Hertzian Electromagnetic Potentials Author(s): W. H. McCrea Source: Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, Vol. 240, No. 1223 (Jul. 16, 1957), pp. 447457 Published by: The Royal Society Stable URL: http://www.jstor.org/stable/100050 Accessed: 08/02/2010 20:22 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, noncommercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=rsl. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a notforprofit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.
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Hertzian electromagnetic potentials BY W. H. MCCREA, F.R.S. Royal Holloway College, University of London (Received 1 February 1957) The general properties of the Hertzian electromagnetic potentials and their gauge transformations, recently given by Nisbet, are rederived in tensor form appropriate to the spacetime of special relativity. There appears to be a considerable gain in conciseness and comprehensibility of the general results, although it is recognized that Nisbet's forms may be those suited to particular applications. Whittaker's scalar potentials are briefly considered. The theory refers exclusively to flat spacetime because the results require covariant differentiation to be commutative, but it is formulated in terms of any curvilinear coordinates in such spacetime.
1. INTRODUCTION
A comprehensive treatment of the Hertzian electromagnetic potentials was given in a recent interesting paper by Nisbet (I955). He worked entirely in terms of scalar and vector fields in Euclidean space (o) of three dimensions. This was probably natural, having regard to the applications which he had in view. However, the most concise and symmetrical formulation of general electromagnetic theory is that appropriate to fourdimensional spacetime. This seems to be particularly true of the part of the theory treated by Nisbet, where the threedimensional formulation is bewildering in its complexity. The gain in brevity and ease of apprehension effected by the fourdimensional formulation is so great that it seems worth placing on record even though the results are equivalent to Nisbet's and may have to be translated into his form for special applications. Incidentally, this translation may well be deferred to the latest possible stage, since, among other complications, the arrangement of algebraic signs of the translated terms offers an unfortunately considerable chance of error. Although Nisbet does not employ a fourdimensional formulation, he does mention the relativistic covariance of his results and refers, too, to the work of Laporte & Uhlenbeck (I931), also quoted by Sommerfeld (I935). The latter is a fourdimensional treatment, but it is based upon the theory of spinors which is not required here. The present writer met the problems involved in reviewing (McCrea 1957) the work of the late Sir Edmund Whittaker and in seeking to discover the general significance of Whittaker's (I904, 1951) expression of an electromagnetic field in terms of two scalar functions. The writer then became aware of Nisbet's work, which includes a discussion of Whittaker's contribution. Here the fourdimensional treatment seems to offer only a very little more insight; this is briefly mentioned in ? 8. In ? 2 the general features of the fourdimensional treatment are summarized; in ??3 and 4 the tensors required are defined and their assumed properties stated. In ??5 and 6 are stated in tensor form the general properties of the potentials and of their gauge transformations, corresponding to Nisbet's results, and the four[ 447 ]
W. H. McCrea
448
dimensional derivation of these properties is given in ? 7. This arrangement facilitates the comparison with Nisbet's treatment, while making it possible, if desired, to read the present work without immediate reference to Nisbet's. 2.
FOURDIMENSIONAL
FORMULATION
The procedure need be only formalistic, since the aim is merely to give an alternative expression of Nisbet's results. In particular, all required conditions of differentiability are assumed to be satisfied. Also a solution of any of the differential equations has the meaning, implied in the corresponding part of Nisbet's work, of a function or functions satisfying the equation in the relevant domain of the variables, without any reference to boundary conditions at this stage. We consider the fourdimensional formulation appropriate to the spacetime (S9) of special relativity. The results are to be in tensor form, all tensor indices running through the range 1, 2, 3, 4. As usual, the fundamental tensor for any system of coordinates x1, X2, x3, X4 is denoted by gpq and the value of its determinant by g. First and second covariant derivatives will be denoted by brackets ( )p, ()pq. Covariant differentiation is commutative because 9 is flat; some of the results depend upon this and cannot be taken over without modification into spacetime of nonzero curvature. A set of rectangular coordinates in &, regarded as a spatial section of 9, together with the corresponding time determination, will be denoted by x, y, z, t; these will be called special coordinates in 9. We take the metric in such coordinates to be ds2 =  dx2  dy2  dz2 + dt2.
