Measurements of the polarization angle and orientation of cosmological radio sources may be used to search for unusual effects in the propagation of light through the Universe. Recently, Nodland and Ralston [Phys. Rev. Lett. 78, 3043 (1997)] have claimed to find evidence for a redshift- and direction-dependent rotation effect in existing data. We reexamine these data and argue that there is no statistically significant signal present. We are able to place stringent limits on hypothetical chiral interactions of photons propagating through spacetime.
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Carroll et al. (1997) studied this question.
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