Axion and dilaton + metric emerge jointly from an electromagnetic model universewith local and linear response behavior

# Axion and dilaton + metric emerge jointly from an electromagnetic model universe with local and linear response behavior

## Abstract

We take a quick look at the different possible universally coupled scalar fields in nature. Then, we discuss how the gauging of the group of scale transformations (dilations), together with the Poincaré group, leads to a Weyl-Cartan spacetime structure. There the dilaton field finds a natural surrounding. Moreover, we describe shortly the phenomenology of the hypothetical axion field. — In the second part of our essay, we consider a spacetime, the structure of which is exclusively specified by the premetric Maxwell equations and a fourth rank electromagnetic response tensor density with 36 independent components. This tensor density incorporates the permittivities, permeabilities, and the magneto-electric moduli of spacetime. No metric, no connection, no further property is prescribed. If we forbid birefringence (double-refraction) in this model of spacetime, we eventually end up with the fields of an axion, a dilaton, and the 10 components of a metric tensor with Lorentz signature. If the dilaton becomes a constant (the vacuum admittance) and the axion field vanishes, we recover the Riemannian spacetime of general relativity theory. Thus, the metric is encapsulated in , it can be derived from it.
file CarlBrans8008.tex, 26 Mar 2016

## Contents

1. Dilaton and axion fields

1. Scalar fields

2. Einstein gravity and the energy-momentum current

3. Einstein-Cartan gravity: the additional spin current

4. Dilaton field and dilation current

5. The Weyl-Cartan spacetime as a natural habitat of the dilaton field

6. Axion field

2. An electromagnetic model universe

1. The premetric Maxwell equations

2. A local and linear electromagnetic response

3. Propagation of electromagnetic disturbances

4. Fresnel wave surface

5. Suppression of birefringence: the light cone

6. Axion, dilaton, metric

3. Discussion

“Universally coupled, thus gravitational, scalar fields are still active players in contemporary theoretical physics. So, what is the relationship between the scalar of scalar-tensor theories, the dilaton and the inflaton? Clearly this is an unanswered and important question. The scalar field is still alive and active, if not always well, in current gravity research.”

Carl H. Brans (1997)

## 1 Dilaton and axion fields

### 1.1 Scalar fields

The Jordan-Brans1-Dicke scalar, the dilaton, the axion, the inflaton—scalar fields everywhere—and eventually even one, the scalar, that is, the spinless Higgs boson , which has been found experimentally as heavy as some 134 protons. These different scalar fields2 are not necessarily independent from each other, it could be, for example, that the JBD-scalar can be identified with the dilaton (see Fujii:2003pa ()) or the Higgs boson with the inflaton (see Barvinsky:2009fy (); Bezrukov:2015 ()). Thus, the list of potentially existing universally coupled scalar fields could be somewhat smaller. For the history of the JBD-scalar, one can compare Brans Brans:2005ra () and Goenner Goenner:2012cq () and, for the role of the inflaton in different models, Vennin et al. Vennin:2015eaa ().

### 1.2 Einstein gravity and the energy-momentum current

As remarked by Brans Brans:1996tp () in the quotation above, if universally coupled, the scalar fields are intrinsically related to the gravitational field. In Einstein’s theory of gravity, general relativity (GR), the gravitational potential is the metric , with as (holonomic) coordinate indices. As its source acts the symmetric energy-momentum tensor of matter. This is a second rank tensor, which is generated already in special relativity (SR) with the help of the group of translations in space and in time. Together with the Lorentz transformations , the translations build up the Poincaré group as a semi-direct product: . This is the group of motion in the Minkowski space of special relativity, see Giulini:2008 ().

Accordingly, if one desires to understand gravity from the point of view of the gauge principle, the is an indispensable part of these considerations. However, being only one piece of the , it is suggestive to gauge the complete . This is exactly what Sciama and Kibble did during the beginning 1960s, see Obukhov:2006gea (), (Reader, , Chap.4), and Chen:2015vya ().

### 1.3 Einstein-Cartan gravity: the additional spin current

This gauging of the extends the geometrical framework of gravity. The 4 translational potentials and the 6 Lorentz potentials span a Riemann-Cartan spacetime, enriching the Riemannian spacetime of GR by the presence of Cartan’s torsion; here are (anholonomic) frame indices. Whereas the translational potentials couple to the canonical energy-momentum tensor of matter , the Lorentz potentials couple to the canonical spin current of matter .

