We present evidence for variations in the fine-structure constant from Keck/HIRES spectra of 143 quasar absorption systems over the redshift range 0.2 < z _ { abs } < 4.2 .
This includes 15 new systems , mostly at high- z 
( z _ { abs } > 1.8 ) .
Our most robust estimate is a weighted mean \mbox { { } } \alpha / \alpha = ( -0.57 \pm 0.11 ) \times 10 ^ { -5 } .
We respond to recent criticisms of the many-multiplet method used to extract these constraints .
The most important potential systematic error at low- z is the possibility of very different Mg heavy isotope abundances in the absorption clouds and laboratory : higher abundances of ^ { 25 , 26 } Mg in the absorbers may explain the low- z results .
Approximately equal mixes of ^ { 24 } Mg and ^ { 25 , 26 } Mg are required .
Observations of Galactic stars generally show lower ^ { 25 , 26 } Mg isotope fractions at the low metallicities typifying the absorbers .
Higher values can be achieved with an enhanced population of intermediate mass stars at high redshift , a possibility at odds with observed absorption system element abundances .
At present , all observational evidence is consistent with the varying- \alpha 
results .

Another promising method to search for variation of fundamental constants involves comparing different atomic clocks .
Here we calculate the dependence of nuclear magnetic moments on quark masses and obtain limits on the variation of \alpha and m _ { q } / \Lambda _ { QCD } from recent atomic clock experiments with hyperfine transitions in H , Rb , Cs , Hg ^ { + } 
and an optical transition in Hg ^ { + } .