which an equation of state is derived, in the superluminal density domain the sound wave propagation velocity exceeds (does not exceed) the velocity of light, if the field is a medium with normal (anomal) dispersion. Super-light sound violates causality, causality violations lead to logical paradoxies, so superluminal density regions should be regions of anomal dispersion.     

Double-slit experiment and Bogoliubov’s causality principle

In the Bogoliubov approach the causality principle is the basic constructive element of quantum field theory. At the same time, this principle has obvious classical interpretation. On the other hand, it is well-known Feynman statement that the double-slit experiment is “impossible, absolutely impossible to explain in classical way, and has in it the heart of quantum mechanics. We describe how taking into account of infrared singularities allows to give quite evident interpretation to double-slit experiment. And this interpretation agrees with the Bogoliubov’s causality principle.     

Nonlocality and local causality in the Schrödinger equation with time-dependent boundary conditions

We investigate the nonlocal dynamics of a single particle placed in an infinite well with moving walls. It is shown that in this situation, the Schrödinger equation (SE) violates local causality by causing instantaneous changes in the probability current everywhere inside the well. This violation is formalized by designing a gedanken faster-than-light communication device which uses an ensemble of long narrow cavities and weak measurements to resolve the weak value of the momentum far away from the movable wall. Our system is free from the usual features causing nonphysical violations of local causality when using the (nonrelativistic) SE, such as instantaneous changes in potentials or states involving arbitrarily high energies or velocities. We explore in detail several possible artifacts that could account for the failure of the SE to respect local causality for systems involving time-dependent boundary conditions.     

Wave propagation in axion electrodynamics

In this paper, the axion contribution to the electromagnetic wave propagation is studied. First we show how the axion electrodynamics model can be embedded into a premetric formalism of Maxwell electrodynamics. In this formalism, the axion field is not an arbitrary added Chern–Simon term of the Lagrangian, but emerges in a natural way as an irreducible part of a general constitutive tensor. We show that in order to represent the axion contribution to the wave propagation it is necessary to go beyond the geometric approximation, which is usually used in the premetric formalism. We derive a covariant dispersion relation for the axion modified electrodynamics. The wave propagation in this model is studied for an axion field with timelike, spacelike and null derivative covectors. The birefringence effect emerges in all these classes as a signal of Lorentz violation. This effect is however completely different from the ordinary birefringence appearing in classical optics and in premetric electrodynamics. The axion field does not simple double the ordinary light cone structure. In fact, it modifies the global topological structure of light cones surfaces. In CFJ-electrodynamics, such a modification results in violation of causality. In addition, the optical metrics in axion electrodynamics are not pseudo-Riemannian. In fact, for all types of the axion field, they are even non-Finslerian.     

‘Faster than light’ photons in gravitational fields — Causality,anomalies and horizons

A number of general issues relating to superluminal photon propagation in gravitational fields are explored. The possibility of superluminal, yet causal, photon propagation arises because of Equivalence Principle violating interactions induced by vacuum polarisation in QED in curved spacetime. Two general theorems are presented: first, a polarisation sum rule which relates the polarisation averaged velocity shift to the matter energy-momentum tensor and second, a ‘horizon theorem’ which ensures that the geometric event horizon for black hole spacetimes remains a true horizon for real photon propagation. These results are consequences of an effective action which in QED is valid only for low frequency photons. Their relevance to signal propagation and causality, which are controlled by high frequency propagation, is dependent on the dispersive properties of the modified propagation. This will be discussed elsewhere. A comparison is made with the equivalent results for electromagnetic birefringence and possible connections between superluminal photon propagation, causality and the conformal anomaly are exposed.     

采用大口径投影光学系统监测远场散斑特征参数

大口径投影光学系统采用低成本、大口径菲涅耳透镜制作,可将远场散斑强度分布投影到CCD成像探测器上。通过CCD图像处理,能够对给定孔径上的接收功率、闪烁指数进行量化评估;在接收孔径足够大、保障散斑不会因为光束漂移效应而脱离菲涅耳透镜的条件下,该系统还可以对光束漂移和特征半径进行量化评估。同时讨论了CCD像元响应非均匀性误差及其影响、CCD辐照响应函数和图像几何投影系数的定标方法。实验表明,系统能够对激光大气传输过程中的远场散斑特征参数进行监测。特别对自由空间激光通信系统而言,可以为大气衰减和多种大气湍流效应综合作用下的中值电平慢衰落研究和检测阈值优化设计提供实验数据支撑。     

Propensities,correlations, and metaphysics

An attempt is made to defend realism and the absence of space-like causation in quantum mechanics, by invoking indeterminism and a new necessary condition for stochastic causality, we term robustness. This condition is defended against recent critical attacks by Cartwright and Jones, and by Healey, and the violation of the robustness condition in Bell-type correlation experiments is shown to follow if an appropriate interpretation of the state vector is employed.This paper is dedicated to Sir Karl Popper on the occasion of his 90th birthday.     