The simplest version of the emerging Poincaré gauge models is the Einstein-Cartan theory (EC), a viable gravitational theory competing with GR if highest matter densities are involved. If denotes the Planck length and the Compton wave length of a particle, then deviations of the Einstein-Cartan from Einstein’s theory are expected at length scales of below ; for protons, prevalent in the early cosmos, it is about . According to Mukhanov Mukhanov (), it is exactly this order of magnitude down to which, according to recent cosmological data, GR is known to be valid.

From a gauge theoretical point of view, the EC-theory looks more reasonable than GR since the Einsteinian principles of how to heuristically derive a gravitational theory were followed closely: they were just applied to fermionic matter instead of to macroscopic point particles or Euler fluids or to classical electromagnetism, as Einstein did.

Incidentally, in the EC-theory and, more generally, in the Poincaré gauge theory, the Poincaré and, in particular, the Lorentz covariance are valid locally by construction, similar as in a Yang-Mills theory, we have local covariance. Kostelecký (priv. comm., Jan. 2016) agrees that the “Einstein-Cartan theory maintains local Lorentz invariance.” Then the same is true for a Poincaré gauge theory, which acts likewise in a Riemann-Cartan spacetime with torsion and curvature. However, in an experimental set-up, according to Kostelecký Kostelecky:2007kx (), torsion must be considered as an external field and, according to the “standard lore for backgrounds,” local Lorentz invariance is broken. By the same token, an external magnetic field in electrodynamics breaks local Lorentz invariance. This is, in my opinion, an abuse of language, which conveys the wrong message that the existence of a torsion field violates local Lorentz invariance.

If, for theoretical reasons, one wants to evade the emergence of the Riemann-Cartan spacetime, then one can manipulate, in the underlying Minkowski space, the intrinsic or spin part of the total angular momentum of matter in such a way that it vanishes on the cost of increasing the orbital part of it by the corresponding amount, see Mielke:1989vg (). This procedure is called Belinfante-Rosenfeld symmetrization of the canonical energy-momentum current, which, in general, is defined as an asymmetric tensor by the Noether procedure. Accordingly, by symmetrization the energy-momentum current is made fit to act as a source of the Einstein field equation. In this way, one can effectively sweep the spin and the torsion under the rug and can live happily forever in the paradise of the Riemannian spacetime of GR.

Of course, in the end observations and/or experiments will decide which of the two theories, GR or EC, will survive. We opt for the latter.

### 1.4 Dilaton field and dilation current

The dilaton field entered life as a Nambu-Goldstone boson of broken scale invariance, see Fujii in Fujii:2003pa (). Thus, is related to dilat[at]ions or scale transformations in space and time. But the dilaton also occurs in theories of gravity (JBD) and in string theory, see Di Vecchia et al. DiVecchia:2015jaq (). The , if multiplied (semi-directly) with the scale group, becomes the Weyl group . This 11-parameter group is an invariance group for massless particles in special relativity. The translations, via Noether’s theorem, generate the conserved energy-momentum tensor , the Lorentz transformations the conserved total angular momentum tensor , and the dilation the conserved total dilation current ,

 ∂kΣik = 0, (1) ∂kJijk = ∂kτijk−Σ[ij]=0, (2) ∂kΥk = ∂kΔk+Σkk=0, (3)

see Lopu (); Kopczynski:1988jq () particularly for . Thus, if a universal coupling is assumed, then should have the intrinsic dilation current as its source; for theories in Weyl spaces in which does not play a role, see Scholz Scholz:2014kba (); Scholz:2014tba ().

There are numerous field theoretical models under way which, if scale or dilation invariance is implemented, have conformal invariance as a consequence; for a more recent review see Nakayama Nakayama:2015 (). Hence, jumping to conformal invariance, before one understood scale invariance, is probably not a very good strategy. For this reason we confine ourselves here to scale invariance, to the dilaton, and to the 11-parametric Weyl group. But it should be understood that the light cone is also invariant under the 15-parametric conformal group, see Barut & Ra̧czka Barut () and Blagojević Blagojevic:2002 () and, for a historical account, Kastrup Kastrup:2008 ().

