Translation:The Fundamental Equations for Electromagnetic Processes in Moving Bodies
The Fundamental Equations for Electromagnetic Processes in Moving Bodies (1908) by , translated from German by Meghnad Saha and Wikisource 
German Original: Die Grundgleichungen für die elektromagnetischen Vorgänge in bewegten Körpern (1908), Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, MathematischPhysikalische Klasse, pp. 53–111

Contents
 1 INTRODUCTION.
 2 PART I. Consideration of the Limiting Case Æther.
 3 PART II. ELECTROMAGNETIC PHENOMENA.
 3.1 § 7. Fundamental Equations for bodies at rest.
 3.2 § 8. The Fundamental Equations for Moving Bodies.
 3.3 § 9. The Fundamental Equations in Lorentz's Theory.
 3.4 § 10. Fundamental Equations of E. Cohn.
 3.5 § 11. Typical Representations of the fundamental equations.
 3.6 § 12. The Differential Operator Lor.
 3.7 § 13. The Product of the Fieldvectors fF.
 3.8 § 14. The Ponderomotive Force.
 4 APPENDIX. Mechanics and the RelativityPostulate.
INTRODUCTION.[edit]
At the present time, different opinions are being held about the fundamental equations of Electrodynamics for moving bodies. The Hertzian^{[1]} forms must be given up, for it has appeared that they are contrary to many experimental results.
In 1895 H. A. Lorentz^{[2]} published his theory of optical and electrical phenomena in moving bodies; this theory was based upon the atomistic conception (vorstellung) of electricity, and on account of its great success appears to have justified the bold hypotheses, by which it has been ushered into existence. In his theory^{[3]}, Lorentz proceeds from certain equations, which must hold at every point of "Æther"; then by forming the average values over "physically infinitely small" regions, which however contain large numbers of electrons, the equations for electromagnetic processes in moving bodies can be successfully built up.
In particular, Lorentz's theory gives a good account of the nonexistence of relative motion of the earth and the luminiferous "Æther"; it shows that this fact is connected with the covariance of the original equation, at certain simultaneous transformations of the space and time coordinates; these transformations have obtained from H. Poincaré^{[4]} the name of Lorentztransformations. The covariance of these fundamental equations, when subjected to the Lorentztransformation, is a purely mathematical fact; I will call this the Theorem of Relativity; this theorem rests essentially on the form of the differential equations for the propagation of waves with the velocity of light.
Now without recognizing any hypothesis about the connection between "Æther" and matter, we can expect these mathematically evident theorems to have their consequences so far extended — that thereby even those laws of ponderable media which are yet unknown may anyhow possess this covariance when subjected to a Lorentztransformation; by saying this, we do not indeed express an opinion, but rather a conviction, — and this conviction I may be permitted to call the Postulate of Relativity. The position of affairs here is almost the same as when the Principle of Conservation of Energy was postulated in cases, where the corresponding forms of energy were unknown.
Now if hereafter, we succeed in maintaining this covariance as a definite connection between pure and simple observable phenomena in moving bodies, the definite connection may be styled the Principle of Relativity.
These differentiations seem to me to be useful for enabling us to characterise the present day position of the electrodynamics for moving bodies.
H. A. Lorentz has found out the Relativity theorem and has created the Relativity postulate as a hypothesis that electrons and matter suffer contractions in consequence of their motion according to a certain law.
A. Einstein^{[5]} has brought out the point very clearly, that this postulate is not an artificial hypothesis but is rather a new way of comprehending the timeconcept which is forced upon us by observation of natural phenomena.
The Principle of Relativity has not yet been formulated for electrodynamics of moving bodies in the sense characterized by me. In the present essay, while formulating this principle, I shall obtain the fundamental equations for moving bodies in a sense which is uniquely determined by this principle. But it will be shown that none of the forms hitherto assumed for these equations can exactly fit in with this principle.
We would at first expect that the fundamental equations which are assumed by Lorentz for moving bodies would correspond to the Relativity Principle. But it will be shown that this is not the case for the general equations which Lorentz has for any possible, and also for magnetic bodies; but this is approximately the case (if we neglect the square of the velocity of matter in comparison to the velocity of light) for those equations which Lorentz hereafter infers for nonmagnetic bodies. But this latter accordance with the relativity principle is due to the fact that the condition of nonmagnetisation has been formulated in a way not corresponding to the relativity principle; therefore the accordance is due to the fortuitous compensation of two contradictions to the relativity postulate. But meanwhile enunciation of the Principle in a rigid manner does not signify any contradiction to the hypotheses of Lorentz's molecular theory, but it shall become clear that the assumption of the contraction of the electron in Lorentz's theory must be introduced at an earlier stage than Lorentz has actually done.
In an appendix, I have gone into discussion of the position of Classical Mechanics with respect to the relativity postulate. Any easily perceivable modification of mechanics for satisfying the requirements of the Relativity theory would hardly afford any noticeable difference in observable processes; but would lead to very surprising consequences. By laying down the relativity postulate from the outset, sufficient means have been created for deducing henceforth the complete series of Laws of Mechanics from the principle of conservation of energy (and statements concerning the form of the energy) alone.
§ 1. NOTATIONS.[edit]
Let a rectangular system (x, y, z, t,) of reference be given in space and time. The unit of time shall be chosen in such a manner with reference to the unit of length that the velocity of light in space becomes unity.
Although I would prefer not to change the notations used by Lorentz, it appears important to me to use a different selection of symbols, for thereby certain homogeneity will appear from the very beginning. I shall denote the vector
 electric force by , the magnetic induction by , the electric induction by and the magnetic force by ,
so that are used instead of Lorentz's respectively.
I shall further make use of complex magnitudes in a way which is not yet current in physical investigations, i.e., instead of operating with t, I shall operate with it, where i denotes . If now instead of (x, y, z, it), I use the method of writing with indices, certain essential circumstances will come into evidence; on this will be based a general use of the suffixes (1, 2, 3, 4). The advantage of this method will be, as I expressly emphasize here, that we shall have to handle symbols which have a purely real appearance; we can however at any moment pass to real equations if it is understood that of the symbols with indices, such ones as have the suffix 4 only once, denote imaginary quantities, while those which have not at all the suffix 4, or have it twice denote real quantities.
An individual system of values of x, y, z t, i. e., of shall be called a spacetime point.
Further let denote the velocity vector of matter, the dielectric constant, , the magnetic permeability, the conductivity of matter, while denotes the density of electricity in space, and the vector of "Electric Current" which we shall come across in §7 and §8.
PART I. Consideration of the Limiting Case Æther.[edit]
§ 2. The Fundamental Equations for Æther.[edit]
By using the electron theory, Lorentz in his above mentioned essay traces the Laws of Electrodynamics of Ponderable Bodies to still simpler laws. Let us now adhere to these simpler laws, whereby we require that for the limiting case , they should constitute the laws for ponderable bodies. In this ideal limiting case , we shall have . At every space time point x, y, z, t we shall have the equations:
I shall now write for x, y, z, it and
for
Further I shall write
for
i.e., the components of and along the three axes; now if we take any two indices h, k out of the series
therefore
,
, 
Then the three equations comprised in (I), and the equation (II) multiplied by i becomes
(A) 
On the other hand, the three equations comprised in (III) multiplied by i, and equation (IV) multiplied by 1, become
(B) 
By means of this method of writing we at once notice the perfect symmetry of the 1st as well as the 2nd system of equations as regards permutation with the indices (1,2,3,4).
