Most of the successful physical theories rely on the constancy of few fundamental quantities ( such as the speed of light , c , the fine-structure constant , \alpha , the proton to electron mass ratio , \mu , etc ) , and constraining the possible time variations of these fundamental quantities is an important step toward a complete physical theory .
Time variation of \alpha can be accurately probed using absorption lines seen in the spectra of distant quasars .
Here , we present the results of a detailed many-multiplet analysis performed on a new sample of Mg ii systems observed in high quality quasar spectra obtained using the Very Large Telescope .
The weighted mean value of the variation in { \bf \alpha } derived from our analysis over the redshift range { \bf 0.4 \leq z \leq 2.3 } is { \bf \Delta \alpha / \alpha } = { \bf ( -0.06 \pm 0.06 ) \times 10 ^ { -5 } } .
The median redshift of our sample ( z \simeq 1.55 ) corresponds to a look-back time of 9.7 Gyr in the most favored cosmological model today .
This gives a 3 \sigma limit , { \bf - 2.5 \times 10 ^ { -16 } ~ { } { yr } ^ { -1 } \leq ( \Delta \alpha / \alpha \Delta t ) \leq + 1 % .2 \times 10 ^ { -16 } ~ { } { yr } ^ { -1 } } , for the time variation of \alpha , that forms the strongest constraint obtained based on high redshift quasar absorption line systems .