Both currents, the intrinsic dilation current and the energy-momentum current are related to external groups, to the dilation (scale) and to the translation groups, respectively. This is the reason for their universality.

### 1.5 The Weyl-Cartan spacetime as a natural habitat of the dilaton field

We only tried to make a strong case in favor of the EC-theory in order to repeat the corresponding arguments for the dilation group. Gauging the Weyl group yields a Weyl-Cartan spacetime. The classical paper in that respect is the one of Charap and Tait, see (Reader, , Chap.8). A universally coupled massless scalar field induces a Weyl covector as the corresponding dilation potential willy nilly. This is the type of spacetime Weyl used (with vanishing torsion) for his failed unified theory of 1918. Here the Weyl space with the connection is resurrected for the dilation current, instead of for the electric current, see Blagojevic:2002 ():

 \lx@stackrelW∇igjk=−Qigjk,WΓijk=RCΓijk+12(Qigjk+Qjgki−Qkgij); (4)

is the connection of the Riemann-Cartan space. Again, as in the case of the Lorentz group, one can manipulate the total dilation current and can transform its intrinsic part into an orbital part by modifying in this case the trace of the energy-momentum current. Then, again, one can stay within the realm of the Riemannian space of GR, see Callan, Coleman, and Jackiw Callan:1970ze ().

As we mentioned already, the gauge theoretical answer was given by Charap and Tait Charap:1973fi (). Again, which approach will succeed is eventually a question to experimental verification.

We see, if the JBD-scalar is interpreted as a dilaton, then we would expect that the Weyl-Cartan spacetime is its arena. Clearly this does only provide the kinematics of the theory. The dynamics would depend on the exact choice of the dilaton Lagrangian.

Recently, Lasenby and Hobson Lasenby:2015dba () wrote an in-depth review of gauging the Weyl group and, moreover, formulated an “extended Weyl gauge theory.” Also within their framework, the Weyl-Cartan space, and a straightforward extension of it, play an important role, see also Haghani et al. Haghani:2014zra (). Definite progress has also been achieved in the study of equations of motion within the scalar tensor theories of gravity, see Obukhov and Puetzfeld Obukhov:2014mka (); Puetzfeld:2014qba (); Puetzfeld:2015jha (). The breaking of scale invariance in the more general approach of metric-affine gravity was studied in Hehl:1989ij (), for example; for somewhat analogous breaking mechanisms, see Mielke:2011zza (); Mielke:2011zz (); Mielke:2013mla ().

### 1.6 Axion field

Dicke did not only introduce in 1961, together with Brans Brans:1961sx (), a scalar field into gravity, but he also discussed, in 1964, and pseudoscalar or axial scalar field in the context of gravitational theory, see (Dicke:1964, , Appendix 4, p.51, Eq.(7)).

Subsequently, in the early 1970s, Ni Ni:1973 () investigated matter coupled to the gravitational field and to electromagnetism and looked for consistency with the equivalence principle. He found it possible to introduce in this context a new neutral pseudoscalar field accompanying the metric field, see also Ni:1977zz (); Ni:1984 (); Balakin:2009rg (); Ni:2014cca (). Later, in the context of the vacuum structure of quantum chromodynamics, a light neutral psedoscalar, subsequently dubbed “axion” was hypothesized, see Weinberg (Weinberg, , pp.458–461). Similar as Ni’s field, the axion couples also to the electromagnetic field, see Wilczek’s paper Wilczek:1987mv () on “axion electrodynamics”.

The axion field is of a similar universality as the gravitational field. In other words, the axion belongs to the universally coupled scalar fields. Let in electrodynamics, and denote the excitation and the field strength, respectively. The constitutive relation characterizing the axion field (in elementary particle terminology it is called ) reads birkbook (),

 Hij=12αϵijklFklor{Da=αBa,Ha=−αEa, (5)

see also Hehl:2007ut () for the corresponding formalism; here is the totally antisymmetric Levi-Civita symbol with , moreover, . Clearly, the axion embodies the magnetoelectric effect par excellence. It is a pseudoscalar under 4-dimensional diffeomorphisms.