§ 3. The Theorem of Relativity of Lorentz.[edit]
It is wellknown that by writing the equations I) to IV) in the symbol of vector calculus, we at once set in evidence an invariance (or rather a (covariance) of the system of equations A) as well as of B), when the coordinate system is rotated through a certain amount round the nullpoint. For example, if we take a rotation of the axes round the zaxis. through an amount , keeping fixed in space, and introduce new variables instead of , where
and introduce magnitudes
where
and , where
,
, , 
then out of the equations (A) would follow a corresponding system of dashed equations (A') composed of the newly introduced dashed magnitudes.
So upon the ground of symmetry alone of the equations (A) and (B) concerning the suffixes (1, 2, 3, 4), the theorem of Relativity, which was found out by Lorentz, follows without any calculation at all.
I will denote by a purely imaginary magnitude, and consider the substitution
(1) 
Putting
(2) 
where , and is always to be taken with the positive sign.
Let us now write
(3)  , 
then the substitution 1) takes the form
(4) 
the coefficients being essentially real.
If now in the abovementioned rotation round the zaxis, we replace 1, 2, 3, 4 throughout by 3, 4, 1, 2, and by , we at once perceive that simultaneously, new magnitudes , where
and , where
,
, , 
must be introduced. Then the systems of equations in (A) and (B) are transformed into equations (A'), and (B'), the new equations being obtained by simply dashing the old set.
All these equations can be written in purely real figures, and we can then formulate the last result as follows.
If the real transformations 4) are taken, and x', y', z', t' be taken as a new frame of reference, then we shall have
(5)  ,
, 
furthermore
(6) 
and
(7)  ^{[6]} 
Then we have for these newly introduced vectors with components ; and the quantity a series of equations I'), II'), III'), IV) which are obtained from I), II), III), IV) by simply dashing the symbols.
We remark here that e are components of the vector , where is a vector in the direction of the positive zaxis, and , and is the vector product of and ; similarly are the components of the vector .
The equations 6) and 7), as they stand in pairs, can be expressed as.
,
, 
If denotes any other real angle, we can form the following combinations : —
(8) 
, 
(9) 
§ 4. Special LorentzTransformation.[edit]
The role which is played by the zaxis in the transformation (4) can easily be transferred to any other axis when the system of axes are subjected to a transformation about this last axis. So we came to a more general law: —
Let be a vector with the components , and let . By we shall denote any vector which is perpendicular to , and by , we shall denote components of in direction of and .
Instead of x, y, z, t, new magnetudes x,' y,' z,' t' will be introduced in the following way. If for the sake of shortness, is written for the vector with the components x, y, z in the first system of reference, for the same vector with the components x', y', z' in the second system of reference, then for the direction of we have
(10)  , 
and for every perpendicular direction
(11)  , 
and further
(12) 
The notations and are to be understood in the sense that with the directions , and every direction perpendicular to in the system x, y, z are always associated the directions with the same direction cosines in the system x', y', z' ,
A transformation which is accomplished by means of (10), (11), (12) with the condition will be called a special Lorentztransformation. We shall call the vector, the direction of the axis, and the magnitude of the moment of this transformation.
If further and the vectors , in the system x', y', z' are so defined that,
(13)  , 
(14)  , 
further^{[7]}
(15)  , 
Then it follows that the equations I), II), III), IV) are transformed into the corresponding system with dashes.
The solution of the equations (10), (11), (12) leads to
(16)  . 
Now we shall make a very important observation about the vectors and . We can again introduce the indices 1, 2, 3, 4, so that we write instead of x,' y,' z,' it' , and instead of . Like the rotation round the zaxis, the transformation (4), and more generally the transformations (10), (11), (12), are also linear transformations with the determinant +1, so that
(17)  d. i. 
is transformed into
On the basis of the equations (13), (14), we shall have
transformed into or in other words,
(18)  , 
is an invariant in a Lorentztransformation.
If we divide by this magnitude, we obtain the four values
so that
(19)  . 
It is apparent that these four values, are determined by the vector and inversely the vector of magnitude follows from the 4 values , where are real, real and positive and condition (19) is fulfilled.
The meaning of here is, that they are the ratios of to
(20) 
The differentials denoting the displacements of matter occupying the spacetime point to the adjacent spacetime point.
After the Lorentztransfornation is accomplished the velocity of matter in the new system of reference for the same spacetime point x', y', z', t' is the vector with the ratios as components.
Now it is quite apparent that the system of values
is transformed into the values
in virtue of the Lorentztransformation (10), (11), (12).
The dashed system has got the same meaning for the velocity after the transformation as the first system of values has got for before transformation.
If in particular the vector of the special Lorentztransformation be equal to the velocity vector of matter at the spacetime point , then it follows out of (10), (11), (12) that
Under these circumstances therefore, the corresponding spacetime point has the velocity after the transformation, it is as if we transform to rest. We may call the invariant as the restdensity of Electricity.
§ 5. Spacetime Vectors. Of the 1st and 2nd kind.[edit]
If we take the principal result of the Lorentz transformation together with the fact that the system (A) as well as the system (B) is covariant with respect to a rotation of the coordinatesystem round the null point, we obtain the general relativity theorem. In order to make the facts easily comprehensible, it may be more convenient to define a series of expressions, for the purpose of expressing the ideas in a concise form, while on the other hand I shall adhere to the practice of using complex magnitudes, in order to render certain symmetries quite evident.
Let us take a linear homogeneous transformation,
(21) 
the Determinant of the matrix is +1, all coefficients without the index 4 occurring once are real, while , are purely imaginary, but is real and , and transforms into . The operation shall be called a general Lorentz transformation.
If we put then immediately there occurs a homogeneous linear transformation of x, y, z, t in x', y', z', t' with essentially real coefficients, whereby the aggregrate transforms into , and to every such system of values x, y, z, t with a positive t, for which this aggregate , there always corresponds a positive t'; this last is quite evident from the continuity of the aggregate x, y, z, t.
The last vertical column of coefficients has to fulfill, the condition
(22) 
If then , and the Lorentz transformation reduces to a simple rotation of the spatial coordinate system round the worldpoint.
If are not all zero, and if we put
On the other hand, with every set of value of which in this way fulfill the condition 22) with real values of , we can construct the special Lorentztransformation (16) with as the last vertical column, — and then every Lorentztransformation with the same last vertical column supposed to be composed of the special Lorentztransformation, and a rotation of the spatial coordinate system round the nullpoint.
The totality of all LorentzTransformations forms a group.
Under a spacetime vector of the 1st kind shall be understood a system of four magnitudes with the condition that in case of a Lorentztransformation it is to be replaced by the set , where these are the values obtained by substituting for in the expression (21).
Besides the timespace vector of the 1st kind we shall also make use of another spacetime vector of the first kind , and let us form the linear combination
(23) 
with six coefficients . Let us remark that in the vectorial method of writing, this can be constructed out of the four vectors
and the constants and at the same time it is symmetrical with regard the indices (1, 2, 3, 4).
