Interaction between two point-like charges in nonlinear electrostatics

We consider two point-like charges in electrostatic interaction between them within the framework of a nonlinear model, associated with QED, that provides finiteness of their field energy. We find the common field of the two charges in a dipole-like approximation, where the separation between them $R$ is much smaller than the observation distance $r:$ with the linear accuracy with respect to the ratio $R/r$, and in the opposite approximation, where $R\gg r,$ up to the the term quadratic in the ratio $r/R$. The consideration fulfilled proposes the law $a+b R^{1/3}$ for the energy, when the charges are close to one another, $R\rightarrow 0$. This leads to the singularity of the force between them to be $R^{-2/3}$, which is weaker than Coulomb law $R^{-2}$.


I. INTRODUCTION
Recently a class of nonlinear electrodynamic models was studied [1] wherein the electrostatic field of a point charge is, as usual, infinite in the point, where the charge is located, but this singularity is weaker than that of the Coulomb field, so that the space integral for the energy stored in the field converges (and also the scalar potential is finite in the position of the charge [2]). This class unites Lagrangians [2] - [4] that grow with the field invariant F = (B 2 − E 2 ) /2 faster than (−F) w , w > 3/2 (see Ref. [2] also for a subtler estimate of the boundary of the necessary growth). Thus, the simple quadratic effective Lagrangian first considered in a different aspect in [5] is also included into the class under consideration.
The infiniteness of the field near the charge distinguishes the class under consideration from many other models (see the papers [6] and references therein) with finite self-energy of the point charge, allied to their famous prototype, the Born-Infeld model [7], where the finiteness of the field is achieved at the cost of square-root nonanalyticity of the Lagrangian that supplies an infinity to the Maxwell equation. The most popular application [6], [8] of these models is to combine them with the General Relativity in order to study their effect on the initial singularity and on the evolution of the Universe. In contrast to the Born-Infeld model, the models from the class of Ref. [1] refer to nonsingular Lagrangians that follow for instance from the Euler-Heisenberg (E-H) effective Lagrangian [9] of QED truncated at any finite power of its Taylor expansion in the field. This allows us to identify the self-coupling constant of the electromagnetic field with a definite combination of the electron mass and charge and to propose that such models may be used to extend QED to the extreme distances smaller than those for which QED may be thought of as a perfectly adequate theory.
More advanced approaches based on the Euler-Heisenberg Lagrangian that do not depend upon any assumption of smallness of its field argument (the background field) and do not hence appeal to expansion of the Lagrangian in powers of the background fields received attention, as well, under the restriction, however, that not-too-fast-varying in space and time fields are studied as solutions of the nonlinear Maxwell equations. Among the nonlinear effects studied, there are the linear and quadratic electric and magnetic responses of the vacuum with a strong constant field in it to an applied electric field [10], with the emphasis on the magneto-electric effect [11,12] and magnetic monopole formation [13]. Also self-interaction of electric and magnetic dipoles was considered with the indication that the electric and magnetic moments of elementary particles are subjected to a certain electromagnetic renormalization [14] after being calculated following a strong-interaction theory, say, QCD or lattice simulations. Interaction of two laser beams against the background of a slow electromagnetic wave was studied along these lines, too [15]. The finiteness of the field energy allows one to develop a soliton view on a moving point charge [3], [16].
In the present paper we are extending the consideration to cover the electrostatic problem of a system of two point charges that interact following nonlinear Maxwell equations stemming from the Lagrangian quadratic in the the field invariant F. Their common field is not, of course, just a linear combination of the individual fields of the two charges. The nonlinear problem is outlined in the next Section II, where we present the nonlinear Maxwell equations and give them the form in Subsection II B apt for finding the approximate solutions of Section III. Once the field energy is finite it is possible in principle to define the attraction or repulsion force between charges as the derivative of the field energy with respect to the distance R between them. Contrary to the standard linear electrodynamics, this is evidently not the same as the product of one charge by the field strength produced by the other! This rule holds true only if one of the charges is much smaller in value than the other.
In Section III we develop the procedure of finding the solution to the static two-body problem in two opposite approximations determined by the ratio of the distance R, to the coordinate of the observation point r 1 . Where this ratio is small, R/r 1, we find in Subsection III A the leading expression for the common field, which makes the nonlinear correction to electric dipole, and the corresponding potential. In Subsection (III B)) the opposite case r/R 1 is considered, first, also in the leading approximation (Subsubsection III B 1). The simplifying circumstance that makes these approximations easy to handle is that it so happens that one needs, as a matter of fact, to solve only the second (following the classification of Ref. [17]) Maxwell equation, the one following from the least action principle, while the first one, the Bianchi identity, [∇ × E] = 0, is trivially satisfied. The situation becomes far more complicated in the next-to-leading approximation (r/R) 2 developed in Subsubsection III B 2. In developing the above approximations no assumption was made on whether the nonlinear scale determined by the self-coupling of the electromagnetic field is 1 Throughout the paper, Greek indices span Minkowski space-time, Roman indices span its threedimensional subspace. Boldfaced letters are three-dimensional vectors, same letters without boldfacing and index designate their lengths, except the coordinate vector x = r, whose length is denoted as r. The large or small as compared to r or R. Their use in the expression for the energy of the two charges as a function of the separation R in the limit R → 0, i.e. when the charges are so close to one another that R is much less than the nonlinearity scale, allows to make a preliminary estimation confirmed by another approach to be reported in a separate publication that for small separation, the energy of the system of two point charges can be presented as a+bR 1/3 , where a and b are finite constants depending only on the two charges (in QED they include the electron mass and charge). Hence the force between two point-like charges turns to infinity following the law R −2/3 . This formula replaces, in the given nonlinear model, the Coulomb law R −2 for the force between two point charges.