In electrotechnical terms, the axion behaves like the (nonreciprocal Tellegen) gyrator of network analysis, see Kupf (); Russer (); also the perfect electromagnetic conductor (PEMC) of Lindell & Sihvola PEMC (); Sihvola:2008 () represents an analogous structure. Metaphorically speaking, as we see from (5), the axion “rotates” the voltages into the currents . In SI, we have the units . Thus, carries the physical dimension of an admittance. Now, in the Maxwell Lagrangian, we find an additional piece , a term, which was perhaps first discussed by Schrödinger (Schro, , pp.25 to 26). If were a constant, the field equations would not change.

As we already remarked, is a 4-dimensional pseudoscalar. The same is true for the von Klitzing constant . And this covariance is a prerequisite for its universal meaning. Phenomenologically, the quantum Hall effect (QHE) can also be described by a constitutive law of the type (5), see (birkbook, , Eq.(B.4.60)).

It is possible to apply the constitutive relation (5) directly to a solid, too. By the evaluation of experiments we have shown Hehl:2007ut () that in the multiferroic CrO (chromium sesquioxide) we have a nonvanishing axion piece of up to , where is the vacuum admittance of about . This fact demonstrates that there exist materials with a nonvanishing, if small, (pseudoscalar) axion piece. This may be considered as a plausibility argument in favor of a similar structure emerging in fundamental physics. If the were found, it would not be an unprecedented structure, see in this context also Ni et al. Ni:1999di ().

In matter-coupled supergravity models, there are examples in which a dilaton and an axion are contained simultaneously in the allowed particle spectrum, see Freeman and Van Proeyen (Freedman, , p.451). However, in the next section we will demonstrate that in a fairly simple classical model of an electromagnetic universe, the axion can emerge jointly with a dilaton and the metric.

More recently, there have been attempts to relate the axion field to the torsion of spacetime, see, for example Mielke et al. Mielke:2006zp () and Castillo-Felisola et al. Castillo:2015ema (). To us, this assumed link between the internal symmetry of the axion with the external translation symmetry related to the torsion appears to be artificial and not supported by physical arguments.

## 2 An electromagnetic model universe

### 2.1 The premetric Maxwell equations

We consider a 4-dimensional differentiable manifold. The electromagnetic field is specified by its excitation , a 2nd rank antisymmetric contravariant tensor density, and by its field strength , a 2nd rank antisymmetric covariant tensor; the electric current is a contravariant vector density, see Post Post (). On this manifold, the Maxwell equations read

 ∂kHik=Jk,∂[iFjk]=0; (6)

the brackets denote antisymmetrization of the corresponding indices with as a factor, see Schouten (); for the Tonti-diagram of (6), compare (Tonti, , p.315).

In none of these equations the metric tensor nor the connection are involved. Still, these equations are valid and are generally covariant in the Minkowski space of special relativity, in the Riemann space of general relativity, and in the Riemann-Cartan or Weyl-Cartan space of gravitational gauge theories. The Maxwell equations (6) as such, apart from a historical episode up to 1916, see Einstein:1916Max (); Meaning (), have no specific relation to the Poincaré or the Lorentz group.

Perlick Perlick:2010ya () has shown that the initial value problem in electrodynamics can be particularly conveniently implemented by means of the premetric form of the Maxwell equations.

In contrast to most textbook representations, no “comma goes to semicolon rule” is required. The Maxwell equations (6) are just universally valid for all forms of electrically charged matter. Incidentally, this represents also a simplifying feature for numerical implementations. The price one has to pay is to introduce, as Maxwell did, the excitation , besides , as an independent field quantity and to note that it is a tensor density. From a phenomenological point of view, this is desirable anyway, since the excitation has an operational definition of its own, namely as charge/length () and current/length (), respectively, which is independent from the definition of the field strength as force/charge () and force/current (). For a rendition in the calculus of exterior differentiable forms, one can compare with the axiomatic scheme in birkbook () and Delphenich:2009 (), see also Delphenich:2015 ().

Let us stress additionally that , , and can be defined in a background independent way.

The Maxwell equations (6) are based on the conservation laws of electric charge (unit in SI “coulomb”) and magnetic flux (unit in SI “weber”). Charge and flux are 4-dimensional scalars. They induce the structure of the excitation and the field strength . In this context, the field strength is operationally defined via the Lorentz force density , the current being directly observable and the force and its measurement known from mechanics.