(2.1)
In these coordinates the raising or lowering of a tensor index merely changes the sign of the component if the index is 1, 2 or 3, and makes no changes if the index is 4. By a transformation of special coordinates we mean a transformation of x, y, z to another set of rectangular coordinates without change of t, the coefficients in the transformation being independent of t. Under such a transformation, (a) an invariant inY9 is an invariant in ?, (b) if V, is a vector inS9, then the set of components (Vx,V2, V3)tranforms as a vector in & while the component V4behaves as an invariant in ?, (c) if Wpqis a skew tensor (sixvector) inV9, then each of the sets of components (W, W24, W4, W3), (W3, W31,W2) transforms as a vector in d. If a valid tensor relation in y be translated into relations between scalars or vectors in g according to these rules, then the validity of the latter relations is assured. Conversely, if V is a given vector in g having components V1,V2, 3 in any given special coordinates, and v is an invariant in g, we may define a vector Vyin Y having components (Vl,V2,V3,v) in the given system of special coordinates. Also, if U, V are two given vectors in ? having components (U1, U2,U3), (V,,V2,V3)in the special coordinates, we may define a skew tensor W1qin 92 such that in these coordinates (WL,4 W24,W34)= (U1, 2, U3) and (W2, W31,W12)= (VI,VT,V3). Such definitions will be expressed by writing = (V, v);
W=
(U, V),
W =Wqp.
(2.2)
Hertzian
449
potentials
electromagnetic
It must be emphasized that the components may be identified in the manner stated only in special coordinates, the components of Vp,Wpqin any other coordinates being then determined by the laws of tensor transformation. By such means, suitable relations between scalars and vectors in S can be translated into tensor relations in Y. We recall the existence of the LeviCivita tensor yPqrs, the components of which in any coordinate system are + ( g) if p, q, r, s is an even or odd permutation of 1, 2, 3, 4 and are zero otherwise; thus (g)zyPqrs= epqrs,the usual permutation symbol. The covariant derivatives of this tensor are zero. If Wpqis any skew tensor, the dual tensor *Wpqis defined by Wpq =
(23)
'pqrsWrs;
that is, a contravariant (skew) tensor is derived from Wpqin accordance with (23) and its indices can then be lowered in the usual way to give *Wp, * WPq, WVq.It is convenient to extend the notation to apply also if dummy suffixes are present; thus if Wpq= Xpqtyt, say, defines a skew tensor we may write * Wpq=
*XPqt yt
= *Xpqt
(24)
The dual of the dual is the original tensor with its sign reversed (on account of the occurrence of (g)l rather than g~ in the definition). As is clearly permissible, we regard a tensor as being defined by either an expression for the tensor itself or an expression for its dual. Finally, we note the following identities: (a) If Wpqis any skew tensor then (Wq)pq = 0.
(2.5)
(b) If a tensor Fpqis defined in terms of a vector A, by *Fpq = yPqrs(As)r,
then where
(F1)q, = (A)qp F]2
(26)
(2.7)
2Ap,
is the tensor form of the d'Alembertian operator defined by 2 2grs(
(28)
)rs
In (2 6) there is summation over r, s; but if p, q are fixed (p q), then r, s are uniquely determined so that p, q, r, s is an even permutation of 1, 2, 3, 4, and (26) becomes
(_ g ) *Fpq = (As)r (At)s = As, r Ar, s
(2.9)
where the comma denotes a simple derivative. Hence from (2.3) Fpq =Aq, A,
q
or
,pq (Aq)pg
(Ap)r,
whence, on forming the covariant derivative and contracting, (Fp (2.)q
()pq
_g(Ap)rq.
This is (2 7), it being noted that the order of covariant differentiation is permutable.
W. H. McCrea
450
(c) If WPqis any skew tensor, then  *qrs (wt
Ppqrs(*Wt) r
++2
Wq.
