Does the missing mass problem signal the breakdown of Newtonian gravity?

We find dispersion laws for the photon propagating in the presence of mutually orthogonal constant external electric and magnetic fields in the context of the $\theta $-expanded noncommutative QED. We show that there is no birefringence to the first order in the noncommutativity parameter $% \theta .$ By analyzing the group velocities of the photon eigenmodes we show that there occurs superluminal propagation for any direction. This phenomenon depends on the mutual orientation of the external electromagnetic fields and the noncommutativity vector. We argue that the propagation of signals with superluminal group velocity violates causality in spite of the fact that the noncommutative theory is not Lorentz-invariant and speculate about possible workarounds.

The exact solution for the chiral field superpropagator in a constant external Abelian field is given.

The dynamical stability of disk galaxies is sensitive to whether their anomalous rotation curves are caused by dark matter halos or Milgromian dynamics (MOND). We investigate this by setting up a MOND model of M33. We first simulate it in isolation for 6 Gyr, starting from an initial good match to the rotation curve (RC). Too large a bar and bulge form when the gas is too hot, but this is avoided by reducing the gas temperature. A strong bar still forms in 1 Gyr, but rapidly weakens and becomes consistent with the observed weak bar. Previous work showed this to be challenging in Newtonian models with a live dark matter halo, which developed strong bars. The bar pattern speed implies a realistic corotation radius of 3 kpc. However, the RC still rises too steeply, and the central line-of-sight velocity dispersion (LOSVD) is too high. We then add a constant external acceleration field of 8.4 × 10−12 m s−2 at 30° to the disk as a first-order estimate for the gravity exerted by M31. This suppresses buildup of material at the center, causing the RC to rise more slowly and reducing the central LOSVD. Overall, this simulation bears good resemblance to several global properties of M33, and highlights the importance of including even a weak external field on the stability and evolution of disk galaxies. Further simulations with a time-varying external field, modeling the full orbit of M33, will be needed to confirm its resemblance to observations.

This work explores the conditions that allow the injection and pinning of different magnetic domain walls (DWs) in Permalloy nanostripes with notches of different shapes. The injection is done under a constant external field by applying a 10 ns current pulse through an adjacent current line. The type of DW is identified by its anisotropic magnetoresistance (AMR). We find that, while a quasi-static pinning (nucleating at zero field and propagating the DW to the notch by slowly increasing the external field) would allow to distinguish different types of DWs pinned at the notch, a dynamic pinning (nucleating, propagating, and pinning the DW under a constant non-zero magnetic field) makes the discrimination of different DWs very difficult. Micromagnetic simulations indicate that the AMR of the different types of DWs become quite similar to each other as the injection field increases. This might explain why at large injection fields, usually only one or two values of resistance are detected, depending on the shape of the notch. Therefore, caution should be taken when establishing a one-to-one relationship between a type of DW and a value of resistance, especially if the injection is done under a non-zero external magnetic field.

Quantum electrodynamics in an external constant field

Massive scalar and spin 1/2 fields are considered on a two-dimensional space-time with constant background curvature and in the presence of an external constant field. For each case, the Euler–Heisenberg effective action and the pair creation rate and their dependence on both the curvature and external field is determined exactly.

Within the QCD sum-rule approach in an external field, we calculate the baryon matrix element of isovector-scalar current, $H_{\rm B}=\langle B|\overline{u}u- \overline{d}d|B\rangle/2M_{\rm B}$, for octet baryons, which appears in the response of the correlator of baryon interpolating fields to a constant isovector-scalar external field. The sum rules are obtained for a general baryon interpolating field with an appropriate form for the phenomenological ansatz of the spectral density. The key phenomenological input is the response of the quark condensates to the external field. To first order in the quark mass difference $\delta m=m_d-m_u$, the non-electromagnetic part of the baryon isospin mass splitting is given by the product of $\delta m$ and $H_{\rm B}$. Therefore, QCD sum-rule calculation of $H_{\rm B}$ leads to an estimate of the octet baryon isospin mass splittings. The resulting values are comparable to the experimental values; however, the sum-rule predictions for $H_{\rm B}$ are sensitive to the values of the response of the quark condensates to the external source, which are not well determined.

The interaction and the transformation of the electromagnetic and gravitational waves and the variation of those amplitudes are analyzed theoretically, where the Einstein-Maxwell field is assumed. Two waves are defined such that the electromagnetic vector potentials and the gravitational tensor potentials and their first derivatives are continuous while their second derivatives are discontinuous at a wave front. It is proved that the concomitant electromagnetic and gravitational waves can interact with each other if an external electromagnetic field exists. The global behaviors of interaction and transformation of the two concomitant waves are also investigated, where a Lorentz metric space-time is assumed. The variation formulas for amplitude are derived. The amplitudes may grow or decay according to a factor depending on the mean and Gaussian curvatures of an initial wave front and to a factor depending on the external electromagnetic field. When there is no external electromagnetic field, there is no interaction between the two waves and they propagate independently, while when an external field exists, the electromagnetic wave may transform into the gravitational wave and vice versa. In a case of a weak constant external electromagnetic field, where a flat space may be assumed, the amplitude of a wave varies sinusoidally with respect to the distance measured along its ray. The length of interchangeability is defined by the length in which a wave is completely transformed into its dual wave, and it is inversely proportional to the strength of an external electromagnetic field.