The charge and its conservation is the anchor of electrodynamics. Its current defines, by means of the Lorentz force density , the field strength , which allows to define the magnetic flux . Faraday’s induction law is an incarnation of magnetic flux conservation.

Some people have no intuition about the conservation of a quantity that is defined in 3 dimensions by integration over a 2-dimensional area , since we usually associate conservation with a quantity won by 3-dimensional volume integration, namely . Some mathematics education about dimensions will enable us to understand the induction law as a “continuity equation.”

Summing up: the premetric Maxwell equations are a close-knit structure, the 4-dimensional diffeomorphisms covariance holds it all together. Clearly, a metric as well as a connection are alien to the Maxwell equations.

### 2.2 A local and linear electromagnetic response

In order to fill the Maxwell equations with life, one has to relate to :

 Hij=Hij(Fkl) (7)

If we assume this functional to be local, that is, depends only on , and linear homogeneously, then we find

 Hij=12χijklFklwithχijkl=−χijlk=−χjikl; (8)

here the field represents the electromagnetic response tensor density of rank 4 and weight , with the physical dimension . An antisymmetric pair of indices corresponds, in 4 dimensions, to 6 independent components. Thus, can be understood as a matrix with 36 independent components.

We want to characterize the electromagnetic model spacetime by this response tensor field with 36 independent components.3 This is the tensor density defining the structure of spacetime. It transcends the metric and/or the connection.

We decompose the matrix into its 3 irreducible pieces. On the level of , this induces birkbook (); Delphenich:2006bm ()

 χijkl = (1)χijkl+(2)χijkl+(3)χijkl. (9) 36 =

The third part, the axion part, is totally antisymmetric , with the pseudoscalar , see also RaabSihvola (). The skewon part is defined according to . Under reversible conditions, (8) can be derived from a Lagrangian, then . The principal part fulfills the symmetries and .

The local and linear response relation now reads

 Hij=12((1)χijkl+(2)χijkl+αϵijkl)Fkl, (10)

and, split in space and time birkbook (); Hehl:2007ut (),

 Da = (εab−ϵabcnc)Eb+(γab+sba−δabscc)Bb+αBa, (11) Ha = (μ−1ab−ϵabcmc)Bb+(−γba+sab−δbascc)Eb−αEa; (12)

here are the 3-dimensional Levi-Civita symbols. The 6 permittivities , the 6 permeabilities were already known to Maxwell. The 8 magnetoelectric pieces (its trace vanishes, ) were found since 1961, see Astrov Astrov (). Eventually, the hypothetical skewon piece birkbook () carries 3 permittivities , the 3 permeabilities , and the 9 magnetoelectric pieces . Equivalent response relations were formulated by Serdyukov et al. (Serdyukov:2001, , p.86) and studied in quite some detail, see also de Lange and Raab Lange:2015zja ().

Suppose we have as special case a vacuum spacetime described by a Riemannian metric . Then the response tensor turns out to be

 χijkl=(1)χijkl=2λ0√−ggi[kgl]j%andHij=λ0√−gFij, (13)

with the vacuum admittance . Thus, we recover known structures, and we recognize that the relation (8) represents a natural generalization of the vacuum case. The metric can be considered as some kind of a square root of the electromagnetic response tensor .

We should keep in mind that a local and homogeneous electromagnetic response like (8) can be, if the circumstances require it, generalized to nonlocal and/or to nonlinear laws. Examples of nonlocal laws have been proposed by Bopp and Podolsky4 and by Mashhoon.5 Nonlinear laws are due to Heisenberg and Euler,6 Born and Infeld,7 and Plebański.8 Fresnel surfaces for the nonlinear case were found by Obukhov and Rubilar Obukhov:2002xa (), for example. More recently, Lämmerzahl et al. Lammerzahl:2012kw () and Itin et al. Itin:2014uia () investigated electrodynamics in Finsler spacetimes. In the premetric framework, this corresponds to a nonlocal constitutive law, see (Itin:2014uia, , Eq.(3.29)), somewhat reminiscent of the Bopp-Podolsky scheme.