(210)
Let the skew tensor Vpqbe defined such that * Vpq = fpqrs(wst)tr+
2* W .
(211)
Proceeding as in obtaining (2.9) with a similar determination of p, q, r, s, (2.11) becomes (g)
+ D2W *VPq = (W')t(W)t = gt(Wsu, r + Wur,s+ Wrs,u)
(2.12)
= gP(Wsp,r + Wpr,s + Wrs,p)t + gq(JVsqr, + Wr,
Wrs,q)t,
(2.13)
since the expression in brackets in (2.12) vanishes if u is equal to r, s, so that contributions arise only from u = p, q; there is not summation over p, q in (2.13). Again using the permutation properties of p, q, r, s we find that Wp,r + Wpr,s+Wrs,
* = (g) (* Wr,r + WSP, + * WPq)
= (g)
while similarly the second such quantity in f(213) is + (i)9f*WPu),,. yields *
q=
*Vpq=
or
 Vr =
which gives or
gtP(
*
(* wf),,
Thus 1213)
Wqu)u + gtq(* WPU)t
(*Wq)up + (*Wpu)uq, ?Prqs(*WptU)
FVpq= Vfqrs(*Wst)r,
(2.14)
where full summation is restored, the last form resulting merely from appropriate rearrangement of the indices. Identifying the values of VPqfrom (211) and (214), and using the notation illustrated in (24), we obtain (210). 3.
DEFINITION
OF FIELD TENSORS
The same notation as Nisbet's for the parameters associated with an electromagnetic field in g will be used except for the employment here of units such that his factors 47rand c are absorbed. The physical meanings of the parameters emerge as we proceed in subsequent paragraphs. Here it is sufficient to define tensors, related to quantities in J in accordance with (22), as follows: chargecurrent vector field intensities
J = (j,p), Hpq = (D, H),
(31)
Fpq = (E, B),
(33)
polarization tensor vector potential
Ppq = ( P, M), Ap = (A,  ),
(3.4) (3.5)
Hertzian
WP = (IIe, Hm),
(36)
first stream potential
Qp = ( Qe, Qn),
(37)
second stream potential
Rp = (Re, Rm).
(38)
potential
(32)
Hertzian electromagnetic potentials
451
All of the secondorder tensors are skew. The allocation of signs is arbitrary; some follow the standard convention and others are Nisbet's choice. In his work, it might have been more natural to interchange the roles of Qm, Rmin (3 7) and (3 8), but we adhere to his actual usage. 4.
POSTULATED FIELD RELATIONS
We shall refer to equations in Nisbet's paper by his numbering prefixed by N; it must be remembered that his factors 47T and c have to be omitted in making comparison with our formulation. Maxwell's equations (N22), (N 21) are = (4.1) (HPq)q JP, (*FPq) = 0. The constitutive relations (N 23) are
(42)
Hp + =pq=Fpq.
(4.3)
The stream potentials are such that Hpq = Qpq is some particular integral of (41) and *Fpq = *Rpq is some particular integral of (42); but these particular integrals do not necessarily also satisfy (43). These properties are expressed by (N 212), (N 224). 5. PROPERTIES
OF THE POTENTIALS
The usual representation (N 2.4) of the field by a vector potential is given by *Fpq = prAs(As)r
(5.1)
The Lorentz condition (N 25) on AP is an(d the wave equation for Av is
(AP) = 0,
+ p + (Pp)q = 0. 2AJP
(5.2) (5.3)
Using (3.1), (3.4) and (3.5) this translates into 12A = j +P+curlM,
(N27)
q2 = pdiv
(N 26)
P.
The meaning of these relations is that for all Av the tensor Fpqderived from (51) yields a solution of Maxwell's equations (42), while, if AP is a solution of (53) satisfying the auxiliary condition (52), then the tensor Hpqrelated to this Fpqby (43) is a solution of Maxwell's equations (4.1). The representation by a Hertzian potential is
Ap = (5p4) r(P)q Hpq = Vpqr(*Wts% Qpq *FP
with the wave equation
= qPvrs(Wt)t + *Rpq,
+ +rQ Qpq a 2W P,+ Oq Rpq 3PQJ _Z  ?.
