Do the fundamental constants vary in space or time?

4 problems

• Is $\alpha$ drifting today at the $10^{-19}$ per year level?

  Open

  Two clocks whose ticking depends differently on $\alpha$ would slowly drift apart if $\alpha$ changed. So far no drift has been seen at about one part in $10^{18}$ per year.

• Does the proton-to-electron mass ratio vary in time or space?

  Open

  Molecular vibrations depend on the ratio of proton to electron mass, while atomic transitions mostly do not, so comparing them tests whether this ratio changes. Both laboratory molecular clocks and ancient molecular absorption in distant galaxies are used.

• Is there a spatial dipole in $\alpha$ from quasar absorption spectra?

  Contested

  Light from distant quasars passes through gas clouds that absorb at wavelengths set by $\alpha$ at that place and time. Some analyses claimed $\alpha$ differs slightly in opposite directions on the sky, while newer instruments see no change.

• Sensitivity factor $K$ of the $\mathrm{Th}-229$ nuclear transition to $\alpha$

  Partially resolved

  The thorium-229 nucleus has an unusually low-energy excited state that can be reached by a laser, and its frequency may react to changes in $\alpha$ thousands of times more than atomic clocks do. How large that factor is depends on nuclear-structure numbers still poorly known.