Metrology studies and time variation of fundamental constants

The issue

The issue of a possible variability of fundamental physical constants has been put on the agenda of contemporary physics with recent claims on evidence for a variation of the fine structure constant alpha, based on comparisons of laboratory data on atomic multiplet structures and the observations of the same features in the absorption spectra of quasars. Webb et al. [Phys. Rev. Lett. 87, 091301 (2001)] find a 4 sigma indication for a change in alpha for observations on quasars on the northern hemisphere. Below the results from the Sydney-based group are graphically displayed [from, J. Webb, Physics World, April 2003].

In contrast other groups [Phys. Rev. Lett. 92, 121302 (2004)] recently reported an essential null result from observations on the southern hemisphere.

In the literature there is a continuing debate on the appropriateness of searching for temporal variations in physical law, and whether it is most suitable to search for variations of dimensionless constants. For this discussion we refer to a paper by Duff. There are two dimensionless constants that are currently subject in searches for changes:
1) the fine structure constant alpha, which is the fundamental constant governing the strength of the electromagnetic interaction, and
2) the proton-to-electrom mass ratio Mp/me, which is dimensionless, although not really a fundamental constant: in its value of 1836.15267261 (85) [see: P. J. Mohr and B. N. Taylor, Rev. Mod. Phys. 77, 1 (2005)] various properties of the nuclear, weak and electromagnetic force are contained.
As was explained in the work by Flambaum and coworkers [Phys. Rev. Lett. 82, 888 (1999)] all atomic (and ionic) spectral lines depend in a certain way on the fine structure constant; its specific dependence can be expressed in terms of a parameter q, the value of which can be derived in relativistic atomic structure calculations. Sensitivities to a change in alpha are displyed in the figure below for a number of spectral lines in various atoms and ions.

For comparisons with quasar spectral data spectral accuracies on the order of 10-7 or better are required but for many lines these accuracies have not been reached yet in laboratory studies. Berengut et al.(Sydney) have produced a list of lines to be measured more accurately in order to facilitate searches for alpha variation. One of our aims is to measure some of the relevant missing lines to an accuracy, such that their laboratory frequencies can be considered exact for the purpose of comparing with quasar spectra.
Our group is involved in the search for temporal variations of these constants by performing accurate laboratory spectroscopy of atoms and molecules. There are two strategies to probe a possible variation of a fundamental constant:
1) Via reasonable accurate laboratory spectral measurements in the modern epoch and comparing the results with spectra that have travelled to us over long times and long distances; such spectra exist in the light of quasi-stellar obejcts. by this means it is possible to compare the physical laws and constants of today with those of 12 Billion years ago (for z=3 redshifts).
2) By making comparisons of laboratory spectra recorded with time intervals a several years. In the latter case a much higher precision in the measurements is required. This aim is currently being pursued by several groups specifically for determining a temporal variation of the fine structure constant alpha.

See further:

Laboratory studies for probing a variation of alpha

Secondly we are involved in measurement of accurate line positions in the dipole allowed spectrum of the H2 molecule. The Lyman and Werner band systems are the strongest absorption features in the spectrum of hydrogen and these lie in the range of the extreme ultraviolet (90-110 nm), at least under laboratory conditions (z=0). Based on these studies we try to estimate:

The possibility of a temporal change in the proton-electron mass ratio: Mp/me