Variations in fine structure constant
joe lavelle
about Delta alpha/alpha of -0.7 E-5 at the 4 sigma confidence level.
Is anyone familiar with a theory (or class of theories) that would
predict a variation of this magnitude?
Thanks,
joe lavelle
Jonathan Thornburg
joe lavelle <jlav...@cox.rr.com> wrote:
>In an article in the August 27 PRL, Webb et al. report a variation of
>about Delta alpha/alpha of -0.7 E-5 at the 4 sigma confidence level.
The actual paper can be viewed online from the PRL web pages
http://prl.aps.org/
(cookies required! :( :( ). I believe this particular paper can be
found at
http://link.aps.org/abstract/PRL/v87/e091301
The abstract reads as follows:
We describe the results of a search for time variability of the fine
structure constant alpha using absorption systems in the spectra of
distant quasars. Three large optical data sets and two 21 cm and mm
absorption systems provide four independent samples, spanning ~23%
to 87% of the age of the universe. Each sample yields a smaller alpha
in the past and the optical sample shows a 4 sigma deviation:
delta alpha/alpha = -0.72+/-0.18×10-5 over the redshift range
0.5<z<3.5. We find no systematic effects which can explain our
results. The only potentially significant systematic effects push
delta alpha/alpha towards positive values; i.e., our results would
become more significant were we to correct for them.
This result appears to contradict a fair number of other experiments
which have placed *very* tight bounds (much better than 1e-10 per year)
on any possible time variations in alpha (along with various other
elementary constants). (References given below.) In particular,
the paper by Varshalovich et al (cited below) reaches the opposite
conclusion, that the data show no statistically significant evidence
for a time variation, down to a limit of
|\dot\alpha/\alpha| < (1.4e-14)/yr
My personal opinion is to be somewhat dubious of the Webb et al result,
since so many other experiments have found null results at much higher
sensitivities. But I await with interest comments from those with a
more detailed knowledge of this area (which I don't have).
References to experimental bounds on time variations of alpha (and
other elementary constants):
In article <Dqu5...@murdoch.acc.Virginia.EDU> (3 May 1996),
Steve Carlip (then sjca...@ucdavis.edu), wrote about observations of
# hydrogen hyperfine splitting
# (the famous 21cm transition), Mg II fine structure absorption
# lines, and resonance transition lines, at red shift z=.5 (see
# Wolfe et al., Phys. Rev. Lett. 37 (1976) 179). These results restrict
# the variation of the log of the fine structure constant to less
# than 4x10^-12 per year.
There are also measurements of isotope ratios in a uranium deposit in
Oklo (Gabon) which operated as a natural nuclear reactor about 1.8 billion
years ago (the branching ratios of various nuclear reactions depended
on various nuclear energy levels at that time, which in turn depended
on alpha at that time).
And there is http://arXiv.org/abs/physics/0004062 which reports:
| Testing cosmological variability of fundamental constants
|
| Authors: D. A. Varshalovich, A. Y. Potekhin, A. V. Ivanchik (Ioffe
| Phys.-Tech. Inst., St.Petersburg)
| Comments: 9 pages, 2 figures, 2 tables, LaTeX using aipproc.sty
| (included). In: X-ray and Inner-Shell Processes, R.W. Dunford, D.S.
| Gemmel, E.P. Kanter, B. Kraessig, S.H. Southworth, L. Young (eds.),
| AIP Conf. Proc. (AIP, Melville, 2000) vol. 506, p. 503
| Subj-class: Atomic Physics; General Physics
|
| One of the topical problems of contemporary physics is a possible
| variability of the fundamental constants. Here we consider possible
| variability of two dimensionless constants which are most important
| for calculation of atomic and molecular spectra (in particular, the
| X-ray ones): the fine-structure constant \alpha=e^2/\hbar c and the
| proton-to-electron mass ratio \mu=m_p/m_e. Values of the physical
| constants in the early epochs are estimated directly from
| observations of quasars - the most powerful sources of radiation,
| whose spectra were formed when the Universe was several times
| younger than now. A critical analysis of the available results
| leads to the conclusion that present-day data do not reveal any
| statistically significant evidence for variations of the
| fundamental constants under study. The most reliable upper limits
| to possible variation rates at the 95% confidence level, obtained
| in our work, read:
| |\dot\alpha/\alpha| < (1.4e-14)/yr,
| |\dot\mu/\mu| < (1.5e-14)/yr
| on the average over the last ten billion years.
--
-- Jonathan Thornburg <jth...@thp.univie.ac.at>
Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
Golm, Germany http://www.aei.mpg.de/~jthorn/home.html
"Imagine a restaurant that assigns armed guards to escort your credit-card
to the cash-register and back, then tacks all the carbons to the
employee-bulletin-board, right inside an un-locked back door"
-- Mike Albaugh's metaphor for the "security" of
SSL-encrypted credit card transactions on the web
Robert A. McNees
A time variation of the fine structure constant is one of the many
problems that has to be addressed in theories of Quintessence.
Quintessence postulates that there is a dynamical agent responsible for
the negative-pressure component driving the observed acceleration of the
universe, as opposed to (or in addition to) a static cosmological constant.
Many Quintessence theories make use of a scalar field slowly rolling
down some sort of potential that asymptotes to zero. The "slow roll"
condition places the field in a damped regime where the potential energy
density dominates the kinetic energy density, resulting in a "perfect
fluid" with a negative pressure. Invariably these theories require the
field to be very, very light, which often introduces a fine tuning
problem comparable to the one they sought to solve (i.e. why is the
cosmological constant so small compared to all the supposedly relevant
particle physics scales).
After you introduce this field you need to explain why we haven't seen
it; such a light mass field should produce a noticeable long range force
unless all of its couplings to matter are either very small or of a
special form. For instance, we would expect a scalar to couple to the EM
field. Since the (approximately spatially homogenous) scalar field is
"rolling slowly" it changes only slightly over long periods of time.
Thus, we might expect to see a gradual change in what we measure as the
fine structure constant. The only problem is that in order for such a
change to agree with data we have limiting the time evolution of \alpha
the coupling of the quintessence field to the EM field would need to be
unnaturally small, if not zero.
Since quintessence models present attractive solutions to some important
cosmogical questions a lot of people (including the authors of the paper
you refer to) have gone back to look and see how tightly our data
contrains the change in \alpha over time. Understanding this limit may
help us understand how a hypothetical quintessence couples to normal
matter, which in turn illuminates its role in cosmic evolution and
particle physics.
Luc Bourhis
If you wrote about [1], Magueijo claims his varying speed of light
theory can accomodate Webb et al's result [2].
[1] J.K. Webb et al, "A Search for Time Variation of the Fine Structure
Constant", Phys.Rev.Lett. 82 (1999) 884
[2] John D. Barrow and Joao Magueijo, astro-ph/9907354.
--
Luc J. Bourhis