II. NONLINEAR MAXWELL EQUATIONS
A. Nonlinear Maxwell equations as they originate from QED It is known that QED is a nonlinear theory due to virtual electron-positron pair creation by a photon. The nonlinear Maxwell equation of QED for the electromagnetic field tensor produced by the classical source J µ (x) may be written as, see e.g. [10].
Here L (F, G) is the effective Lagrangian (a function of the two field invariants F = (1/4)F µν F µν and G = (1/4)F µν F µν ), of which the generating functional of one-particleirreducible vertex functions, called effective action [18], is obtained by the space-time known as the E-H effective action [9]. It is valid to the lowest order in the fine-structure constant α, but with no restriction imposed on the the background field, except that it has no nonzero space-time derivatives. Two-loop expression of this local functional is also available [19].
The dynamical Eq. (2.1), which makes the "second pair" of Maxwell equations, may be completed by postulating also their "first pair" whose fulfillment allows using the 4-vector potential A ν (x) for representation of the fields: . This representation is important for formulating the least action principle and quantization of the electromagnetic field. From it, Eq. (2.2) follows identically, unless the potential has the angular singularity like the Dirac string peculiar to magnetic monopole. In the present paper we keep to Eq. (2.2), although its local denial is not meaningless, as discussed in Refs. [13], where a magnetic charge is produced in nonlinear electrodynamics.
We are going now to separate the electrostatic case. This may be possible if the reference frame exists where all the charges are at rest, J 0 (x) = J 0 (r) (We denote r = x). Then in this "rest frame" the spacial component of the current disappears, J (x) = 0, and the purely electric time-independent configuration F ij (r) = 0 would not contradict equation With the magnetic field equal to zero, the invariant G = (E · B) disappears, too.
In a theory even under the space reflection, to which class QED belongs, also we have = 0, since the Lagrangian should be an even function of the pseudoscalar G.
Then we are left with the equation for a static electric field with the constant external charge J 0 (r) and the zero argument set for the second field invariant G. In the rest of the paper we shall be basing on this Lagrangian in understanding that it may originate from QED as described above or, alternatively, be given ad hoc to define a certain model. In the latter case, if treated seriously as applied to short distances near a point charge where the field cannot be considered as slowly varying, in other words, beyond the applicability of the infrared approximation of QED outlined above, the Lagrangian (2.4) may be referred to as defining an extension of QED to short distances once L (F,0) is the E-H Lagrangian (or else its multi-loop specification) restricted to G = 0. L (F,0) grows with −F as (−F) w , the field energy is finite provided that w > 3/2. The derivation of this condition is given in [2] and in [4]. As a matter of fact a more subtle In the present paper we confine ourselves to the simplest example of the nonlinearity generated by keeping only quadratic terms in the Taylor expansion of the E-H Lagrangian in powers of the field invariant F where the constant and linear terms are not kept, because their inclusion would contradict the correspondance principle that does not admit changing the Maxwell Lagrangian L Max = −F for small fields. The correspondence principle is laid into the calculation of the E-H Lagrangian via the renormalization procedure.
Finally, we shall be dealing with the model Lagrangian quartic in the field strength with γ being a certain self-coupling coefficient with the dimensionality of the fourth power of the length, which may be taken as where e and m are the charge and mass of the electron, if L is chosen to be the E-H oneloop Lagrangian. We do not refer to this choice henceforward. Generalization to general Lagrangians can be also done in a straightforward way.
The second (2.3) and the first (2.2) Maxwell equations for the electric field E with Lagrangian (2.5) are Denoting the solution of the linear Maxwell equations as E lin (r) we write the solution of (2.7), in the following way [10] - [14] 1 + The second Maxwell equation (2.9) may be conveniently written in the form to be exploited later where the function ξ(x) is defined as the real solution to the cubic equation Its explicit form is given by the Cardano formula: We substitute (2.10) in the first Maxwell equation (2.8) to get: where the prime designates the derivative with respect to the argument. Taking into account the relations In the center-symmetric case of a single point charge considered in [1], [3], [2], [16], one has Ω(r) = 0, as a solution to the equation (2.15). This simplification makes the exact solution possible. The equality Ω(r) = 0 holds as well in the axial-symmetric problem of two point charges within the approximations linear with respect to the ratios R/r or r/R to be considered in Subsections III A and III B of the next Section. In these cases it will be sufficient to present the solution of the differential part of Eq. (2.7) in the form (2.9) setting Ω(r) = 0 in it, then the first Maxwell equation (2.8) is fulfilled automatically. On the contrary, within the next order of (r/R) 2 the pseudovector function Ω(r) is nontrivial, which makes the axial-symmetric "quadrupole-like" solution found in Subsection III B 2 for the field of two point-like charges more sophisticated.

