LOG#033. Electromagnetism in SR.Posted: 2012/10/07
The Maxwell’s equations and the electromagnetism phenomena are one of the highest achievements and discoveries of the human kind. Thanks to it, we had radio waves, microwaves, electricity, the telephone, the telegraph, TV, electronics, computers, cell-phones, and internet. Electromagnetic waves are everywhere and everytime (as far as we know, with the permission of the dark matter and dark energy problems of Cosmology). Would you survive without electricity today?
The language used in the formulation of Maxwell equations has changed a lot since Maxwell treatise on Electromagnetis, in which he used the quaternions. You can see the evolution of the Mawell equations “portrait” with the above picture. Today, from the mid 20th centure, we can write Maxwell equations into a two single equations. However, it is less know that Maxwell equations can be written as a single equation using geometric algebra in Clifford spaces, with , or the so-called Kähler-Dirac-Clifford formalism in an analogue way.
Before entering into the details of electromagnetic fields, let me give some easy notions of tensor calculus. If , how does transform under Lorentz transformations? Let me start with the tensor components in this way:
Note, we have used with caution:
1st. Einstein’s convention: sum over repeated subindices and superindices is understood, unless it is stated some exception.
2nd. Free indices can be labelled to the taste of the user segment.
3rd. Careful matrix type manipulations.
We define a contravariant vector (or tensor (1,0) ) as some object transforming in the next way:
where denotes the Jabobian matrix of the transformation.
In similar way, we can define a covariant vector ( or tensor (0,1) ) with the aid of the following equations
Contravariant tensors of second order ( tensors type (2,0)) are defined with the next equations:
Covariant tensors of second order ( tensors type (0,2)) are defined similarly:
Mixed tensors of second order (tensors type (1,1)) can be also made:
We can summarize these transformations rules in matrix notation making the transcript from the index notation easily:
1st. Contravariant vectors change of coordinates rule:
2nd. Covariant vectors change of coordinates rule:
3rd. (2,0)-tensors change of coordinates rule:
4rd. (0,2)-tensors change of coordinates rule:
5th. (1,1)-tensors change of coordinates rule:
Indeed, without taking care with subindices and superindices, and the issue of the inverse and transpose for transformation matrices, a general tensor type (r,s) is defined as follows:
We return to electromagnetism! The easiest examples of electromagnetic wave motion are plane waves:
Indeed, the cuadrivector K can be “guessed” from the phase invariant ( since the phase is a dot product):
where is the four dimensional nabla vector defined by
Now, let me discuss different notions of velocity when we are considering electromagnetic fields, beyond the usual notions of particle velocity and observer relative motion, we have the following notions of velocity in relativistic electromagnetism:
1st. The light speed c. It is the ultimate limit in vacuum and SR to the propagation of electromagnetic signals. Therefore, it is sometimes called energy transfer velocity in vacuum or vacuum speed of light.
2nd. Phase velocity . It is defined as the velocity of the modulated signal in a plane wave, if , we have
where k is the modulus of . It measures how much fast the phase changes with the wavelength vector.
From the definition of cuadrivector wave length, we deduce:
Then, we can rewrite the distinguish three cases according to the sign of the invariant :
a) . The separation is spacelike and we get .
b). The separation is lightlike or isotropic. We obtain .
c). The separation is timelike. We deduce that . This situation is not contradictory with special relativity since phase oscillations can not transport information.
3rd. Group velocity . It is defined like the velocity that a “wave packet” or “pulse” has in its propagation. Therefore,
where we used the Planck relationships for photons and , with
4th. Particle velocity. It is defined in SR by the cuadrivector
5th. Observer relative velocity, V. It is the velocity (constant) at which two inertial observes move.
There is a nice relationship between the group velocity, the phase velocity and the energy transfer, the lightspeed in vacuum. To see it, look at the invariant:
Deriving this expression, we get
so we have the very important equation
Other important concept in electromagnetism is “light intensity”. Light intensity can be thought like the “flux of light”, and you can imagine it both in the wave or particle (photon corpuscles) theory in a similar fashion. Mathematically speaking:
so where u is the energy density of the electromagnetic field and c is the light speed in vacuum. By Lorentz transformations, it can be showed that for electromagnetic waves, energy, wavelength, energy density and intensity change in the following way:
The relativistic momentum can be related to the wavelength cuadrivector using the Planck relation . Under a Lorentz transformation, momenergy transforms . Assign to the wave number vector a direction in the S-frame:
In matrix notation, the whole change is written as:
Using the first two equations, we get:
Using the first and the third equation, we obtain:
Dividing the last two equations, we deduce:
This formula is the so-called stellar aberration formula, and we will dedicate it a post in the future.
If we write the first equation with the aid of frequency f (and ) instead of angular frequency,
where we wrote the frequency of the source as and the frequency of the receiver as . This last formula is called the relativistic Doppler shift.
