**9. Outlook**

62 Trends in Electromagnetism – From Fundamentals to Applications

θ


(in the direction of ∇*φ*)


Fig. 4. Space contribution to the local polarization rotation angle -- [Σμ=13*φ,* μΔxμ] = |∇*φ*| cos *θ* Δx0. The time contribution is *φ,0* Δx0. The total contribution is (|∇*φ*| cos *θ* + *φ,0*) Δx0. (Δx0 >

In a medium with optical activity, the direction of a linearly polarized beam will rotate as it propagates through the medium. A medium subjected to magnetic field becomes optically

Cosmic polarization rotation is neither dichroism nor birefringence. It is more like optical activity, with the rotation angle independent of wavelength. Conforming to the common

Now we review and compile the constraints of various analyses from CMB polarization

In 2002, DASI microwave interferometer observed the polarization of the cosmic background (Kovac et al., 2002). E-mode polarization is detected with 4.9 σ. The TE correlation of the temperature and E-mode polarization is detected at 95% confidence. This correlation is expected from the Raleigh scattering of radiation. However, with the (pseudo)scalar-photon interaction under discussion, the polarization anisotropy is shifted differently in different directions relative to the temperature anisotropy due to propagation; the correlation will then be downgraded. In 2003, from the first-year data (WMAP1), WMAP found that the polarization and temperature are correlated to more than 10 σ (Bennett *et al*

Further results and analyses of CMB polarization observations came out after 2006. In Table 2, we update our previous compilations (Ni 2008, 2010). Although these results look different at 1 σ level, they are all consistent with null detection and with one another at 2 σ

active and the associated polarization rotation is called Faraday rotation.

usage in optics, one should not call it cosmic birefringence -- *a misnomer*.

2003). This gives a constraint of about 10-1 for *Δφ* (Ni, 2005a, 2005b).

0).

observations.

level.

We have looked at the foundations of electromagnetism in this chapter. For doing this, we have used two approaches. The first one is to formulate a Parametrized Post-Maxwellian framework to include QED corrections and a pseudoscalar photon interaction. We discuss various vacuum birefringence experiments – ongoing and proposed -- to measure these parameters. The second approach is to look at electromagnetism in gravity and various experiments and observations to determine its empirical foundation. We found that the foundation is solid with the only exception of a potentially possible pseudoscalar-photon interaction. We discussed its experimental constraints and look forward to more future experiments.

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