  Murphy et al. (2003a, MNRAS, 345, 609) claim to find evidence of cosmological
variations of the fine structure constant $\alpha$ in the spectra of
intervening QSO absorption line systems. We find that this result is affected
by systematic effects. The $\alpha$ values estimated in individual line systems
depend on the set of atomic transitions used and therefore the quoted
dependence on the cosmic age may reflect the fact that different sets of
transitions are used at different redshifts. A correlation between line shifts
and relative optical depths of the atomic transitions is also present. This
correlation is very tight for a high-redshift subsample and accounts for the
anomalous dispersion of the $\alpha$ values found by Murphy et al. (2003a) in
this subsample. The above correlations are consistent with a scenario in which
gravitational redshift, caused by the gravity of the dark halo, contributes to
the shift in frequency of individual components. Gravitational redshift causes
differential spectral shifts of the same order as magnitude of those measured.
In the presence of line misidentification, these shifts can be interpreted in
terms of a variable $\alpha$. In order to verify the gravitational redshift
hypothesis, a direct access to Murphy et al. (2003a) data, or to a large amount
of new high resolution data, is necessary.