### 2.3 Propagation of electromagnetic disturbances

The obvious next step in evaluating the physics of our model of spacetime is to look how electromagnetic disturbances propagate in this spacetime. One can either consider the short wave-length limit of the electromagnetic theory, the WKB-approximation, or one can study, as we will do here, the propagation of electromagnetic disturbances with a technique developed by Hadamard; for a general outline, see (Truesdell, , Chap. C).

Hadamard describes an elementary wave as a process that forms a wave surface. Across this surface, the electromagnetic field is continuous, but the derivative of the field has a jump. The direction of a jump is given by the wave covector. The subsequent integration produces the rays, with the wave vectors as tangents to rays, see for our case Kiehn:1991 (); Fukui (); birkbook (); Itin:2009aa (). In the meantime, our methods have been improved, see Favaro (); Baekler:2014kha (); Favaro:2015jxa ().

Out of the electromagnetic response tensor density we can define, with the help of the covariant Levi-Civita symbol , the premetric “diamond” (single) dual and the diamond double dual, respectively:

 χ⋄ijkl:=12χijcdϵcdkl,⋄χ⋄ijkl:=12ϵijabχ⋄abkl=14ϵijabχabcdϵcdkl. (14)

The covariant Levi-Civita symbol carries weight and weight . Thus, the double dual has weight , too. Performing the double dual apparently corresponds to a lowering of all four indices of —and this is achieved without having access to a metric of spacetime.

After this preparation, it is straightforward to define the (premetric) 4th rank Kummer tensor density, which is cubic in , as Baekler:2014kha ()

 Kijkl[χ]:=χaibj⋄χ⋄acbdχckdl. (15)

It has weight and obeys the symmetry

At each point in spacetime, the wave covectors of the electromagnetic waves span the Fresnel wave surfaces, which are quartic in the wave covectors according to

 Kijkl[χ]qiqjqkql=K(ijkl)[χ]qiqjqkql=0. (16)

The Tamm-Rubilar (TR) tensor density Rubilar:2007qm (); birkbook (), with the conventional factor , is defined by

 Gijkl[χ]:=16K(ijkl)[χ]=16χa(ij|b⋄χ⋄acbdχc|kl)d. (17)

It is totally symmetric and carries 35 independent components. By straightforward algebra it can be shown that the axion field drops out from the TR-tensor:

 Gijkl[χ]=Gijkl[(1)χ+(2)χ]; (18)

see in this connection also Itin:2007cv () and the references given there. The effect of the skewon piece on light propagation has been studied in Obukhov:2004zz (). Ni Ni:1977zz () was the first to understand that the axion field doesn’t influence the light propagation in the geometrical optics limit. Note that has independent components, exactly as —probably not by chance.

Accordingly, the totally symmetric TR-tensor , with its 35 independent components, can, up to a factor, be observed by optical means, that is, the TR-tensor—in contrast to the Kummer tensor, as far as we know—has a direct operational interpretation.

### 2.4 Fresnel wave surface

The (generalized) Fresnel equation

 Gijkl[χ]qiqjqkql=0, (19)

determines a Fresnel wave surface. A trivial test for checking the correctness of (19) is to substitute the response tensor for the Maxwell-Lorentz vacuum electrodynamics (13) into the TR-tensor of (19). One finds straightforwardly , that is, two light cones that collapse onto each other. The decomposition of (19) into space and time can be found in (birkbook, , (D.2.44)).

For illustration, following Schaefer (); Baekler:2014kha (), see also Itin:2009fj (), we will display a classical example of such a surface. In Eqs.(11) and (12), we choose an anisotropic permittivity tensor with three different principal values and assume trivial vacuum permeability, whereas all magnetoelectric moduli—with the possible exception of the axion —vanish,

 (εab)=⎛⎜⎝ε1000ε2000ε3⎞⎟⎠and(μ−1ab)=μ−10⎛⎜⎝100010001⎞⎟⎠. (20)

Substitution into the Fresnel equation yield the quartic polynomial

 (α2x2+β2y2+γ2z2)(x2+y2+z2) −[α2(β2+γ2)x2+β2(γ2+α2)y2+γ2(α2+β2)z2]+α2β2γ2=0, (21)

with the 3 parameters9 , and with as the vacuum speed of light.

