One way to study the properties of electromagnetism over vast distances is to investigate the polarization characteristics of electromagnetic waves that have traveled through large regions of the universe. As an electromagnetic wave moves forward in a straight line through space, its electric field oscillates perpendicularly to the wave's line of travel. Normally, such an electromagnetic wave is unpolarized, that is, the electric field oscillates in all possible directions perpendicular to the line of propagation of the wave. However, sometimes the wave might be "plane-polarized," which means that its electric field - in addition to oscillating perpendicularly to the line of travel of the wave - oscillates predominantly within a fixed plane that contains the wave's line of travel. This plane is called the polarization plane of the radiation.
In the cosmos, there are many galaxies that emit so-called "electromagnetic synchrotron radiation." This radiation is highly plane-polarized. Such radiation is produced by a galaxy as charged particles are moving at relativistic speeds in circular orbits within the galaxy. This type of motion causes a charged particle to emit a highly plane-polarized electromagnetic wave. In its journey through the cosmic expanse, such a plane-polarized wave passes through localized regions of space that are filled with magnetized plasmas of charged particles, like ions and electrons. The interaction between a magnetized plasma and the plane-polarized wave produces a rotation of the polarization plane of the wave. This rotation is called "Faraday rotation," and is a well-understood physical process.
The handedness and strength of the Faraday polarization rotation depend on the orientation and strength of the magnetic field in the plasma, the plasma density, and the wavelength of the wave. There will always be a component of the magnetic field in the plasma that is parallel to the wave's line of travel. If this component points in the same direction as the propagation direction of the wave, the rotation of the wave's polarization plane will be counterclockwise, as observed from a point on the wave's line of travel where the wave is approaching you. If the magnetic field component points oppositely to the wave's propagation direction, the Faraday polarization rotation is clockwise. The magnitude of the polarization rotation depends on the magnitude of the magnetic field component along the wave's line of propagation, the density of charged particles making up the plasma, and on the wavelength of the wave.
Since 1994, John Ralston and I have studied data published by several independent research groups on the plane-polarization of radio waves emitted by synchrotron galaxies. Since Faraday rotation depends on the frequency of the wave, one can deduce how much of a wave's polarization rotation is caused by the Faraday effect. In this way, we removed the part of the polarization rotation in the data that was Faraday rotation. Surprisingly, we found that a wave's polarization plane undergoes an additional rotation that is very different from Faraday rotation.
The startling observation we made was that the additional polarization rotation is anisotropic in nature, as it depends systematically on the direction that the plane-polarized electromagnetic wave moves through space. This global, anisotropic dependency of the polarization rotation revealed itself as we systematically searched through the totality of all directions on the sky as seen from Earth. The directional dependency of the effect is analogous to that of a so-called "anisotropic," or "birefringent," crystal interacting with polarized light passing through it. Such a crystal influences the polarization of light that traverses it in a way that depends on the straight-line direction that the light takes through the crystal.
In particular, we found that the rate of rotation of the polarization plane caused by the new effect depends on the angle between the direction of travel of the polarized wave and a fixed direction in space, pointing approximately toward the constellation Sextans from Earth. The more parallel the direction of straight-line travel of the wave is with this fixed direction, the greater the rotation of the polarization plane of the wave. The amount of polarization rotation is also proportional to the distance of travel of the wave. These are the only two dependencies of the rotation.
The curious, anisotropic effect is illustrated in the diagram above. In this diagram, Earth is at the center, and the direction toward Sextans is represented by a red "anisotropy axis." The axis extends from Earth toward Sextans in one direction, and toward the constellation Aquila in the opposite direction. A plane-polarized radio wave emitted by Galaxy A (green) travels in a straight line toward Earth in a direction almost parallel to the anisotropy axis (red). On the other hand, a plane-polarized radio wave emitted by Galaxy B (blue) approaches Earth in a direction almost perpendicular to the anisotropy axis.
As the two waves propagate along straight lines through space, their planes of polarization rotate around those lines, as represented by the green and blue helices. The distances of travel are the same for both waves, but the wave traveling nearly parallel to the anisotropy direction (green wave) has its polarization plane rotated more than the wave traveling in a direction nearly perpendicular to the anisotropy direction (blue wave). In general, we find that the polarization rotation increases systematically as a wave's direction of travel approaches that of the fixed anisotropy direction (red line). For illustrative purposes, the rotation effect in this diagram is exaggerated. The actual effect is extremely tiny: we find that, on the average, one full revolution of the polarization plane is completed after the wave has voyaged for about ten billion years.
It is important to note that the anisotropy axis running through Aquila, Earth and Sextans, as shown in the figure, only represents a direction, or, in the vernacular of Mathematics, a vector, in space. Any other axis - possibly vastly remote from Earth, Sextans and Aquila - parallel to the anisotropy axis shown here, will suffice in defining the anisotropy vector. No particular location in space, like the location of Earth for example, is relevant - only directions are relevant.
As in any analysis of experimental data, analysis of the synchrotron radiation data pinpointed only approximately the orientation of the anisotropy axis. We found that the data strongly indicated that the anisotropy axis lies within an "anisotropy cone" that has its vertex at Earth, its central axis pointing from Earth to Sextans, and its surface making a 20 degree angle with the central axis. The data provided no support for an anisotropy vector pointing anywhere outside this cone. In the opposite direction, from Earth to Aquila, the same axis is confined within a similar cone, so that the anisotropy cone is really a "double cone."
In the figure above, the double anisotropy cone is shown in red, positioned with its vertex at the Blue Planet, at the center of the figure, and opening up toward the constellation Sextans in one direction, and toward the constellation Aquila in the opposite direction. Our data consisted of 160 radio galaxies, shown as yellow dots. The most distant galaxies in the data are about 7 billion light years away.
In a curious way, the anisotropy direction reveals itself as that orientation of the needle of a cosmic compass around which the polarization plane of electromagnetic radiation twists the most as the radiation journeys through the fabric of space. It is interesting to note that the constellation Sextans stands for the sextant, the ancient navigational instrument by which seafarers would orient themselves. Aquila, by the way, is the messenger from Heaven - the mythological Eagle leading souls to immortality.
Since the new polarization rotation we observe has such a systematic dependence on the direction of travel of the radiation, it is implausible that it is generated by cosmic ions and fields via some mechanism similar to the Faraday effect, or some other effect that depends on physical matter in the Universe. One may therefore surmise that it is Vacuum itself that flaunts a form of electromagnetic anisotropy - similar to the anisotropy exhibited by many crystals.
In the language of the Quantum Field Theory branch of Physics, one can show that the residual polarization rotation can be generated by a coupling of the so-called "electromagnetic field tensor" to a new, four-dimensional vacuum field, whose "spatial part" is the anisotropy vector we discovered. Furthermore, when subjected to coordinate transformations such as "time reversal" and "space inversion," this new field behaves in the same manner that the intrinsic spin of an atom or elementary particle does, when the atom or particle is subjected to such transformations. One may therefore affix some sort of "spin" to the new vacuum field.
However, at this point in time, the question of what is truly underlying the effect we see is as wide open as Space itself. In nature, there are many experimentally verified imperfections, like "parity violations" of kaon decays for example. The fundamental conclusion from our finding is that our world seems to exhibit another special type of imperfection, or asymmetry, called "anisotropy." Nature never ceases to amaze us.