Poincaré, Einstein, and Poincaré with Decoherence

Arthur Wightman in 2002. I was in his class on the Lorentz group in 1960,
and I learned this twobytwo representation from him. 
 People do not like to do Lorentz transformations because their fourbyfour
transformation matrixes are two clumsy for them. Things could become quite
different when they can be represented by twobytwo matrices. Then then
become parts of sentences.
I had my first exposure to the twobytwo representation while I was in Wightman's class on the Lorentz group in 1960 when I was a secondyear graduate student at Princeton. Here is the note I took on the twobytwo representation of the Minkowski fourvector
Later, I had to study this representation more thoroughly from Naimark's book on Linear Representations of the Lorentz Group, when I was writing a book on the Theory and Applications of the Poincaré Group, with Marilyn Noz. since I cannot find earlier references than Naimark's work, I choose to call this twobytwo formalism Naimark representation.
 This Naimark representation is not only useful in special relativity, but
also in other areas of physics, including Stokes parameters,
Poincaré sphere,
density Matrix, Feynman's rest of the universe. I am still looking for other
places where this representation can play its role.
 In this Naimark, the spacetime four vector takes
the form, and its Lorentz
transformation can be
written as
 Here is the explicit expression for this
transformation.
This G matrix is unimodular with six independent parameters. It is possible to translate this twobytwo matrix into a corresponding fourbyfour transformation matrix applicable to the Minkowskian fourvector.
Click here to see how.
However, for all practical purposes, it is enough to use four transformation matrices and their fourbyfour counterparts.  With this Naimark representation, we can clearly see what Poincaré did,
and what Einstein did for the formulation of special relativity.
Click here for the story.
In 1939, Wigner considered subsets of the G matrix which leaves a given momentum invariant. They are different for massive, massless, and superluminal particles. These subgroups are called Wigner's little groups. As you probably know, this is my favorite subject. Wigner used to become very happy whenever I talked about this subject.
Not many particle physicists seem to know that Wigner's little groups dictate internal spacetime symmetries of elementary particles. This is particularly true for those who claim to be studying the internal spacetime symmetries.
It may be easier to explain this aspect of Wigner's little group if I use the Naimark representation. Click here for an explanation.
Perhaps, it could be even easier if we use this formalism for another subject of physics. In polarization optics, we use Jones vectors and Stokes parameters and their transformation matrices.
Let us study Polarization optics with the Naimark representation of the Lorentz group.
 The Jones vector consists of the x and y components of electric
field for a light wave propagating along the z direction. These electric
field component
can have different phases and they can be changed. The electric fields can be
rotated around the direction of the wave propagation. In traditional textbooks,
the polarizer matrix is discussed. However, it is an extreme case of the attenuation
matrix with two different attenuation rates. Thus, this polarizer matrix can be
replaced with a squeeze matrix.
Click here for the formulas.
Click on the figure to expand it.  Stokes parameters. The Jones vector with its two components cannot address
the issue of whether two orthogonal electric fields are coherent with each other.
This is the reason why we need the twobytwo coherency matrix. Its offdiagonal
elements measure the degree of coherence. Since the parameter for this is
necessarily smaller than one and greater than zero, we choose this number to
be cos\chi, and call the angle \chi the coherence angle. The
degree of decoherence is zero when \chi = 0, and is maximum when
\chi = 90^{o} .
However, the determinant of the coherency matrix remains invariant under these transformations, and the value of the determinant is

(ab)^{2} (sin\chi)^{2} .
This quantity is invariant under all available transformations in the Lorentz group, and is therefore a Lorentzinvariant quantity. As this table indicates, the Lorentzinvariant quantity in the mass of the particle in spacetime physics. Thus, this decoherence parameter occupies a very important place from the mathematical point of view.
 We can now place the
four Stokes parameters into the coherency matrix, as we did
for the spacetime coordinate into the Naimark representation of the Lorentz
group. These parameters can be computed from the elements of the coherency
matrix.
The radius of the Poincaré sphere takes a value between its maximum and it minimum, depending on the decoherence parameter.

S_{1}, S_{2}, and S_{3 }

R = [S_{1}^{2} + S_{2}^{2} +
S_{3}^{2}]^{1/2}
This radius takes its maximum value of S_{0} when the decoherence angle is zero, and takes its minimum value when the angle is 90^{o}. This is what the Poincaré sphere is all about.
However, if we include Lorentz boosts applicable to the twobytwo coherency matrix, all these variables are shaken up. However, its determinant

S_{0}^{2}  R^{2} =
(ab)^{2} (sin\chi)^{2}
 The twobytwo coherency matrix can also serve as the density matrix, if it is
sp normalised that its trace is one. I also learned this while I was in
Wightman's class in 1960. See the
1960 Les Houches Lectures. Since the coherency matrix is
Hermitian, it can be diagonalized. It is then straightforward to compute
the entropy. The entropy depends only on the decoherence angle, and
therefore invariant under Lorentz transformations on the coherency matrix.
Here are the formulas.
 Since this density matrix is determined by (sin\chi)^{2} , we
can consider another density matrix with (cos\chi)^{2 } and
the corresponding Poincaré sphere. It is clear that the entropy increases
in the first Poincaré sphere, the entropy for the second
Poincaré sphere should decrease. Thus, these two Poincaré
spheres could lead to another example
of Feynman's rest of the universe.
With Sibel Baskal, I published a paper on possible enlargement of spacetime group to accommodate this aspect of Feynman's rest of the universe. You may click here for the paper. I like to study more. It is an interesting subject.
 If you are not accustomed to read physics from webpages, you can go to a written
article on this subject.
Click here. This paper
is based on my talk presented at the Fedorov Memorial Symposium: International
Conference "Spins and Photonic Beams at Interface," dedicated to the 100th
anniversary of F.I.Fedorov (Minsk, Belarus, 2011). I would like to thank Professor
Sergei Kilin for inviting me to this conference. My way of thinking him is to
make a webpage for him and his city.
It is also my pleasure to acknowledge what I learned from Arthur Wightman while I was a student at Princeton. He was not my thesis advisor, but he has been kind and helpful to me throughout my professional life. I mentioned Wigner and Wheeler many times before, but my thesis advisor was Sam Treiman. Here is my photo with him taken in 1987. Since I gained so much from Princeton, I maintain a webpage for the campus of the University. This page seems to be popular among those high school students who want to go to Princeton for their undergraduate education.
Y. S, Kim (March 2012)
copyright@2010 by Y. S. Kim, unless otherwise specified. The image of Feynman is from the public domain.
Click here for his home page.