(5.5) (5.6)
(57)
W. H. McCrea
452
With the definitions of ? 3 these translate, respectively, to give A = I + curl m,
q =div He;
e curl m Q, H = curl H + grad div n 
(N29)
fD = curl curl
+
Q.;
 curl fE = grad div 11 I+ Re, ,B = curl Ae + curl curl 1m  Rm;
(N 220) (N 221) (N 218) (N 219)
2IIe = P + Q + R,,
(N 222)
2Im  M + Qm+ Rn.
(N223)
The meaning of (55) to (57) is that for any skew tensor Wpq,and Rpq as defined, the tensor Fpq derived from (56) satisfies Maxwell's equations (42), while, with Qpq as defined, the condition (57) is sufficient for the tensor Hpq determined by (55) to satisfy Maxwell's equations (41) together with the constitutive relations (43). 6,
GAUGE
TRANSFORMATIONS
OF THE POTENTIALS
Following Nisbet's terminology, gauge transformations of the first, second and third kinds apply to invariants, vectors and secondorder tensors, respectively. We are not concerned here with the transformation of the first kind, which is merely the multiplication of an invariant by an exponential factor. A transformation of the second or third kind is effected by means of an invariant or a vector, respectively, which is arbitrary so far as the definition of the transformation is concerned, but which may be subjected to various conditions for particular applications. The transformation of the third kind, in the present context, is due to Nisbet. In the definitions that follow, the original value of any variable is distinguished by the affix 0 from its transformed value. Applied to the vector potential, a transformation of the second kind is defined by A = A + (X)p
(61)
where X is an arbitrary invariant. But, if Ap, A? satisfy the condition (5.2), then we require (62) [2X = 0, and, if this is satisfied, the wave equation (5.3) is gaugeinvariant. Applied to the stream potentials and the Hertzian potential, Nisbet's transformations of the third kind are defined by Qpq = Qopq+ pqrs(G)r,,
(6.3)
Rpq = ROpq+*4rrs(Ls)r,
(6.4)
Wpq = WOpq+ Pqr(rs)r + *rpqrs(As)r
(6.5)
where in the first instance Gp, Lp, Up, Ar are arbitrary vectors.
Hertzian
electromagnetic
453
potentials
If Qj, ROq are stream potentials, then so are Qpq,Rpq defined by (6.3) and (6.4). The electromagnetic field Hp2, Fpqderived in accordance with (55) and (5.6) from Wpq,Qpq,RBpgiven by (63) to (6.5) is the same as that derived from Wpq,Qq, ROq, provided 2r G (6.6)
+ ()p,
Lp=D2Ap
(67)
where 6, 0 are arbitrary invariants. The wave equation (5.7) is also gaugeinvariant under these conditions. If the vector A. is such that (68) _ A?~ (6p, (?8) where Cis an arbitrary invariant, then the transformation (6.5) of Wpqinduces a transformation in Ap of the form (6.1) with X =  (AP) 
(69)
C
In this case it follows from (67) that L, must be zero, or the gradient of an invariant which makes zero contribution in (6.4). As regards the expression of the results in Nisbet's form, (61) gives (N 3 1) with the sign of X reversed and (6.2) is (N 3.2). With the identifications expressed in the manner of (22) by p= (G,g),
Lp=(L,1),
rp =(r,
),
A =(A,A),
(6.10)
(6.3) to (6.5) become, respectively, in special coordinates Qe  Q0 + curl G, R =R? Lgrad,,
Q
= Q? Ggradg;
Rm= Rm curlL;
no + curl r AgradA, } = grad ycurl A. nm no e=
(N 33) (N34) (N 312)
Inserting the values (6.3) and (6.4) in (55) to (57) we obtain (N 38, 9), (N 36, 7) and (N 310, 11), respectively; the results are, of course, the same as are got by using the values of Qe, etc., from (N 33, 4) as quoted in (N 218) to (N 223). Equations (66) and (6.7) are equivalent to (N315, 16), and equations (6.8) and (69) to (N 313, 14), with minor changes of notation for the invariants. Formulae like (5 5) to (5 7), (6.3) to (6 5), compared with their translations into the threedimensional expressions, demonstrate the relative brevity of the fourdimensional description which in turn renders the significance of the results much more immediately evident. 7.
