The modified electromagnetism and the emergent longitudinal wave

The classical theory of electromagnetism has been revisited and the possibility of longitudinal photon wave is explored. It is shown that the emergence of longitudinal wave is a consequence of Lorenz gauge (condition) violation. Proca, Vlaenderen&Waser and Arbab theories are investigated.


Introdution
Maxwell's equations are one of the biggest edifices that mankind had come to establish. Maxwell unified electromagnetism with light in a set of consistent equation dubbed in Maxwell equation. These equation prematurely predicted that speed of light in vacuum is constant. In an elegant theory, Salam, Weinberg and Glashow unified electromagnetism with weak interaction. The ultimate goal of unifying all interactions into a single universal theory is underway. Some theoretical prejudices advocated by Proca in extending maxwell's theory to encompass massive photons was expounded in 1967. This later theory will be of importance, at cosmic scale, as some compelling investigation usher in that direction. Of these investigation is the missing mass (dark matter) in the universe. It is remarked by Schrodinger that if the photon had a mass then the Black-Body formula will be modified and the energy density of these photons would be 3/2 times the ordinary one. This because these photon would have a longitudinal polarization besides the transverse one. Longitudinal waves have been shown to have high penetration power. This character rendered them to be of great importance and will find wider applications in communication systems. Some endeavors to generalize Maxwell's equations to remedy these shortcoming have been proposed by following a quaternionic generalizations. In the framework of these attempts, the shortcoming present in the original Maxwell's formulation can be avoided. In this work, we will outline the different approaches and explore these interrelations. We will see that whenever the Lorenz gauge (condition) is violated, longitudinal waves will be inevitable. We also analyze the status of the experimental evidence supporting the presence of these waves.

Maxwell's equation
Maxwell's equations describe the behavior of the electric and magnetic fields. These equations are and and Vector and scalar potential The electric and magnetic fields are defined as It is belied that vector and scalar potentials, A and ϕ, are more fundamental than E and B. For this reason, we describe the electromagnetic interaction in terms of these potentials, rather than the electric and magnetic fields. This is apparent, at the quantum level, in the Aharonov-Bohm effect where when two beams of electrons are sent through region with same B, but different A, experience an interference pattern when collected at the detector. This occurs because the phase factor of the electron wavefunction depends on A. Substituting eq.(3) in (1) and (2), we obtain and 1 c 2 These are the Maxwell's equations in terms of the vector and scalar potentials.

Lorenz gauge
The electric and magnetic fields are invariant under the gauge transformations, This allows the freedom of taking the (Lorenz gauge) condition, In this case one has the condition that It is interesting to see that the electric and magnetic fields can not be uniquely specified by A and ϕ. For this reason these two fields are treated as mere mathematical constructs and bear no physical meaning, but E and B do.

Vlaenderen and Waser approach
Vlaenderen and Waser have recently adopted a biquaternionic formulation of Maxwell's equation and introduced a scalar function (S) that is a measure of the violation of Lorenz gauge. They remarked that this scalar field mimics the Maxwell's displacement current. For this case, they assumed that Accordingly, they obtain the following equations and The scalar S satisfies the wave equation upon using the continuity equation. Besides this, S is found to represent an electroscalar wave having a Poynting vector S E, i.e., along the electric field direction.

Proca's equations
Proca generalized Maxwell's equations by introducing a massive photon filed. The resulting equations are and universal quantum equations We have recently introduced a system of unified quantum wave equations describing matter waves. Like Maxwell's formulation, in this formulation the particle is described by scalar and vector potentials. These equations read Here ψ and ψ are the scalar and vector fields (potentials) representing the material wave nature that endorsed by de Broglie of a given particle (mass). We assume that any wave that carries a material nature should satisfy these equations. When m = 0 the material nature disappears and we obtain a wave traveling at the speed of light.
The analogy among these paradigms Differentiation partially the first equation w.r.t. t, taking the divergence of the second equation and subtracting the resulting two equations yield This is the familiar Klein-Gordon equation of massive spin zero particle. Thus, if the photon had a non-zero mass, as hypothesized by Proca, then this photon wave must be governed by eq.(18).
Integrating the second equation in eq.(17) yields But from Stokes theorem, Therefore, eq.(17) can be written as we consider here f (t) = constant. The above equation is nothing but the electromotive force. Hence, the scalar potential ϕ represents the electromotive force. It is evident from eq.(20) that if the Lorenz gauge is satisfied, then the photon must be massless (µ 2 γ = 0), or otherwise φ m = 0 . Moreover, the non-satisfaction of Lorenz gauge would lead to massive photon and non-zero magnetic flux. Equation (18) and (20) imply that the magnetic flux satisfies the Klein-Gordon equation, where the flux has a mass, µ 2 γ . Thus, the magnetic flux would propagate as a bosonic particle with spin zero. If we assume now the scalar and vector potentials satisfy the matter wave equation, i.e., eq.(15), then 1 We consider here −S instead of S This equation can be compared with eq.(9), in which case, S = − 2m h ϕ. If we compare eqs. (9), (17), (18) and (22) we will get This implies that the scalar potential of the massive photon, ϕ, satisfies the Klein-Gordon equation. Now differentiating partially eq.(22) with respect to time and using (3) yield Taking the gradient of eq.(22) and using eq.(3) and the second equation in eq.
(2), we obtain where we have used the vector identity, It is interesting to see that if the photon had a mass (m = 0) in vacuum, then the Lorenz gauge in eq.(22) would not be satisfied. This encourages us to associate a non-zero mass of the photon to the breaking of Lorenz gauge. In vacuum, ρ = 0 and J = 0, so that eqs.(24) and (25), using eq.(3), become Taking the gradient of eq.(22) and using eq.(3) and the second equation in eq.(2), we obtain 1 c 2 This is a quite surprising result. It is evident from eqs.(26) and (27) that spatial and temporal variations of the scalar field is the source of the ϕ and A waves. Now apply the gauge transformation, eq.(6), in eq.(22) to get This equation is know as the Telegrapher's equation. It describes the electric signals in telephone and telegraph lines. However, by making the substitution, Λ(r, t) = e −(mc 2 t/h) χ(r, t), eq.(29) can be written as which is the ordinary Klein-Gordon equation of massive scalar field χ. This equation is a consequence of gauge invariance.