We study the orbits in the MOND theory within a dwarf galaxy of mass Md at a distance of ~100kpc from a neighboring galaxy of mass Mg, such as ours. It is assumed that a second mass m<<Md is gravitationally bound to Md by a previously calculated potential for the MOND theory. This potential is obtained for a free falling mass Md in a constant external gravitational acceleration field. The numerical technique of surfaces of section is used to study the stability of the phase-space orbits in the dwarf galaxy. Equatorial orbits with sufficiently small eccentricities e<0.65 are found to be stable with respect to small chang es in the initial conditions. (The equatorial plane is perpendicular to the direction of the external gravitational acceleration, which is along the line joining Md and Mg.) For decreasing values of the conserved component of the angular momentum, in the direction of the external gravitational acceleration, equatorial stability is lost.

The susceptibility for the brownian motion in a cosine potential is proportional to the one sided Fourier transform of the velocity autocorrelation function whereas the polarizability for the rotation of a dipole in a constant external field is proportional to the one sided Fourier transform of the time derivative of the sin- and cos-autocorrelation function. The one sided Fourier transform of these autocorrelation functions can be expressed by matrix continued fractions. They are evaluated for large, medium and even for very small damping constants, thus obtaining various susceptibilities practically in the whole region of friction constants. Furthermore the connection to the zero friction limit case is discussed.

The nonlinear mathematical model of behavior of controllable viscosity fluid (CVF) under applied external field is presented. A large family of these fluids is commonly used to control responding forces of dampers in vibration control applications. The responding force of a damper with CVF has two components. The first one - uncontrollable - is proportional to the viscosity of a base fluid and velocity of its motion, the second one, which is controllable, depends on the strength of the applied external field. Both are involved in the process of dissipation of unwanted energy from the vibrating systems. An equivalent damping factor based on the principle of energy dissipated during one cycle of damper work under a constant strength external field was calculated. When mass or stiffness is variable the equivalent damping factor can be set accordingly by adjusting the strength of external field to have vibrating damped system purposely/continuously working in the critical or other chosen state. This paper also presents cases of applying periodically changing strengths of an external field synchronized with cycles of periodical motion of the vibrating system to continuously control the damping force within each cycle.

The effect of a pulsed (with a duration on the order of 1 ms) and of a constant external electric field on plasma-chemical processes in the lower ionosphere at altitudes of 70–100 km above the Earth’s surface is considered. Interest in these externally stimulated physical settings is associated with traditional radiophysical research and problems of radio-chemical atmospheric physics, which has been actively in recent years due to its various scientific and technical applications. A new page in this field of science (unlike the use of spurious radio signals) opens up the possibility of the purposeful field impact on a GPS receiver on the improvement of work and the further development of satellite navigation systems, which, undoubtedly, has not been discussed previously in the literature. In this work, on the basis of the numerical simulation, the concentrations of the main charged particles and the electron temperature in the presence of a strong external electric field have been calculated at an altitude of 100 km.

The feasibility of a new method for direct overwrite with a power-modulated laser beam under a constant external bias field Hex fixed in the same direction as an initializing field Hinit was demonstrated by means of a dynamic disk tester. TbFeCoCr amorphous films with compensation temperature (Tcomp) far below room temperature were employed, but coercivity falls below 200 Oe at 200 °C, while the saturation magnetization remains high. We found that written pits are formed by a demagnetization field Hd in the region of a weak Hex, whereas for a stronger Hex they are formed by a effective external field Heff(=Hex−Hd) fixed in the same direction as Hinit. By using a power modulation between 9 and 5 mW under a constant field of −100 Oe, we were able to demonstrate direct overwrite, although the readout carrier-to-noise ratio was low (18 dB).

The general integral of the equations of motion of a relativistic string in a constant external field is given. TheN-soliton excitations are constructed and a particular solution (one-soliton excitation) is discussed. The one soliton excitation gives rise to a motion analogous to the helicoidal motion of an electron in a constant magnetic field.

The effect of homogeneous and constant external electric fields on the structural stability of chignolin, a typical β-hairpin peptide, has been studied for 10ns using molecular dynamics simulations. The peptide aligns quickly its total dipole moment with the external electric field and then a constant stress is applied on its charged residues. For weaker fields this has mixed effects on the creation, destruction or strength of hydrogen bonds, but ultimately for strong fields chignolin unfolds starting by the separation of the terminal residues. Since the function of proteins is critically linked to their three-dimensional structures, these findings indicate that exposure to external electric fields may induce changes in conformation that can potentially initiate severe cellular dysfunction.