  Over 40 years after the calculation of the 1-loop QED corrections to the muon
lifetime, new theoretical developments have made it possible to obtain an
analytic expression for the complete 2-loop QED contributions in the Fermi
theory. The exact result for the effects of virtual and real photons, virtual
electrons, muons and hadrons as well as e+e- pair creation is
  Delta Gamma^(2)=Gamma_0(alpha/pi)^2[(156815/5184)-(1036/27)zeta(2)
  -(895/36)zeta(3)+(67/8)zeta(4)
  +53zeta(2)ln(2)-(0.042+/-0.002)]
  where Gamma_0 is the tree-level width. This eliminates the theoretical error
in the extracted value of the Fermi coupling constant, G_F, which was
previously the source of the dominant uncertainty. The new value is
  G_F=(1.16637 +/- 0.00001) x 10^-5 GeV^-2
  The overall error has been roughly halved and is now entirely experimental.
Several experiments are planned for the next generation of muon lifetime
measurements and these can proceed unhindered by theoretical uncertainties.
