  A detection of the primordial gravitational wave background is considered to
be the ``smoking-gun '' evidence for inflation. While super-horizon waves are
probed with cosmic microwave background (CMB) polarization, the relic
background will be studied with laser interferometers. The long lever arm
spanned by the two techniques improves constraints on the inflationary
potential and validation of consistency relations expected under inflation. If
gravitational waves with a tensor-to-scalar amplitude ratio greater than 0.01
are detected by the CMB, then a direct detection experiment with a sensitivity
consistent with current concept studies should be pursued vigorously. If no
primordial tensors are detected by the CMB, a direct detection experiment to
understand the simplest form of inflation must have a sensitivity improved by
two to three orders of magnitude over current plans.