The corresponding surface is drawn in Fig.1. As an example of a Fresnel surface for a more exotic material, we provide one for the so-called PQ-medium of Lindell Lindell:PQ (). It may turn out that this response tensor can only be realized with the help of a suitable metamaterial, see Shamonina (). Corresponding investigations are underway by Favaro Favaro:2015privcomm ().

Let us shortly look back on what we have achieved so far: We have formulated the Maxwell equation in a premetric way. For the response tensor only local and linear notions are used, no distances or angles were mentioned nor implemented. Under such circumstances, electromagnetic disturbances propagate in a birefringent way in accordance with the Fresnel wave surfaces, such as presented in Figs.1 and 2.

How can we now bring in distances and angles, which are concepts omnipresent in everyday life? The answer is obvious, we have to suppress birefringence.

### 2.5 Suppression of birefringence: the light cone

Looking at the figures, it is clear that we have to take care that both shells in each Fresnel wave surface become identical spheres. Then light propagates like in vacuum. For this purpose, we can solve the quartic Fresnel equation (19) with respect to the frequency , keeping the 3–covector fixed. One finds four solutions, for the details please compare Lammerzahl:2004ww (); Itin:2005iv (). To suppress birefringence, one has to demand two conditions. In turn, the quartic equation splits into a product of two quadratic equations proportional to each other. Thus, we find a light cone at each point of spacetime.

Perhaps surprisingly, we derived also the Lorentz signature, see birkbook (); Itin:2004qr (); ItinFriedman (). This can be traced back to the Lenz rule, which determines the relative sign of the two terms in the induction law, as compared to the relative sign in the Ampère-Maxwell law. The Lorentz signature can be understood on the level of classical electrodynamics, no appeal to quantum field theory, which is widespread in the literature, is necessary.

Globally in the cosmos, birefringence is excluded with high accuracy, see the observations of Polarbear Ade:2015cao () and the discussion of Ni Ni:2014qfa ().

### 2.6 Axion, dilaton, metric

At the premetric level of our framework, besides the principal piece, first the skewon and the axion fields emerged. Only subsequently the light cone was brought up. The skewon field was phased out by our insistence of the vanishing birefringence in the vacuum. Accordingly, the axion field and the light cone survived the suppression of the birefringence.

The light cone does not define the metric uniquely. Rather an arbitrary function is left over:

 λ(x)gij(x)qiqj=0. (22)

The light cone is invariant under the 15-parametric conformal group. The 4 proper conformal transformation correspond to a reflection at the unit circle and, as such, are of a nonlocal nature. As a consequence, if two frames are related to each other by a proper conformal transformation and one frame is inertial, the other one is accelerated with respect to the former one. Accordingly, there is an operational distinction possible between a proper conformal and a dilation or scale transformation. Thus, only the 11 parameter Weyl subgroup of the 15 parameter conformal group is based on local transformations.

If we compare our result in (22) with vacuum response in (13), we recognize, not forgetting the axion field, that we find the following response equation for vanishing birefringence:

 Hij=[λ(x)dilaton√−ggik(x)gjl(x)+α(x)axionϵijkl]Fkl. (23)

Because of the presence of the dilation within the Weyl group, it is natural to identify the function with the dilaton field.10

In the calculus of exterior differential forms, see birkbook (), the twisted excitation 2-form and the untwisted field strength 2-form , together with the twisted current 3-form , obey the Maxwell equations and . By means of the metric, we can introduce the Hodge star operator. Then the response relation (23) becomes even more compact Hehl:2005hu (); Favaro:2010ys ():

 H=[λ(x)⋆+α(x)]F. (24)

Eqs.(23) and (24) represent the end result of investigating an electromagnetic spacetime model with local and linear response and without birefringence. The three fields , , and come up together with a reasonable interpretation. At least in the way we defined them here, , , and are all three descendants of electromagnetism.

As we have argued in Sec.1.5, the dilaton seems to be at home in the Weyl-Cartan spacetime. Our results (23) or (24) are consistent with this expectation, that is, we believe that these equations are valid in a Weyl-Cartan spacetime.

What are we told by experiments and observations? The axion has not been found so far, so we can provisionally put . Moreover, under normal circumstances, the dilaton seems to be a constant field and thereby sets a certain scale, that is, , where is the admittance of free space, the value of which is, in SI-units, . Under these conditions, we are left with the response relation of conventional Maxwell-Lorentz electrodynamics,

 Hij=λ0√−gFijorH=λ0⋆F. (25)

The possible generalizations are apparent.

