Is the Universe Birefringent?
This page contains links to various papers having to do with the
interesting question of whether light propagates isotropically through
Borge Nodland and John Ralston claimed in Physical Review
Letters (78, 3043, 1997) to have detected rotation in the polarization of
radio waves from distant galaxies, as if the Universe were
(sort of, but not really, like a giant piece of
Daniel Eisenstein and I published a brief
comment explaining an error in this paper. We concluded that there is
at this point no demonstrated evidence that this effect is real.
Our comment, which was published in
Physical Review Letters (vol. 79, p. 1957),
is available from this site in either
HTML or PostScript
format. The PostScript format will look nicer and be easier to
read; the HTML is provided for those whose Web browsers don't
know how to handle PostScript.
If you really don't like the HTML version, but your machine won't
let you view the PostScript, you can always download
free PostScript previewer.
If you have trouble getting these files, or if you want more
information, send me e-mail.
Here are some related sources of information:
- Nodland and Ralston's paper is available from
preprint archive as
- In addition to the above paper, Nodland and Ralston
description of their results.
- Dr. Nodland has written a
technical description of their findings.
- Nodland and Ralston have distributed a
detailing why they disagree with our comment.
Our reply to this document is available
from this site, in either
HTML or PostScript
- Ralston and Nodland have written an
the status of their claim.
- Jain and Ralston have performed an analysis
of some different data to search for a correlation between
polarization angle and position on the sky. They claim a statistically
- Sean Carroll and George Field have written a
paper in which they analyze the data set used by
Nodland and Ralston. They find
no statistically significant evidence of birefringence.
Incidentally, Carroll and Field, together with Roman Jackiw, originally
collected this data set and used it to search for effects like
the one Nodland and Ralston claim to have found. See Phys. Rev. D 41:1231
(1990) for details.
- Carroll has written an on-line summary of the key
results of the above paper.
- J.P. Leahy has written a comment in which
high-resolution data on distant radio sources is used to set very
stringent limits on the possible amount of birefringence. According
to Leahy, the maximum possible amount of birefringence is some 30 times lower
than the level claimed by Nodland and Ralston.
- Nodland and
Ralston have responded
to Leahy's comment. Leahy has written a reply to this
response and appended it to his original
paper by Wardle, Perley, and Cohen uses similar techniques
to Leahy's and reaches similarly strong conclusions.
release describing the paper is available from the
National Radio Astronomy Observatory.
Loredo et al. use Bayesian statistical techniques to analyze
the Nodland-Ralston data. They do not detect birefringence; in
fact, they set an upper limit several times smaller than the Nodland-Ralston
- There have been at least seven theoretical papers, by
Obukhov et al.,
Dobado and Maroto,
Ralston et al.,
Mansouri and Nozari
attempting to explain the Nodland-Ralston
effect. (Yes, it is the same Ralston; Nodland is a coauthor.
Incidentally, Obukhov et al.'s model is, contrary
to their assertion, strongly ruled out by observations of the
microwave background. In an act of shameless self-promotion, I
urge you to check out a
paper I wrote
to see why.)
- Ned Wright did some calculations to assess the statistical
significance of the Nodland-Ralston result and concluded that
the effect was not statistically significant.
his conclusions in his
tutorial. (Click on the "old news of the universe" link.)
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