PROOFS OF THE FORMULAE
The tensor *FPq defined by (5.1) or (26) clearly satisfies Maxwell's equations (42) identically for arbitrary Ap. If we then impose the condition (52) on A., (2.7) becomes (7.1) (FPq) =  2AP. Therefore, on substituting for Hpq from (43) in Maxwell's equations (4.1) we get immediately the wave equation (5.3). 29
Vol. 240. A.
W. H. McCrea
454
If now the vector potential Ap is defined in terms of an arbitrary skew tensor Wpqby (54), the condition (5.2) is automatically satisfied by virtue of (25). The tensor *Fpq derived from this Ap according to (5.1) gives the first term of the righthand member of (5.6), but, following Nisbet, we may add explicitly any particular solution of (42) and so obtain (56) in the form stated. Since (5 2) is satisfied, by a further application of the identity expressed by (7 1) we obtain immediately from (56) 2 W2 2(
(Fpq)q =
)q + (RPq)q.
(7.2)
Therefore, substituting from (43) in (41) we have from (72)  o2(Wp)
+ (Rpq)q (Ppq)q = JP.
(73)
But QPqis a particular integral of (4.1) and so (QP)q = JP.
(74)
(02Wpq + ppq + QpP Rp)q = 0,
(75)
Combining (73) and (74) we have
and a sufficient condition for this is (57). Using (56) and (57) in (43) we then obtain HPq = FpI  Ppq =
=
=
*qr8(Wst)tr
+ Rpq  pPq
*pqrs(W)r
+
]2 Wpq + Qpq
pqrs(*Wst)t + Qp,
(76)
using (210) in the last step; this is (5.5). Proceeding to the gauge transformations, the properties of the transformation (61) are immediate; we consider those of the transformations defined by (63) to (6.5). When Wpqis given by (65) let Hpqbe derived from Wpqin accordance with (5.5) and let Hq be similarly
derived from Wpq with stream potential
Qq. Then,
if we introduce tensors defined by rpq = rpqrs(rs)r, Apq = Wpq = WOpq+ r
(65) becomes
pqrs(As)r,
+ *APQ,
(77) (78)
and (5.5) gives HPq =
PQrs(*Wst)tr QP
= HOq Now from (7.7)

Qop + Qpq+ ypqrs(*
t)

prs(A^)tr
(At)t = gfsu(AUI)t= gsurul(m(Am)t = 0
(79) (710)
since one factor is symmetric and the other is antisymmetric in 1, t. Also, using (27) applied to *(*rpq)=
 pq, we have
(* 8t)t=  (rt)s+ 2s, and, further, Finally, from (63),
Pwrs(rt)t = 0.
Qpq_ QOpq=apq(rs(s)r.
(711) (712)
(713)
Hertzian electromagneticpotentials
455
Therefore, from (710) to (713), the result (79) becomes
HPq= HOpq+,
pqrs([E2rs +^s)r.
The last term is the tensor curl of a vector and vanishes if and only if the vector is the gradient of an invariant, i.e. say, E2r
+8 = (6).
This is (6.6) and a similar procedure yields (6.7). Again from (7.8) we have F2Wpqa= [2 Wopqf+[2rpQ
+E 2*Apq.
From the definitions (77) and the properties of the operator
(714) ?2,
=
[]2rpq
pqrs([2rs)r,
2 *Apq =
*D22APq = *fpqrs(E]2As),
and from (5 7), (63) and (6 4) + ROpq C]2Wpq = _ ppq _ QOpq = _ ppq _ Qpq+ Rpt + ylpqrs(s),
_
*PqrS(Ls)r.