The combined gauge transformation
We will consider here the combined gauge transformations in which both ϕ & A, as well as, ρ & J are transformed. Interestingly, Proca equations are invariant under these combined gauge transformations. These are given by and Under these gauge transformations, Proca's equations become gauge invariant and that the mass term is no longer problematic. Moreover, these transformations leave the system of the continuity equations invariant as well, as long as Λ satisfies the wave equation in eq.(8) [ref. ].

The status of experimental evidence of longitudinal wave
There are some appealing experiments that amount to the existence of longitudinal wave. An electric longitudinal wave has been observe by ... et al. in ..... This wave is characterized by the vanishing magnetic component. The enormous success in quantum electrodynamics has lead to the acceptance of the concept of massless photons. Also, the nonzero photon mass would give rise to a wavelength (and frequency) dependence of the speed of light in free space, a more rapid (exponential or Yukawa) falloff of magnetic dipole fields with distance than the usual inverse cube dependence [1], the possibility of longitudinal electromagnetic waves. This wave is characterized by the vanishing magnetic and electric field components [2]. A massive photon is equivalent to a violation of Coulomb's Law [3]. Hence, a Yukawa factor e −r/λ in the 1/r electrostatic and magnetostatic potential terms would appear. The magnetic field term will contain correction terms related to the de Broglie wavelength λ γ =h/m γ . Another way to test the existence of the photon mass and hence the longitudinal waves is the limit of the power in the inverse of the distance distance in Coulomb law, namely 1/r 2+ε . One of the consequences of ε = 0is the decrease of the electromagnetic waves as the wavelength increases which is due to the smallness of ε. A limit of ε = 5.1×10 −20 was set by Lou et al [4]. According to the uncertainty principle the upper limit on the photon mass can be obtained from m γ ≈h/(∆t c 2 ). Using the age of the universe as 10 10 years one obtains m γ ≈ 10 −66 g.
De Broglie noticed that photon mass leads to a larger speed of violet light than that of the red one [5]. He concluded that during the eclipse in a double star system the color of the appearing star would change from violet to red. On the other hand, Schrdinger pointed out that magnetic field of the Earth would be exponentially cut off at distances of the order of the photon Compton wave length [6]. From the observed altitude of auroras he concluded that λ γ > 10 4 . Other data have shown more limits also. Because of limitations involved in experiments performed on the Earth, astrophysical measurement could promise better limits on photon masses. One of the consequences of photons being massive is the dispersion of star light. According to measurements performed on planet Jupiter the upper limit of Compton wavelength is λ γ = 3.14 × 10 11 cm [7]. Based on analyzing the Earth's magnetic field the upper limit on photon mass is m γ = 1 × 10 −48 g [8]. Also from geomagnetic field Fischbach et al found a limit on the upper mass of the photon as m γ ≤ 10 −48 g [9]. According to Proca equation, the velocity of light depends on its frequency. From the measurement on the radio signals from the Crab Nebula it was found that the upper limit on the mass of the photon is m γ ≤ 10 −44 g [10]. Also, hydrodynamic waves coupling to the interplanetary magnetic fields could be useful in finding the mass of the photon. Ryutov (1997) set a limit of m γ ≤ 10 −51 g based on these studies [11]. Other limits on the photon mass are obtained from studying the current density and plasma of the interstellar media (m γ ≤ 10 −53 g) [12]. Modern theories that appeal to the concept of spontaneous symmetry breaking assume that particles, which are massless above a certain critical temperature T c , acquire mass below this temperature. This can be applied to photons. From the cryogenic photon mass experiment performed by Ryan et al an upper limit mass on photons were found to be m γ = 1.5 × 10 −42 . From the symmetry breaking point of view and the experiment, one can speculate that photons can be massless above a certain temperature [13]. Superconductivity is characterized by perfect conductivity and perfect diamagnetism.
BCS theory can explain both of these fundamental properties of superconductors. However, the London equations can be easily derived and used to understand the expulsion of flux within the material without appealing to the broader BCS theory. Such a derivation relies solely on postulating that the Cooper pairs in the superconducting state maintain their ground state momentum of p = 0. It has been possible to identify an effective photon mass that is directly related to the London penetration depth of the applied field [14].
In 2008 a Russian group performed a measurement on the volt value triggered by the radiation coming from solar eclipse. It was noticed the registered signal does not depend on the orientation of the tourmaline plates used in front of the detector [15]. This could be an indication of the longitudinal wave component and, hence, the non-zero mass of the photon . Another experimental evidence for the existence of the massive photons is the Tesla coil. The coil is a device which produces high frequency alternating current with high voltage and low current. The coil is used to produce energy which can be transmitted to a device without any wires. A neon light can lit without wiring connection, hence. This can be explained in terms of the longitudinal component of the lightwave.