If we subject and simultaneously to the Lorentz transformation (21), the combination (23) is changed to.
(24) 
where the coefficients depend solely on and the coefficients .
We shall define a spacetime Vector of the 2nd kind as a system of sixmagnitudes with the condition that when subjected to a Lorentz transformation, it is changed to a new system which satisfies the connection between (23) and (24).
I enunciate in the following manner the general theorem of relativity corresponding to the equations (I) — (IV), — which are the fundamental equations for Æther.
If x, y, z, it (space coordinates, and time it) is subjected to a Lorentz transformation, and at the same time (convectioncurrent, and charge density × i) is transformed as a space time vector of the 1st kind, further (magnetic force, and electric induction × i) is transformed as a space time vector of the 2nd kind, then the system of equations (I), (II), and the system of equations (III), (IV) transforms into essentially corresponding relations between the corresponding magnitudes newly introduced info the system.
These facts can be more concisely expressed in these words: the system of equations (I, and II) as well as the system of equations (III) (IV) are covariant in all cases of Lorentztransformation, where is to be transformed as a space time vector of the 1st kind, is to be treated as a vector of the 2nd kind, or more significantly, —
is a space time vector of the 1st kind, is a spacetime vector of the 2nd kind.
I shall add a few more remarks here in order to elucidate the conception of spacetime vector of the 2nd kind. Clearly, the following are invariants for such a vector when subjected to a group of Lorentz transformation.
(25)  , 
(26)  , 
A spacetime vector of the second kind , where and are real magnitudes, may be called singular, when the scalar square , i.e. , and at the same time , i.e. the vector and are equal and perpendicular to each other; when such is the case, these two properties remain conserved for the spacetime vector of the 2nd kind in every Lorentztransformation.
If the spacetime vector of the 2nd kind is not singular, we rotate the spacial coordinate system in such a manner that the vectorproduct coincides with the zaxis, i.e. . Then
Therefore is different from , and we can therefore define a complex argument in such a manner that
If then, by referring back to equations (9), we carry out the transformation (1) through the angle and a subsequent rotation round the zaxis through the angle , we perform a Lorentztransformation at the end of which , and therefore and shall both coincide with the new xaxis. Then by means of the invariants and the final values of these vectors, whether they are of the same or of opposite directions, or whether one of them is equal to zero, would be at once settled.
§ 6. Concept of Time.[edit]
By the Lorentz transformation, we are allowed to effect certain changes of the time parameter. In consequence of this fact, it is no longer permissible to speak of the absolute simultaneity of two events. The ordinary idea of simultaneity rather presupposes that six independent parameters, which are evidently required for defining a system of space and time axes, are somehow reduced to three. Since we are accustomed to consider that these limitations represent in a unique way the actual facts very approximately, we maintain that the simultaneity of two events exists of themselves.^{[8]} In fact, the following considerations will prove conclusive.
Let a reference system x, y, z, t for space time points (events) be somehow known. Now if a space point at the time be compared with a space point P(x, y, z) at the time t and if the difference of time , (let ) be less than the length 'AP, i.e. less than the time required for the propagation of light from A to P, and if , then by a special Lorentz transformation, in which AP is taken as the axis, and which has the moment q, we can introduce a time parameter t' which (see equation 11, 12, § 4) has got the same value for both spacetime points , and P, t. So the two events can now be comprehended to be simultaneous.
Further, let us take at the same time , two different spacepoints A, B, or three spacepoints A, B, C which are not in the same spaceline, and compare therewith a space point P, which is outside the line AB, or the plane ABC at another time t, and let the time difference be less than the time which light requires for propagation from the line AB, or the plane ABC to P. Let q be the ratio of by the second time. Then if a Lorentz transformation is taken in which the perpendicular from P on AB, or from P on the plane ABC is the axis, and q is the moment, then all the three (or four) events and P, t are simultaneous.
If four spacepoints, which do not lie in one plane are conceived to be at the same time to, then it is no longer permissible to make a change of the time parameter by a Lorentztransformation, without at the same time destroying the character of the simultaneity of these four space points.
To the mathematician, accustomed on the one hand to the methods of treatment of the polydimensional manifold, and on the other hand to the conceptual figures of the socalled nonEuclidean Geometry, there can be no difficulty in adopting this concept of time to the application of the Lorentztransformation. The paper of Einstein which has been cited in the Introduction, has succeeded to some extent in presenting the nature of the transformation from the physical standpoint.
PART II. ELECTROMAGNETIC PHENOMENA.[edit]
§ 7. Fundamental Equations for bodies at rest.[edit]
After these preparatory works, which have been first developed on account of the small amount of mathematics involved in the limitting case , let us turn to the electromagnetic phenomena in matter. We look for those relations which make it possible for us — when proper fundamental data are given — to obtain the following quantities at every place and time, and therefore at every spacetime point as functions of x, y, z, t: — the vector of the electric force , the magnetic induction , the electrical induction , the magnetic force , the electrical spacedensity , the electric current (whose relation hereafter to the conduction current is known by the manner in which conductivity occurs in the process), and lastly the vector , the velocity of matter.
The relations in question can be divided into two classes.
Firstly — those equations, which, — when , the velocity of matter is given as a function of x, y, z, t, — lead us to a knowledge of other magnitude as functions of x, y, z, t — I shall call this first class of equations the fundamental equations —
Secondly, the expressions for the ponderomotive force, which, by the application of the Laws of Mechanics, gives us further information about the vector as functions of x, y, z, t.
For the case of bodies at rest, i.e. when , the theories of Maxwell (Heaviside, Hertz) and Lorentz lead to the same fundamental equations. They are ; —
(1) The Differential Equations: — which contain no constant referring to matter: —
; 
(V)  , 
where = dielectric constant, = magnetic permeability, = the conductivity of matter, all given as function of x, y, z, t. is here the conduction current.
By employing a modified form of writing, I shall now cause a latent symmetry in these equations to appear. I put, as in the previous work,
and write for ,
further
for ,
and
for ;
lastly we shall have the relation , (the letter f, F shall denote the field, s the (i.e. current).
Then the fundamental Equations can be written as
(A) 
(B) 
§ 8. The Fundamental Equations for Moving Bodies.[edit]
We are now in a position to establish in a unique way the fundamental equations for bodies moving in any manner by means of these three axioms exclusively.
The first Axion shall be, —
When a detached region of matter is at rest at any moment, therefore the vector is zero, for a system x, y, z, t — the neighbourhood may be supposed to be in motion in any possible manner, then for the spacetime point x, y, z, t the same relations (A) (B) (V) which hold in the case when all matter is at rest, snail also hold between , the vectors and their differentials with respect to x, y, z, t.
The second axiom shall be : —
Every velocity of matter is < 1, smaller than the velocity of propagation of light.
The third axiom shall be : —
The fundamental equations are of such a kind that when x, y, z, it are subjected to a Lorentz transformation and thereby and are transformed into spacetime vectors of the second kind, as a spacetime vector of the 1st kind, the equations are transformed into essentially identical forms involving the transformed magnitudes.
Shortly I can signify the third axiom as ; —
and are spacetime vectors of the second kind, is a spacetime vector of the first kind.
This axiom I call the Principle of Relativity.
In fact these three axioms lead us from the previously mentioned fundamental equations for bodies at rest to the equations for moving bodies in an unambiguous way.