To a problem of faster-than-light motion

A version of electrodynamics admitting faster-than-light motion with a real mass is constructed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 15–19, February, 2006.     

On the sensitivity of coherent gradient sensing: Part II—An experimental investigation of accuracy in fracture mechanics applications

The sensitivity of the transmission ‘Coherent Gradient Sensing’ (CGS) technique is investigated experimentally in relation to the study of deformations near the tips of cracks in three-point bend specimens. Fringe data from these experiments are interpreted as gradients (geometric interpretation) and finite differences (physical interpretation) of hydrostatic stress fields. These data are used to compare the accuracy of the geometric interpretation of fringes with the physical interpretation of fringes obtained using increased sensitivity in order to confirm the theoretical findings from Part I of this investigation. Also, a least-squares fitting technique was used on the fringe data obtained from the region outside of the near tip 3D zone in order to investigate issues of K-dominance of the stress field in this region.Results from the experimental investigations reported in this paper indicated that increasing the sensitivity of the CGS technique improved the quantity and quality of fringe data. However, the apparent size of the region on the image plane dominated by 3D effects increased and the differences between the geometric and physical interpretations of CGS fringe data were increased as well. In addition, the stress field outside of the 3D zone was determined not to be strictly K-dominant, as was predicted by results from previous experiments.     

A linear relativistic model of processes with causal faster-than-light effects

To get a synthesis of causal faster-than-light effects and signals that do not propagate faster than light by using local, covariant, linear equations of motion, we propose the following hypothesis. Free fields that propagate signals according to the Klein-Gordon, Dirac, Proca or Maxwell equations, are actually describing only smoothed-out, average properties of underlying causal transport processes of point like entities with arbitrary four-momenta, the states of which are described by a scalar, spinor or four-vector field that satisfies a local, covariant, linear transport equation. An example of such a linear, causal, covariant transport process is shown to display causal faster-than-light effects, to propagate signals not faster than light, and to contain the Klein-Gordon equation as a limiting case. An analogous transport model displays causal, four-vector, faster-than-light effects, and also distinctive four-vector, long-range and short-range effects that do not propagate faster than light.     

Superluminal propagation in resonant dissipative media

We analyse the superluminal propagation of optical narrow-band pulses at resonances in dissipative media. We find that, for a broad class of optical systems holding this type of lossy faster-than-light transmission capability, the output waveform is an attenuated, time-advanced version of the input which can be interpreted as the result of the interference of two scaled replicas of the input having a positive relative delay. This analysis is shown to apply, among other scenarios, both in the propagation in a passive bulk medium at an electronic resonance and in a dielectric waveguide coupled to a lossy micro-ring resonator.     

Faster-than-c Signals, Special Relativity, and Causality

Motivated by the recent attention on superluminal phenomena, we investigate the compatibility between faster-than-c propagation and the fundamental principles of relativity and causality. We first argue that special relativity can easily accommodate—indeed, does not exclude—faster-than-c signaling at the kinematical level. As far as causality is concerned, it is impossible to make statements of general validity, without specifying at least some features of the tachyonic propagation. We thus focus on the Scharnhorst effect (faster-than-c photon propagation in the Casimir vacuum), which is perhaps the most plausible candidate for a physically sound realization of these phenomena. We demonstrate that in this case the faster-than-c aspects are “benign” and constrained in such a manner as to not automatically lead to causality violations.     

Tunnelling of neutral spin-1/2 particles through magnetic fields

The tunnelling of a neutral magnetic dipole moment (neutron or neutrino) through a magnetic field barrier is studied without the weak field approximation. The laws for the change of polarization during the reflection and transmission from vacuum to field and from field to vacuum and during the propagation inside the field are determined. The ambiguities and controversies concerning the interpretation of the change of polarization as due to translational phase of due to spinor transformation law are clarified and possible new experiments are suggested.     

Linear transformations in unitary geometric algebra

The interpretation of complex eigenvalues of linear transformations defined on a real geometric algebra presents problems in that their geometric significance is dependent upon the kind of linear transformation involved, as well as the algebraic lack of universal commutivity of bivectors. The present work shows how the machinery of geometric algebra can be adapted to the study of complex linear operators defined on a unitary space. Whereas the well-defined geometric significance of real geometric algebra is not lost, the primary concern here is the study of the algebraic properties of complex eigenvalues and eigenvectors of these operators.Dedicated to David Hestenes on his 60th birthday.     

Design,development and first experiments on the X‐ray imaging beamline at Indus‐2 synchrotron source RRCAT,India

A full‐field hard X‐ray imaging beamline (BL‐4) was designed, developed, installed and commissioned recently at the Indus‐2 synchrotron radiation source at RRCAT, Indore, India. The bending‐magnet beamline is operated in monochromatic and white beam mode. A variety of imaging techniques are implemented such as high‐resolution radiography, propagation‐ and analyzer‐based phase contrast imaging, real‐time imaging, absorption and phase contrast tomography etc. First experiments on propagation‐based phase contrast imaging and micro‐tomography are reported.