Recent detections of high-redshift absorption by both atomic hydrogen and molecular gas in the radio spectra of quasars have provided a powerful tool to measure possible temporal and spatial variations of physical ‘ constants ’ in the Universe .

We have compared the frequency of high-redshift Hydrogen 21 cm absorption with that of associated molecular absorption in two quasars to place new ( 1 sigma ) upper limits on any variation in y \equiv g _ { p } \alpha ^ { 2 } ( where \alpha is the fine structure constant and g _ { p } is the proton g-factor ) of | \Delta y / y| < 5 \times 10 ^ { -6 } at redshifts z = 0.25 and z = 0.68 .
These quasars are separated by a comoving distance of 3000 Mpc ( for H _ { 0 } = 75 \hbox { $ { \thinspace km } { \thinspace s } ^ { -1 } { \thinspace Mpc } ^ { -1 % } $ } and q _ { 0 } = 0 ) .
We also derive limits on the time rates of change of | \dot { g _ { p } } / g _ { p } | < 1 \times 10 ^ { -15 } { \thinspace y } ^ { -1 } and | \dot { \alpha } / \alpha| < 5 \times 10 ^ { -16 } { \thinspace y } ^ { -1 } between the present epoch and z = 0.68 .
These limits are more than an order of magnitude smaller than previous results derived from high-redshift measurements .