According to the second axiom, the magnitude of the velocity vector is < 1 at any spacetime point. In consequence, we can always write, instead of the vector , the following set of four allied quantities
with the relation
(27) 
From what has been said at the end of § 4, it is clear that in the case of a Lorentztransformation, this set behaves like a spacetime vector of the 1st kind, and we want to call it spacetime vector velocity.
Let us now fix our attention on a certain point x, y, z of matter at a certain time t. If at this spacetime point , then we have at once for this point the equations (A), (B) (V) of § 7. If , then there exists according to 16), in case , a special Lorentztransformation, whose vector is equal to this vector and we pass on to a new system of reference x', y', z', t' in accordance with this transformation. Therefore for the spacetime point considered, there arises as in § 4, the new values
(28)  , 
therefore the new velocity vector , the spacetime point is as if transformed to rest. Now according to the third axiom the system of equations for the transformed point x, y, z, t involves the newly introduced magnitude and their differential quotients with respect to x', y', z, t' in the same manner as the original equations for the point x, y, z, t. But according to the first axiom, when these equations must be exactly equivalent to
(1) the differential equations (A'), (B'), which are obtained from the equations (A), (B) by simply dashing the symbols in (A) and (B).
(2) and the equations
(V')  , 
where are the dielectric constant, magnetic permeability, and conductivity for the system x', y', z', t', i.e. in the spacetime point x, y, z, t of matter.
Now let us return, by means of the reciprocal Lorentztransformation to the original variables x, y, z, t, and the magnitudes and the equations, which we then obtain from the last mentioned, will be the fundamentil equations sought by us for the moving bodies.
Now from § 4, and § 6, it is to be seen that the equations A), as well as the equations B) are covariant for a Lorentztransformation, i.e. the equations, which we obtain backwards from A') B'), must be exactly of the same form as the equations A) and B), as we take them for bodies at rest. We have therefore as the first result: —
The differential equations expressing the fundamental equations of electrodynamics for moving bodies, when written in and the vectors are exactly of the same form as the equations for moving bodies. The velocity of matter does not enter in these equations. In the vectorial way of writing, we have
(I)  , 
The velocity of matter occurs only in the auxilliary equations which characterise the influence of matter on the basis of their characteristic constants . Let us now transform these auxilliary equations x, y, z into the original coordinates x, y, z, and t.)
According to formula 15) in § 4, the component of in the direction of the vector is the same us that of , the component of is the same as that of , but for the perpendicular direction , the components of and are the same as those of and , multiplied by . On the other hand and shall stand to , and in the same relation us and to and . From the relation , the following equations follow
(C)  . 
and from the relation we have
(D) 
For the components in the directions perpendicular to , and to each other, the equations are to be multiplied by .
Then the following equations follow from the transfermation equations (12), 10), (11) in § 4, when we replace by .
(E) 
In consideration of the manner in which enters into these relations, it will be convenient to call the vector with the components in the direction of and in the directions perpendicular to the Convection current. This last vanishes for .
We remark that for the equations immediately lead to the equations by means of a reciprocal Lorentztransformation with as vector; and for , the equation leads to , that the "fundamental equations of Æther" discussed in § 2 becomes in fact the limitting case of the equations obtained here with .§ 9. The Fundamental Equations in Lorentz's Theory.[edit]
Let us now see how far the fundamental equations assumed by Lorentz correspond to the Relativity postulate, as defined in §8. In the article on Electrontheory (Ency, Math., Wiss., Bd. V. 2, Art 14) Lorentz has given the fundamental equations for any possible, even magnetised bodies (see there page 209, Eq. XXX', formula (14) on page 78 of the same (part).
Then for moving nonmagnetised bodies, Lorentz puts (page 223, 3) , , and in addition to that takes account of the occurrence of the dielectric constant , and conductivity according to equations
(Eq. XXXIV"', p. 227) (Eq. XXXIII", p. 223) 
Lorentz's are here denoted by while denotes the conduction current.
The three last equations which have been just cited here coincide with eq. (II), (III), (IV), the first equation would be, if is identified with (the current being zero for ),
(29) 
and thus comes out to in in a different form than (1) here. Therefore for magnetised bodies, Lorentz's equations do not correspond to the Relativity Principle.
On the other hand, the form corresponding to the relativity principle, for the condition of nonmagnetisation is to be taken out of (D) in §8, with , not as , as Lorentz takes, but as
(30)  (hier ) 
If we make use of (30) for nonmagnetic bodies, and put accordingly , then in consequence of (C) in §8,
i.e. for the direction of
and for a perpendicular direction ,
i.e. it coincides with Lorentz's assumption, if we neglect in comparison to 1.
Also to the same order of approximation, Lorentz's form for corresponds to the conditions imposed by the relativity principle [comp. (E) § 8] — that the components of , are equal to the components of multiplied by or respectively.
§ 10. Fundamental Equations of E. Cohn.[edit]
E. Cohn^{[9]} assumes the following fundamental equations
(31) 
(32)  , 
where E, M are the electric and magnetic field intensities (forces), are the electric and magnetic polarisation (induction). The equations also permit the existence of true magnetism; if we do not take into account this consideration, is to be put = 0.
An objection to this system of equations, is that according to these, for , the vectors force and induction do not coincide. If in the equations, we conceive E and M and not and as electric and magnetic forces, and with a glance to this we substitute for the symbols , then the differential equations transform to our equations, and the conditions (32) transform into
; 
then in fact the equations of Cohn become the same as those required by the relativity principle, if errors of the order are neglected in comparison to 1.
It may be mentioned here that the equations of Hertz become the same as those of Cohn, if the auxilliary conditions are
(33)  ; 
§ 11. Typical Representations of the fundamental equations.[edit]
In the statement of the fundamental equations, our leading idea had been that they should retain a covariance of form, when subjected to a group of Lorentztransformations. Now we have to deal with ponderomotive reactions and energy in the electromagnetic field. Here from the very first there can be no doubt that the settlement of this question is in some way connected with the simplest forms which can be given to the fundamental equations, satisfying the conditions of covariance. In order to arrive at such forms, I shall first of all put the fundamental equations in a typical form which brings out clearly their covariance in case of a Lorentztransformation. Here I am using a method of calculation, which enables us to deal in a simple manner with the spacetime vectors of the 1st, and 2nd kind, and of which the rules, as far as required are given below.
1°. A system of magnitudes , formed into the matrix
arranged in p horizontal rows, and q vertical columns is called a seriesmatrix,^{[10]} and will be denoted by the letter A.
If all the quantities are multiplied by c, the resulting matrix will be denoted by .
If the roles of the horizontal rows and vertical columns be intercharged, we obtain a series matrix, which will be known as the transposed matrix of A, and will be denoted by A.
If we have a second series matrix B.
then A+B shall denote the series matrix whose members are .
2° If we have two matrices
where the number of horizontal rows of B, is equal to the number of vertical columns of A, then by AB, the product of the matrices A and B, will be denoted the matrix
where
these elements being formed by combination of the horizontal rows of A with the vertical columns of B. For such a point, the associative law holds, where S is a third matrix which has got as many horizontal rows as B (or AB) has got vertical columns.
For the transposed matrix of , we have .
