  Multidimensional cosmologies allow for variations of fundamental physical
constants over the course of cosmological evolution, and different versions of
the theories predict different time dependences. In particular, such variations
could manifest themselves as changes of the proton-to-electron mass ratio
\mu=m_p/m_e over the period of ~ 10^{10} years since the moment of formation of
high-redshift QSO spectra. Here we analyze a new, high-resolution spectrum of
the z=2.81080 molecular hydrogen absorption system toward the quasar PKS
0528-250 to derive a new observational constraint to the time-averaged
variation rate of the proton-to-electron mass ratio. We find |\dot{\mu} / \mu|
< 1.5 \times 10^{-14}/year, which is much tighter than previously measured
limits.