Now, we are going to introduce a very important object in electromagnetism: the electric charge and the electric current. We are going to make an analogy with the momenergy . The cuadrivector electric current is something very similar:
where is the electric current density, and is the charge velocity. Moreover, and where is the electric charge and is the electric charge density number, i.e., the number of “elementary” charges in certain volume. Indeed, we can identify the components of such a cuadrivector:
. We can make some interesting observations. Suppose certain rest frame S where we have , i.e., a frame with equilibred charges , and suppose we move with the relative velocity of the electron (or negative charge) observer. Then and , while the other components are . Then, the charge density current transforms as follows:
1st. Length contraction implies that the charge density increases by a gamma factor, i.e., .
2nd. The crystal lattice “hole” velocity in the primed frame implies the existence in that frame of a current density .
3rd. The existence of charges in motion when seen from an inertial frame (boosted from a rest reference S) implies that in a moving reference frame electric fields are not alone but with magnetic fields. From this perspective, magnetic fields are associated to the existence of moving charges. That is, electric fields and magnetic fields are intimately connected and they are caused by static and moving charges, as we do know from classical non-relativistic physics.
Remember now the general expression of the FORPOWER tetravector, or Power-Force tetravector, in SR:
and using the metric, with the mainly plus convention, we get the covariant componets for the power-force tetravector:
We define the Lorentz force as the sum of the electric and magnetic forces
Noting that , the Power-Force tetravector for the Lorentz electromagnetic force reads:
And now, we realize that we can understand the electromagnetic force in terms of a tensor (1,1), i.e., a matrix, if we write:
where the components of the (1,1) tensor can be read:
We can lower the indices with the metric in order to have a more “natural” equation and to read the symmetry of the electromagnetic tensor (note that we can not study symmetries with indices covariant and contravariant),
Please, note that . Focusing on the components of the electromagnetic tensor as a tensor type (1,1), we have seen that under Lorentz transformations its components change as under a boost with in such a case. So, we write:
From this equation we deduce that:
Example: In the S-frame we have the fields and . The Coulomb force is and the Lorentz force is . How are these fields seen from the S’-frame? It is easy using the above transformations. We obtain that
, , ,
Surprinsingly, or not, the S’-observer sees a boosted electric field (non null!), a boosted magnetic field, a boosted non-null Coulomb force and a null Lorentz force!
We can generalize the above transformations to the case of a general velocity in 3d-space
The last equal in the last two equations is due to the orthogonality of the position vector to the velocity in 3d space due to the cross product. From these equations, we easily obtain:
and similarly with the magnetic field. The final tranformations we obtain are:
In the limit where or , we get that
There are two invariants for electromagnetic fields:
It can be checked that
under Lorentz transformations. It is obvious since, up to a multiplicative constant,
and where we have defined the dual electromagnetic field as
or if we write it in components ( duality sends to and to )
We can guess some consequences for the electromagnetic invariants:
1st. If , then and thus in every frame! This fact is important, since it shows that plane waves are orthogonal in any frame in SR. It is also the case of electromagnetic radiation.
2nd. As can be in the non-orthogonal case either positive or negative. If is positive, then it will be positive in any frame and similarly in the negative case. Morevoer, a transformation into a frame with (null electric field) and/or (null magnetic field) is impossible. That is, if a Lorentz transformation of the electric field or the magnetic field turns it to zero, it means that the electric field and magnetic field are orthogonal.
3rd. If E=cB, i.e., if , then it is valid in every frame.
4th. If there is a electric field but there is no magnetic field B in S, a Lorentz transformation to a pure B’ in S’ is impossible and viceversa.
5th. If the electric field is such that or , then they can be turned in a pure electric or magnetic frame only if the electric field and the magnetic field are orthogonal.
6th. There is a trick to remember the two invariants. It is due to Riemann. We can build the hexadimensional vector( six-vector, or sixtor) and complex valued entity
The two invariants are easily obtained squaring F:
We can introduce now a vector potencial tetravector:
This tetravector is also called gauge field. We can write the Maxwell tensor in terms of this field:
It can be easily probed that, up to a multiplicative constant in front of the electric current tetravector, the first set of Maxwell equations are:
The second set of Maxwell equations (sometimes called Bianchi identities) can be written as follows:
The Maxwell equations are invariant under the gauge transformations in spacetime:
where the potential tetravector and the function are arbitrary functions of the spacetime.
Some elections of gauge are common in the solution of electromagnetic problems:
A) Lorentz gauge:
B) Coulomb gauge:
C) Temporal gauge:
If we use the Lorentz gauge, and the Maxwell equations without sources, we deduce that the vector potential components satisfy the wave equation, i.e.,
Finally, let me point out an important thing about Maxwell equations. Specifically, about its invariance group. It is known that Maxwell equations are invariant under Lorentz transformations, and it was the guide Einstein used to extend galilean relativity to the case of electromagnetic fields, enlarging the mechanical concepts. But, the larger group leaving invariant the Maxwell equation’s invariant is not the Lorentz group but the conformal group. But it is another story unrelated to this post.