3°. We shall have principally to deal with matrices with at most four vertical columns and for horizontal rows.
As a unit matrix (in equations they will be known for the sake of shortness as the matrix 1) will be denoted the following matrix (4 ✕ 4 series) with the elements.
(34) 
For a 4✕4 seriesmatrix, Det A shall denote the determinant formed of the 4✕4 elements of the matrix. If , then corresponding to A there is a reciprocal matrix, which we may denote by so that
A matrix
(35)  , 
Then
(36)  , 
i.e. We shall have a 4✕4 series matrix in which all the elements except those on the diagonal from left up to right down are zero, and the elements in this diagonal agree with each other, and are each equal to the above mentioned combination in (36).
The determinant of f is therefore the square of the combination, by we shall denote the expression
(37) 
4°. A linear transformation
(38) 
which is accomplished by the matrix
will be denoted as the transformation
By the transformation , the expression
is changed into the quadratic form
are the members of a 4✕4 series matrix which is the product of , the transposed matrix of into . If by the transformation, the expression is changed to
we must have
(39) 
has to correspond to the following relation, if transformation (38) is to be a Lorentztransformation. For the determinant of it follows out of (39) that .
From the condition (39) we obtain
(40) 
i.e. the reciprocal matrix of is equivalent to the transposed matrix of .
For as Lorentz transformation, we have further , the quantities involving the index 4 once in the subscript are purely imaginary, the other coefficients are real, and .
5°. A space time vector of the first kind which is represented by the 1✕4 series matrix,
(41) 
is to be replaced by in case of a Lorentz transformation
A spacetime vector of the 2nd kind with components shall be represented by the alternating matrix
(42) 
and is to be replaced by in case of a Lorentz transformation [see the rules in § 5 (23) (24)]. Therefore referring to the expression (37), we have the identity . Therefore becomes an invariant in the case of a Lorentz transformation [see eq. (26) Sec. § 5].
Looking back to (36), we have for the dual matrix
from which it is to be seen that the dual matrix behaves exactly like the primary matrix f, and is therefore a space time vector of the II kind; is therefore known as the dual spacetime vector of f with components .
6°.If w and s are two spacetime vectors of the 1st kind then by (as well as by ) will be understood the combination
(43) 
In case of a Lorentz transformation , since this expression is invariant. — If , then w and s are perpendicular to each other.
Two spacetime rectors of the first kind w, s gives us a 2✕4 series matrix
Then it follows immediately that the system of six magnitudes
(44) 
behaves in case of a Lorentztransformation as a spacetime vector of the II. kind. The vector of the second kind with the components (44) are denoted by [w,s]. We see easily that . The dual vector of [w,s] shall be written as [w,s]*.
If w is a spacetime vector of the 1st kind, f of the second kind, wf signifies a 1✕4 series matrix. In case of a Lorentztransformation , w is changed into , f into , therefore , i.e., wf is transformed as a spacetime vector of the 1st kind. We can verify, when w is a spacetime vector of the 1st kind, f of the 2nd kind, the important identity
(45)  . 
For example, for
;
; 
The fact that in this special case, the relation is satisfied, suffices to establish the theorem (45) generally, for this relation has a covariant character in case of a Lorentz transformation, and is homogeneous in .
After these preparatory works let us engage ourselves with the equations (C,) (D,) (E) by means which the constants will be introduced.
Instead of the space vector , the velocity of matter, we shall introduce the spacetime vector of the first kind w with the components.
where
(46) 
and ..
By F and f shall be understood the space time vectors of the second kind , .
In , we have a space time vector of the first kind with components
The first three quantities are the components of the spacevector
(47)  , 
(48)  , 
Because F is an alternating matrix,
(49)  , 
i.e. is perpendicular to the vector to w; we can also write
(50)  , 
I shall call the spacetime vector of the first kind as the Electric Rest Force.
Relations analogous to those holding between , hold amongst , and in particular wf is normal to w. The relation (C) can be written as
{C} 
The expression (wf) gives four components, but the fourth can be derived from the first three.
Let us now form the timespace vector 1st kind , whose components are
Of these, the first three are the x, y, zcomponents of the spacevector
(51)  , 
and further
(52)  ; 
Among these there is the relation
(53)  , 
which can also be written as
(54) 
The vector is perpendicular to w; we can call it the Magnetic restforce.
Relations analogous to these hold among the quantities and Relation (D) can be replaced by the formula
{D} 
We can use the relations (C) and (D) to calculate F and f from and , we have
and applying the relation (45) and (46), we have
(55)  , 
(56)  , 
i.e.
, etc.
, etc. 
Let us now consider the spacetime vector of the second kind , with the components
,
, 
Then the corresponding spacetime vector of the first kind
vanishes identically owing to equations 49) and 53).
Let us now take the vector of the 1st kind
(57) 
with the components
Then by applying rule (45), we have
(58)  , 
i,e.
The vector fulfills the relation
(59)  , 
which we can write as
and is also normal to w. In case , we have , and
(60)  , 
I shall call , which is a spacetime vector 1st kind the RestRay.
As for the relation E), which introduces the conductivity , we have
This expression gives us the restdensity of electricity (see §8 and §4). Then
(61) 
represents a spacetime vector of the 1st kind, which since , is normal to w, and which I may call the restcurrent. Let us now conceive of the first three component of this vector as the x, y, z coordinates of the spacevector, then the component in the direction of is
and the component in a perpendicular direction is .
This spacevector is connected with the spacevector , which we denoted in § 8 as the conductioncurrent.
Now by comparing with , the relation (E) can be brought into the form
(E)  . 
Lastly, we shall transform the differential equations (A) and (B) into a typical form.
§ 12. The Differential Operator Lor.[edit]
A 4✕4 series matrix
(62) 
with the condition that in case of a Lorentz transformation it is to be replaced by , may be called a spacetime matrix of the II. kind. We have examples of this in : —
 1) the alternating matrix f, which corresponds to the spacetime vector of the II. kind, —
 2) the product fF of two such matrices, for by a transformation , it is replaced by ,
 3) further when and are two spacetime vectors of the 1st kind, the 4✕4 matrix with the ,
 lastly in a multiple L of the unit matrix of 4✕4 series in which all the elements in the principal diagonal are equal to L, and the rest are zero.
We shall have to do constantly with functions of the spacetime point x, y, z, it, and we may with advantage employ the 1✕4 series matrix, formed of differential symbols, —
or
(63) 
Then if S is, as in (62), a spacetime matrix of the II. kind, by lor S' will be understood the 1✕4 series matrix
where
(64) 
When by a Lorentz transformation , a new reference system is introduced, we can use the operator
Then S is transformed to , so by lor' Sis meant the 1✕4 series matrix, whose element are
Now for the differentiation of any function of (x y z t) we have the rule
, 
so that, we have symbolically
Therefore it follows that
(65)  , 
i.e., lor S behaves like a spacetime vector of the first kind.
If L is a multiple of the unit matrix, then by lor L will be denoted the matrix with the elements
(66) 
If is a spacetime vector of the 1st kind, then
(67) 
In case of a Lorentz transformation , we have
i.e., lor s is an invariant in a {scLorentz}}transformation.
In all these operations the operator lor plays the part of a spacetime vector of the first kind.