III. TWO-BODY PROBLEM
By the two point-charge problem we mean the one, where the current j 0 (r) in (2.7) is the sum of delta-functions centered in the positions r = ±R of two charges q 1 and q 2 separated by the distance 2R (with the origin of coordinates x i placed in the middle between the charges) In what follows we shall be addressing this equation as accompanied by (2.8) for the combined field of two charges.
In what follows we shall refer to the field energy density that in the present model (2.5), when there is electric field alone, reads The integral for the full energy of two charges converges since it might diverge only when integrating over close vicinities of the charges.
But in each vicinity the field of the nearest charge dominates, and we know from the previous publication [1] (also to be explained below) that the energy of a separate charge converges in the present model. When the charges are in the same point, R = 0, they make one charge q 1 + q 2 , whose energy converges, too.
A. Small separation r R between charges (dipole approximation) We shall be looking for the solution of (3.1) in the form where E (0) and E (1) are contributions of the zeroth and first order with respect to the ratio R/r 1, respectively. This strong inequality means that the observation point is far from the location of the both charges. So the result of consideration in the present subsection will be the extension of the dipole field to the case, where the point charges self-interact and interact nonlinearly with each other.
The zero-order term is spherical-symmetric, because it corresponds to two charges q 1 , q 2 in the same point that make one charge q 1 + q 2 , namely, we shall find the coefficient functions a, g from Eq. (3.7) and then ascertain that the relation ( 3.6) is obeyed by the solution.