Hence, substituting in (714), Dw2Wp + Pp + QPq RP =
P2s(2rs
+ Gs)r +
*P rs(2As (  L)
= 0,
if (66) and (6.7) are satisfied. Thus the gaugeinvariance of (5.7) is established; this is noted by Nisbet (sentence preceding (N 3.15)). Lastly, if Ap, AO are the vector potentials corresponding to Wpq,WOqin accordance with (5 4), using the form (7.8) with identities like (7*10) and (711) but having the roles of F, A interchanged, we have = (WpJ)q
A
=(Wo)q + (rp)q + (*ApQ)q
where
=A
 (A)qp + D12A
= A
+ (X)p
x =(A)C
provided Ap is such that [2Ap =  ()p. Thus we have obtained (6.8) and (6 9). 8.
WHITTAKER'S POTENTIALS
Whittaker's theorem states that, in a region in which J = 0, PPq= 0, the Hertzian potential WPqmay be taken in special coordinates in the form I/p,q, say, where ? =
^ Y.
,
(8.1) 292
W. H. McCrea
456
the dots denoting zeros and ', 9 being functions of x, y, z, t. Thus the field is expressible entirely in terms of these two functions. There are, of course, three ways of pairing off the rows and columns in (8.1) and it is immaterial which one we select; the form (81) corresponds to (N4.12). Whittaker (1904) proved his result by finding explicit expressions for Y, 9 in the case where the field is produced by moving pointcharges. But Nisbet points out that it holds good for any electromagnetic field in vacuo at points away from the sources. He sketches the proof of this generalization and there is nothing to add from this point of view. Once again, however, the fourdimensional statement of the result makes its significance more plain. The theorem is in fact, if Wpqis a given skew tensor such that D2Wq = 0,
(82)
and if the tensor Y#q has the form determined by (8 1), then the equations
(WK yP),7 ()W = 0
(8.3)
are compatible partial differential equations for F, ( and an invariant X. As stated, the proof is that sketched by Nisbet. Now, in a region where Jp = 0, Ppq = 0, we have from (4.3),Hpq = Fpq,andwe maytake Qpq= 0, Rpq = 0. Then (57)becomes (82) and from (5.6) and (83) *Fpq
=
pqrs(Wst)tr _=
(8.4)
qrs(/s)t.
Thus the field is described in terms of r, W. From (8.3), using (2.5), we have
f(X)pr= that is
Pr(Wp
qj)qr = (W o2]x = 0.
r)Q
=
0 (85)
Using (82) and (85) in (83), we then obtain (L2"i~2 ) = 0.
Thence it can be shown that without loss of generality we may take I2 YpVq = 0, giving 22giv = 0, 2 = 0. The form (8.1) is a canonical form for a general skew tensor, whereby the tensor is reduced to a wrench (Synge I956). Any skew tensor, and so any Hertzian potential, may be reduced to this form at any particular event in y by a suitable choice of coordinates. The point of Whittaker's theorem is that, for an electromagnetic field in free space, a Hertzian potential may be used for which the axis of the equivalent wrench has the same (spacelike) direction at every event in the region of S concerned. By employing nonvanishing stream potentials, Nisbet also formulates a generalization of Whittaker's theorem to regions where Jp * 0, Ppq = 0.
Hertzian electromagneticpotentials
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REFERENCES Laporte, 0. & Uhlenbeck, G. E, I931 Phys. Rev. 37, 1380. McCrea, W. H. I957 J. Lond. Math. Soc. 32, 234. Nisbet, A. I955 Proc. Roy. Soc. A, 231, 250. Sommerfeld, A. I935 In Frank & von Mises, RiemannWebers Differentialgleichungen der Physik, 2nd ed. 2, 790. Brunswick: Vieweg. Synge, J. L. I956 Relativity: the special theory, chap. ix, ?5. Amsterdam: NorthHolland. Whittaker, E. T. 1904 Proc. Lond. Math. Soc. 1, 367. Whittaker, E. T. I95I History of the theories of aether and electricity: the classical theories, p. 409. Edinburgh: Nelson.