If f represents a spacetime vector of the second kind, lor f denotes a spacetime vector of the first kind with the components
So the system o£ differential equations (A) can be expressed in the concise form
{A} 
and the system (B) can be expressed in the form
{B} 
Referring back to the definition (67) for , we find that the combinations and vanish identically, when f and F* are alternating matrices. Accordingly it follows out of (A), that
(68)  , 
(69) 
signifies that of the four equations in (B), only three represent independent conditions.
I shall now collect the results.
Let w denote the spacetime vector of the first kind
F the spacetime vector of the second kind ( = magnetic induction, = Electric force),
f the spacetime vector of the second kind ( = magnetic force,)
= Electric Induction.
s the spacetime vector of the first kind ( = electrical spacedensity,)
= conductivity current,
= dielectric constant,
= magnetic permeability,
= conductivity.
then the fundamental equations for electromagnetic processes in moving bodies are
{A} 
{B} 
{C} 
{D} 
{E}  . 
, and wF, wf, wF*, wf*, which are spacetime vectors of the first kind are all normal to w, and for the system {B}, we have
§ 13. The Product of the Fieldvectors fF.[edit]
Finally let us enquire about the laws which lead to the determination of the vector w as a function of x, y, z, t. In these investigations, the expressions which are obtained by the multiplication of two alternating matrices
are of much importance. Let us write.
(70) 
Then (71)
(71) 
Let L now denote the symmetrical combination of the indices 1, 2, 3, 4, given by
(72) 
Then we shall have
(73) 
In order to express in a real form, we write
(74) 
Now
(75)  ,
, u.s.f. , , u.s.f. , 
(76)  , 
These quantities are all real. In the theory for bodies at rest, the combinations ( are known as Maxwell's Stresses", are known as the Poynting's Vector, as the electromagnetic energydensity, and L as the Langrangian function.
On the other hand, by multiplying the alternating matrices of f and F, we obtain
(77) 
and hence, we can put
(78)  , 
where by L, we mean Ltimes the unit matrix, i.e. the matrix with elements
Since here , we deduce that,
and find, since , we arrive at the interesting conclusion
(79)  , 
i.e. the product of the matrix S into itself can be expressed as the multiple of a unit matrix — a matrix in which all the elements except those in the principal diagonal are zero, the elements in the principal diagonal are all equal and have the value given on the righthand side of (79). Therefore the general relations
(80) 
h, k being unequal indices in the series 1, 2, 3, 4, and
(81) 
for h = 1,2,3,4.
Now if instead of F and f in the combinations (72) and (73), we introduce the electrical restforce , the magnetic restforce and the restray [(55), (56) and (57)], we can pass over to the expressions, —
(82)  , 
(83) 
Here we have
,
. 
The right side of (82) as well as L is an invariant in a Lorentz transformation, and the 4✕4 element on the right side of (83) as well as , represent a space time vector of the second kind. Remembering this fact, it suffices, for establishing the theorems (82) and (83) generally, to prove it for the special case . But for this case , we immediately arrive at the equations (82) and (83) by means (45), (51), (60) on the one hand, and on the other hand.
The expression on the righthand side of (81), which equals
is , because , now referring back to 79), we can denote the positive square root of this expression as .
Since , we obtain for , the transposed matrix of S, the following relations from (78),
(84)  . 
an alternating matrix, and denotes a spacetime vector of the second kind. From the expressions (83), we obtain,
(85) 
from which we deduce that [see (57), (58)].
(86)  , 
(87)  , 
When the matter is at rest at a spacetime point, , then the equation 86) denotes the existence of the following equations
and from 83),
Now by means of a rotation of the space coordinate system round the nullpoint, we can make,
According to 71), we have
(88) 
and according to 83), . In special eases, where vanishes it follows from 81) that
and if and one of the three magnitudes are , the two others . If does not vanish let , then we have in particular from 80)
and if . It follows from (81), (see also 88) that
,
. 
(89) 
is of very great importance for which we now want to demonstrate a very important transformation
According to 78), , and it follows that
The symbol lor denotes a differential process which in lor fF, operates on the one hand upon the components of f, on the other hand also upon the components of F. Accordingly lor fF can be expressed as the sum of two parts. The first part is the product of the matrices (lor f)F, lor f being regarded as a 1✕4 series matrix. The second part is that part of lor fF, in which the diffentiations operate upon the components of F alone. From 78) we obtain
hence the second part of lor fF = the part of , in which the differentiations operate upon the components of F alone. We thus obtain
(90)  , 
where N is the vector with the components
By using the fundamental relations A) and B), 90) is transformed into the fundamental relation
(91) 
In the limitting case , N vanishes identically.
Now upon the basis of the equations (55) and (56), and referring back to the expression (82) for L, and from 57) we obtain the following expressions as components of N,—
(92) 
Now if we make use of (59), and denote the spacevector which has as the x, y, zcomponents by the symbol , then the third component of 92) can be expressed in the form
(93) 
The round bracket denoting the scalar product of the vectors within it.
§ 14. The Ponderomotive Force.[edit]
Let us now write out the relation in a more practical form; we have the four equations
(94) 
, 
(95) 
, 
(96) 
, 
(97) 
. 
It is my opinion that when we calculate the ponderomotive force which acts upon a unit volume at the spacetime point x,y,z,t, it has got x, y, z components as the first three components of the spacetime vector
(98) 
This vector is perpendicular to w; the law of Energy finds its expression in the fourth relation.
The establishment of this opinion is reserved for a separate tract.
In the limitting case , the vector , and we obtain the ordinary equations in the theory of electrons.
APPENDIX. Mechanics and the RelativityPostulate.[edit]
It would be very unsatisfactory if the new way of looking at the timeconcept, which permits a Lorentz transformation, were to be confined to a single part of Physics.
Now many authors say that classical mechanics stand in opposition to the relativity postulate, which is taken to be the basic of the new Electrodynamics.
In order to decide this let us fix our attention upon a special Lorentz transformation represented by (10), (11), (12), with a vector in any direction and of any magnitude q < 1, but different from zero. For a moment we shall not suppose any special relation to hold between the unit of length and the unit of time, so that instead of t, t',q we shall write ct, ct', and , where c represents a certain positive constant, and . The above mentioned equations are transformed into
They denote, as we remember, that is the spacevector x, y, z and the spacevector x', y', z'.
If in these equations, keeping constant, we approach the limit , then we obtain from these
We can, therefore, say that classical mechanics postulates a covariance of Physical laws for the group of homogeneous linear transformations of the expression
(1) 
when .
Now it is rather confusing to find that in one branch of Physics, we shall find a covariance of the laws for the transformation of expression (1) with a finite value of c, in another part for math>c = \infty</math>. It is evident that according to Newtonian Mechanics, this covariance holds for math>c = \infty</math> and not for c = velocity of light. May we not then regard those traditional covariances for only as an approximation consistent with experience, the actual covariance of natural laws holding for a certain finite value of c?
I may here point out that by reforming mechanics, where instead of the Newtonian RelativityPostulate with we assume a relativitypostulate with a finite c, then the axiomatic construction of Mechanics appears to gain considerably in perfection.
The ratio of the time unit to the length unit is chosen in a manner so as to make the velocity of light equivalent to unity.
