  We include the single graviton loop contribution to the linearized Einstein
equation. Explicit results are obtained for one loop corrections to the
propagation of gravitational radiation. Although suppressed by a minuscule
loop-counting parameter, these corrections are enhanced by the square of the
number of inflationary e-foldings. One consequence is that perturbation theory
breaks down for a very long epoch of primordial inflation. Another consequence
is that the one loop correction to the tensor power spectrum might be
observable, in the far future, after the full development of 21cm cosmology.