The inhomogeneity in (3.7)
E lin (r) = q 1 4π satisfies the linear (γ = 0) limit of equation (3.1) ∇ · E lin (r) = q 1 δ 3 (r − R) + q 2 δ 3 (r + R) and also (2.8). The inhomogeneity (3.8) is expanded in R/r as This is the standard monopole+dipole approximation in understanding that d = (q 2 − q 1 ) R is the dipole moment of the two charges, while the dots stand for the disregarded quadrupole and higher multipole contributions.
The zero-order term satisfies the equation with the first term of expansion (3.16) taken for inhomogeneity. This is an algebraic (not differential) equation, cubic in the present model (2.5), solved explicitly for the field E (0) as a function of r in this case, but readily solved for the inverse function r(E (0) ) in any model, this solution being sufficient for many purposes. Even without solving it we see that for small r γ 1/4 the second term in the bracket dominates over the unity, therefore the asymptotic behavior in this region follows from (3.10) to be This weakened -as compared to the Coulomb field (q 1 + q 2 )/(4π)r −2 -singularity is not an obstacle to convergence of the both integrals in This equation is linear and it does not contain derivatives. We use (3.5) as the ansatz. After we obtain two equations, along R and r, with the solutions: where δq = (q 2 − q 1 )/(4π), Q = (q 2 + q 1 )/(4π). From (3.10) we obtain d dr

Hence d dr
With the help of this relation the derivative of (3.12) can be calculated to coincide with (3.13) times r. This proves Eq. (3.6) necessary to satisfy the first Maxwell equation (2.8).
By substituting relations (3.12) and (3.13) in the decomposition (3.5) we finally have for the solution of the both Maxwell equations up to O(R 2 /r 2 ).
The potential corresponding to the electric field (3.14) has the form where V 0 (r) is the potential of the field of one charge [2]:

B. Large separation r R between charges
The quantities that relate to the approximation valid at r R, dealt with in this Subsection, will be supplied with tilde to be distinguished from the corresponding quantities in the previous Subsection III A relating to the opposite approximation.
Let us expand the inhomogeneity (3.8) to the first order in the ratio r/R (without assuming the smallness of R and r as compared to γ 1/4 ): The first term here has a clear meaning of the sum of two oppositely directed Coulomb fields produced in the point r = 0 by the two charges placed far from one another. The second one looks like a dipole field in the variable R with the equivalent "dipole moment" (q 2 + q 1 ) r.
We are looking for solution to equation (3.7) in the form of the expansion in powers of In the zeroth order we have the equation for in form which implies that f /(4πR 2 ) = E (0) (R) is the function obtained from E (0) (r) of the previous Subsection III A by the substitution r → R and q 2 + q 1 → q 2 − q 1 .

Leading (dipole-like) approximation
In the first order, the use of (3.18) turns equation (3.7) to the linear algebraic equation Calculating the second term in the right-hand side (the auxiliary electric field E(r) = (γ/2)E 2 (r)E(r)) with the ansatz (3.17) we obtain from (3.19) two equations for the components of E (1) along R and along r that determine the values , (3.20) where E (0) is the solution of equation ( 16) we observe that in the zero-order term, the difference (q 2 − q 1 )R −2 of the two Coulomb fields in the point r = 0 has been replaced by the nonlinear field E (0) of the equivalent charge q 2 − q 1 , while in the first-order term the "dipole field" (q 2 + q 1 ) r has been modified by two different factors in the terms parallel to r and R.
To be more general, note that the fields (3.20) and (3.14) turn into one another under the simultaneous replacement of the observation coordinate r by the separation R between charges, and of the sum q 2 + q 1 of the charges by their difference q 2 − q 1 . The same symmetry under the interchange r ↔ R, q 2 + q 1 ↔ q 2 − q 1 certainly holds for the linear γ = 0 limits