While now I want to introduce geometrical figures in the manifold of the variables x, y, z, t, it may be convenient to leave y, z out of account, and to treat x and t as any possible pair of coordinates in a plane, refered to oblique axes.
A space time null point () will be kept fixed in a Lorentz transformation. The figure
(2)  , 
The direction of a radius vector OA' drawn from to the point A' of (2), and the directions of the tangents to (2) at A' are to be called normal to each other.
Let us now follow a definite position. of matter in its course through all time t. The totality of the spacetime points x, y, z, t, which correspond to the positions at different times t, shall be called a spacetime line.
The task of determining the motion of matter is comprised in the following problem: — It is required to establish for every spacetime point the direction of the spacetime line passing through it.
To transform a spacetime point P(x, y, z, t) to rest is equivalent to introducing, by means of a Lorentz transformation, a new system of reference x', y', z', t', in which the t' axis has the direction OA', OA' indicating the direction of the spacetime line passing through P. The space t' = const, which is to be laid through P, is the one which is perpendicular to the spacetime line through P. To the increment dt of the time of P corresponds the increment
(3)  ^{[11]} 
of the newly introduced time parameter t'. The value of the integral
when calculated upon the spacetime line from a fixed initial point P° to the variable point P, (both being on the spacetime line), is known as the Propertime of the position of matter we are concerned with at the spacetime point P. (It is a generalization of the idea of Positionaltime which was introduced by Lorentz for uniform motion.)
If we take a body which has got extension in space at time , then the region comprising all the spacetime line passing through shall be called a spacetime filament.
If we have an analytical expression so that is intersected by every space time line of the filament at one pointy — wherebythen the totality of the intersecting points will be called a cross section of the filament. At any point P of such acrosssection, we can introduce by means of a Lorentz transformation a system of reference x' y', z', t', so that according to this
The direction of the uniquely determined t'— axis in question here is known as the upper normal of the crosssection at the point P and the value of for the surrounding points of P on the crosssection is known as the elementary contents (Inhaltslement) of the crosssection. In this sense is to be regarded as the crosssection normal to the t axis of the filament at the point and the volume of the body is to be regarded as the contents of the crosssection.
If we allow to converge to a point, we come to the conception of an infinitely thin spacetime filament. In such a ease, a spacetime line will be thought of as a principal line and by the term Propertime of the filament will be understood the Propertime which is laid along this principal line; under the term normal crosssection of the filament, we shall understand the crosssection upon the space which is normal to the principal line through P.
We shall now formulate the principle of conservation of mass.
To every space R at a time t, belongs a positive quantity — the mass at R at the time t. If R converges to a point x, y, z, t, then the quotient of this mass, and the volume of R approaches a limit , which is known as the massdensity at the spacetime point x, y, z, t.
The principle of conservation of mass says — that for an infinitely thin spacetime filament, the product , where = massdensity at the point of the filament (i.e., the principal line of the filament), dJ = contents of the crosssection normal to the t axis, and passing through , is constant along the whole filament.
Now the contents of the normal crosssection of the filament which is laid through x, y, z, t is
(4) 
and the function
(5) 
may be defined as the restmass density at the position x, y, z, t. Then the principle of conservation of mass can be formulated in this manner: —
For an infinitely thin spacetime filament, the product of the restmass density and the contents of the normal crosssection is constant along the whole filament.
In any spacetime filament, let us consider two crosssections and , which have only the points on the boundary common to each other; let the spacetime lines inside the filament have a larger value of t on Q than on . The finite range enclosed between and shall be called a spacetime sickle^{[WS 1]} is the lower boundary, and is the upper boundary of the sickle.
If we decompose a filament into elementary spacetime filaments, then to an entrancepoint of an elementary filament through the lower boundary of the sickle, there corresponds an exit point of the same by the upper boundary, whereby for both, the product taken in the sense of (4) and (5), has got the same value. Therefore the difference of the two integrals (the first being extended over the upper, the second upon the lower boundary) vanishes. According to a wellknown theorem of Integral Calculus the difference is equivalent to
the integration being extended over the whole range of the sickle, and (comp. (67), § 12)
If the sickle reduces to a point, then the differential equation
(6)  , 
Further let us form the integral
(7)  . 
extending over the whole range of the spacetime sickle. We shall decompose the sickle into elementary spacetime filaments, and every one of these filaments in small elements (It of its propertime, which are however large compared to the linear dimensions of the normal crosssection; let us assume that the mass of such a filament and write and for the 'Propertime' of the upper and lower boundary of the sickle.
Then the integral (7) can be denoted by
taken over all the elements of the sickle.
Now let us conceive of the spacetime lines inside a spacetime sickle as material curves composed of material points, and let us suppose that they are subjected to a continual change of length inside the sickle in the following manner. The entire curves are to be varied in any possible manner inside the sickle, while the end points on the lower and upper boundaries remain fixed, and the individual substantial points upon it are displaced in such a manner that they always move forward normal to the curves. The whole process may be analytically represented by means of a parameter , and to the value , shall correspond the actual curves inside the sickle. Such a process may be called a virtual displacement in the sickle.
Let the point x, y, z, t in the sickle have the values , when the parameter has the value ; these magnitudes are then functions of . Let us now conceive of an infinitely thin spacetime filament at the point x, y, z, t with the normal section of contents and if be the contents of the normal section at the corresponding position of the varied filament, then according to the principle of conservation of mass being the restmassdensity at the varied position,
(8) 
If we now introduce the method of writing with indices, we shall have
(9)  . 
Now on the basis of the remarks already made, it is clear that the value of , when the value of the parameter is , will be: —
(10)  , 
the integration extending over the whole sickle , where denotes the magnitude, which is deduced from
by means of (9) and
therefore: —
(11)  . 
We shall now subject the value of the differential quotient
(12) 
to a transformation. Since each , as a function of vanishes for the zerovalue of the paramater , so in general for .
Let us now put
(13)  , 
then on the basis of (10) and (11), we have the expression (12):
for the system on the boundary of the sickle, shall vanish for every value of and therefore are nil. Then by partial integration, the integral is transformed into the form
the expression within the bracket may be written as
The first sum vanishes in consequence of the continuity equation (6). The second may be written as
whereby is meant the differential quotient in the direction of the spacetime line at any position. For the differential quotient (12), we obtain the final expression
(14)  . 
For a virtual displacement in the sickle we have postulated the condition that the points supposed to be substantial shall advance normally to the curves giving their actual motion, which is , this condition denotes that the is to satisfy the condition
(15) 
Let us now turn our attention to the Maxwellian tensions in the electrodynamics of stationary bodies, and let us consider the results in §§ 12 and 13; then we find that Hamilton's Principle can be reconciled to the relativity postulate for continuously extended elastic media.
At every spacetime point (as in § 18), let a space time matrix of the 2nd kind be known
(16) 
where are real magnitudes.
For a virtual displacement in a spacetime sickle (with the previously applied designation) the value of the integral
(17) 
extended over the whole range of the sickle, may be called the tensional work of the virtual displacement.
The sum which comes forth here, written in real magnitudes, is
. 
we can now postulate the following minimum principle in mechanics.
If any spacetime sickle be bounded, then for each virtual displacement in the sickle, the sum of the massworks, and tension works shall always he an extremum for that process of the spacetime line in the sickle which actually occurs.