## 3 Discussion

Gravity, coupling to all objects carrying energy-momentum, is a truly universal interaction. Electromagnetism is only involved in electrically charged matter. What is curious and what we still do not understand is that the gravitational potential emerges in an electromagnetic context, that is, in studying electromagnetic disturbances, we can suppress birefringence, and then the light cone emerges. And the light cone is essentially involved in general relativity. In other words, we cannot formulate a general-relativistic theory of gravity unless some electric charge is around: electromagnetic waves are a necessary tool for constructing general relativity.

Perlick is not concerned about it. He observes that Perlick:2015 () “…the vacuum Maxwell equations are but one example that have the light cones of the spacetime metric for their characteristics. The same is true of the Dirac equation, the Klein-Gordon equation and others….” Yes, this is true. However, if a metric is not prescribed, we cannot even formulate Dirac’s theory. In contrast, in premetric electrodynamics, if a local and linear response tensor density is assumed, we can derive the metric, as we discussed above. In this sense, electrodynamics is distinguished from Dirac’s theory—and in this, and only in this sense, the premetric Maxwell equations are more fundamental than the Dirac equation.

Accordingly, there seems to be a deep connection between electromagnetism and gravity, even though gravity is truly universal, in contrast to electrodynamics.

###### Acknowledgements.
This project was partly supported by the German-Israeli Research Foundation (GIF) by the grant GIF/No.1078-107.14/2009. I am grateful to Claus Kiefer (Cologne) for helpful remarks re Higgs cosmology, to Yuri Obukhov (Moscow) re questions of torsion and of the Hadamard method, to Helmut Rumpf (Vienna) re the Mukhanov bound of , and to Volker Perlick (Bremen) re gravity versus electromagnetism and what is more fundamental. I would like to thank Ari Sihvola and Ismo Lindell (both Espoo/Helsinki) and Alberto Favaro (London) for the permission to use the image in Fig.2. Many useful comments on a draft of this paper were supplied by Hubert Goenner (Göttingen), Yakov Itin (Jerusalem), Claus Lämmerzahl (Bremen), Ecardo Mielke (Mexico City), Wei-Tou Ni (Hsinchu), Erhard Scholz (Wuppertal), Frederic Schuller (Erlangen), and Dirk Puetzfeld (Bremen). I am grateful to Alan Kostelecký (Bloomington) for a last-minute exchange of emails.

### Footnotes

1. Carl Brans is one of the pioneers of the scalar-tensor theory of gravitation. This essay is dedicated to Carl on the occasion of his 80th birthday with all best wishes to him and his family. During the year of 1998, we had the privilege to host Carl, as an Alexander von Humboldt awardee, for several months at the University of Cologne. I remember with pleasure the many lively discussions we had on scalars, on structures of spacetime, on physics in general, and on various other topics.
2. We skip here the plethora of scalar mesons, they are all composed of two quarks. Thus, the scalar mesons do not belong to the fundamental particles.
3. Schuller et al. Schuller () took the –tensor density, which arises so naturally in electrodynamics, called the tensor proportional to it “area metric”, and generalized it to dimensions and to string theory. For reconstructing a volume element, they have, depending on the circumstances, two different recipes, like, for example, taking the sixth root of a determinant. From the point of view of 4-dimensional electrodynamics, the procedure of Schuller et al. looks contrived to us.
4. See (Feynman, , Sec.28-8).
5. See (birkbook, , Sec.E.2.2).
6. See (birkbook, , Sec.E.2.3).
7. See (birkbook, , Sec.E.2.4).
8. See (birkbook, , Sec.E.2.5).
9. Here, in this context, is not the axion field!
10. In the early 1980s, Ni Ni:1984 () has shown the following: Suppressing the birefringence is a necessary and sufficient condition for a Lagrangian based constitutive tensor to be decomposable into metric+dilaton+axion in a weak gravitational field (weak violation of the Einstein equivalence principle), a remarkable result. Note that Ni assumed the existence of a metric. We, in (23), derived the metric from the electromagnetic response tensor density .

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