Next-to-leading (quadrupole-like) approximation
In this Subsubsection we are studying the next, quadratic in the ratio r/R, term E (2) extending the expansion (3.17). To this end we first extend the expansion (3.16) of the linear field (3.8) E lin (r) to include the corresponding term: where Ω φ ((r · R)/(rR)) is a scalar function of the angle θ between the observation direction and the axis, on which the charges lie, cos θ = (r · R)/(rR). The straightforward calculation yields (we refer to the orts e r = r r , e φ = − r×R |r×R| , e θ = e φ × e r and to the relation (∇ · Ω) = 0) obeyed by (3.22)) From the last relation it follows that it is sufficient to solve equation (2.15) up to the first order in r/R. We expand the right-hand side of equation (2.15) in a series in r/R: (3.26) equation for the function Ω φ (θ): , and ξ is the solution (2.12) of Eq. (2.11).
Note that (3.29) obeys the first Maxwell equation ∇ × E (2) = 0 identically for any coefficients c and d.
Similarly to the coefficient (q 1 − q 2 ) in the third (quadrupole) term in (3.21), the coefficients c(R) and d(R) are odd under the permutation q 1 ↔ q 2 . Note that the seeming singularity at q 1 = q 2 cancels from these coefficients due to the equality κ(R) = 1 that holds in this case. In the linear limit γ = 0 one also has κ(R) = 1, and c(R) and d(R) turn both into q 1 − q 2 , so that (3.29) turns into the last (quadrupole) term in the expansion (3.21) of the linear field.

IV. CONCLUDING REMARKS
We were working within the simplest nonlinear electrodynamics with the self-interaction of the fourth power of electromagnetic field (2.5), which, if needed, may be thought of as resulting from the first nontrivial term of expansion of the Euler-Heisenberg effective Lagrangian in powers of its background field argument F, while the other field invariant is kept vanishing G =0. In this case the coefficient γ, whose dimensionality is [length 4 ], that determines the strength of nonlinearity is expressed as (2.6) in terms of the electron mass and charge. Otherwise it may be considered as arbitrary. Anyway, in our calculation a smallness of γ 1/4 as compared to the two other quantities r and R carrying the dimensionality of length was nowhere assumed.
We considered the electrostatic problem of interaction between two point-like charges q 1 and q 2 placed in the points r = ± R by solving the nonlinear Maxwell equation (3.1), which follows from the least action principle for the Lagrangian (2.4), together with the standard Bianchi identity (2.8). For the small separation between the charges, R r, we found the electric field (3.14) in the approximation, linear with respect to the ratio R/r, which serves the nonlinear extension of the usual dipole field. The result for the corresponding scalar potential is Eq. (3.15). The lines-of-force and equipotential-curves pattern is shown in Fig.   1 in the configuration space r with the parameters chosen in such a way as to make the nonlinearity effect best pronounced. For large separation between the charges, R r, we found the electric field in the approximation, linear (3.20) with respect to the ratio r/R, and quadratic (3.29).
Using the two opposite reprentations (3.14) and (3.20) we can get a rough estimate for the behaviour of the field-energy (3.3), (3.2) in the asymptotic regime R → 0, where the two point charges approach each other infinitely close. According to that estimate, in this regime the energy of the system of two point charges can be presented as where a and b are finite constants depending only on the charges q 1 and q 2 , and on the selfcoupling constant γ. The R-independent term a is the self-energy of the united point-like charge with the value q 1 + q 2 . This is finite, as established in [1]. The behaviour (4.1) is rigorously confirmed following a quite different procedure to be published elsewhere. Although the energy is finite in the limit R = 0, the force F between the two charges defined as the derivative of the energy with respect to the distance is weakly infinite: This formula replaces, in the given nonlinear model, the Coulomb law F ∼ R · R −3 for the force between two point charges. The power 2/3 here is determined by the power 2 in the self-interaction in (2.5).