The meaning is, that for each virtual displacement,
(18) 
By applying the methods of the Calculus of Variations, the following four differential equations at once follow from this minimal principle by means of the transformation (14), and the condition (16).
(19)  , 
whence
(20) 
are components of the spacetime vector 1st kind K = lor S, and is a factor, which is to be determined from the relation . By multiplying (19) by , and summing the four, we obtain , and therefore clearly will be a spacetime vector of the 1st kind which is normal to w. Let us write out the components of this vector as
Then we arrive at the following equations for the motion of matter,
(21) 
and we have also
and
On the basis of this condition, the fourth of equations (21) is to be regarded as a direct consequence of the first three.
From (21), we can deduce the law for the motion of a material point, i.e, the law for the career of an infinitely thin spacetime filament.
Let x, y, z, t denote a point on a principal line chosen in any manner within the filament. We shall form the equations (21) for the points of the normal cross section of the filament through x, y, z, t, and integrate them, multiplying by the elementary contents of the cross section over the whole space of the normal section. If the integrals of the right side be , and if m be the constant mass of the filament, we obtain
(22) 
We may call this vector R the moving force of the material point.
If instead of integrating over the normal section, we integrate the equations over that cross section of the filament which is normal to the t axis, and passes through x, y, z, t, then [See (4)] the equations (22) are obtained, but are now multiplied by ; in particular, the last equation comes out in the form,
The right side is to be looked upon as the amount of work done per unit of time at the material point. In this equation, we obtain the energylaw for the motion of the material point and the expression
may be called the kinetic energy of the material point. Since is always greater than we may call the quotient as the "Gain" (vorgehen) of the time over the propertime of the material point and the law can then be thus expressed; — The kinetic energy of a material point is the product of its mass into the gain of the time over its propertime.
The set of four equations (22) again shows the symmetry in x, y, z, it, which is demanded by the relativity postulate; to the fourth equation however, a higher physical significance is to be attached, as we have already seen in the analogous case in electrodynamics. On the ground of this demand for symmetry, the triplet consisting of the first three equations are to be constructed after the model of the fourth; remembering this circumstance, we are justified in saying, — If the relativitypostulate be placed at the head of mechanics, then the whole set of laws of motion follows from the law of energy.
I cannot refrain from showing that no contradiction to the assumption on the relativitypostulate can be expected from the phenomena of gravitation.^{[12]}
If B*(x*, y*, z*, t*) be a solid (fester) spacetime point, then the region of all those spacetime points B(x, y, z, t)', for which
(23) 
may be called a Rayfigure (Strahlgebilde) of the space lime point B*.
A spacetime line taken in any manner can be cut by this figure only at one particular point; this easily follows from the convexity of the figure on the one hand, and on the other hand from the fact that all directions of the spacetime lines are only directions from B* towards to the concave side of the figure. Then B* may be called the lightpoint of B*.
If in (23), the point B(x, y, z, t) be supposed to be fixed, the point B*(x*, y*, z*, z*) be supposed to be variable, then the relation (23) would represent the locus of all the spacetime points B*, which are lightpoints of B.
Let us conceive that a material point F of mass m may, owing to the presence of another material point F*, experience a moving force according to the following law. Let us picture to ourselves the spacetime filaments of F and F* along with the principal lines of the filaments. Let BC be an infinitely small element of the principal line of F; further let B* be the light point of B, C* be the light point of C on the principal line of F*; so that OA' is the radius vector of the hyperboloidal fundamental figure (23) parallel to B* C*, finally D* is the point of intersection of line B*C* with the space normal to itself and passing through B. The moving force of the masspoint F in the spacetime point B is now the spacetime vector of the first kind which is normal to BC, and which is composed of the vectors
(24) 
in the direction of BD* and another vector of suitable value in direction of B*C*.. Now by is to be understood the ratio of the two vectors in question.
It is clear that this proposition at once shows the covariant character with respect to a Lorentzgroup.
Let us now ask how the spacetime filament of F behaves when the material point F* has a uniform translatory motion, i.e., the principal line of the filament of F* is a line. Let us take the space time nullpoint in this, and by means of a Lorentztransformation, we can take this axis as the taxis. Let x, y, z, t, denote the point B, let denote the proper time of B*, reckoned from O. Our proposition leads to the equations
(25) 
and
(26)  , 
where
(27) 
and
(28) 
In consideration of (27), the three equations (25) are of the same form as the equations for the motion of a material point subjected to attraction from a fixed centre according to the Newtonian Law, only that instead of the time t the proper time of the material point occurs. The fourth equation (26) gives then the connection between proper time and the time for the material point.
Now for different values of the orbit of the spacepoint x, y, z is an ellipse with the semimajor axis a and the eccentricity e. Let E denote the excentric anomaly, T the increment of the proper time for a complete description of the orbit, finally , so that from a properly chosen initial point r, we have the Keplerequation
(29) 
If we now change the unit of time, and denote the velocity of light by c, then from (28), we obtain
(30)  . 
from which, by applying (29),
(31)  , 
the factor is here the square of the ratio of a certain average velocity of F in its orbit to the velocity of light. If now m* denote the mass of the sun, a the semi major axis of the earth's orbit, then this factor amounts to .
The law of mass attraction which has been just described and which is formulated in accordance with the relativity postulate would signify that gravitation is propagated with the velocity of light. In view of the fact that the periodic terms in (31) are very small, it is not possible to decide out of astronomical observations between such a law (with the modified mechanics proposed above) and the Newtonian law of attraction with Newtonian mechanics.
 ↑ Ueber die Grundgleichungen der Elektrodynamik für bewegte Körper. Wiedemanns Ann. 41. p. 869. 1890 (also in: Ges. Werke Bd. I. p. 266. Leipzig 1B92).
 ↑ Versuch einer Theorie der elektrischen und optischen Erscheinungen in bewegten Körpern, Leiden 1895.
 ↑ See. Encyklopädie der math. Wissenschaften, Vol. 2, Art. 14. Weiterbildung der Maxwellschen Theorie. Elektronentheorie.
 ↑ Rend. Circ. Matem. Palermo, t. XXI (1906), p. 129.
 ↑ Ann. d. Phys. 17, p. 891, 1905.
 ↑ The equations (5) are written in a different order, however, equations (6) and (7) in the same order as the equations mentioned before, which amounts to them
 ↑ The brackets shall only summarize the expressions, which are related to the index, and shall denote the vector product of and .
 ↑ Just as beings which are confined within a narrow region surrounding a point on a spherical surface, may fall into the error that a sphere is a geometric figure in which one diameter is particularly distinguished from the rest.
 ↑ Gött. Nachr. 1901, w. 74 (also in Ann. d. Phys. 7 (4), 1902, p. 29).
 ↑ One could think about using Hamilton's quaternion calculus instead of Cayley's matrix calculus, however, Hamilton's calculus seems to me as too narrow and cumbersome for our purposes.
 ↑ The notation with indices and the symbols we again use in the form as it was defined before. (s. § 8 and § 4).
 ↑ In a completely different manner than I do, H. Poincaré (Rend. Circ. Matem. Palermo, t. XXI (1906), p. 129) tried to harmonize Newton's law of attraction with the relativity postulate.
 ↑ Saha used the German word "Sichel" in this edition.
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