  It is argued that quantum states of geometry, like those of particles, should
be coherent on light cones of any size. An exact classical solution, the
gravitational shock wave of a relativistic point particle, is used to estimate
gravitational drag from coherent energy flows, and the expected gravitational
effect of virtual transverse vacuum energy fluctuations on surfaces of causal
diamonds. It is proposed that the appropriately spacetime-averaged
gravitational effect of the Standard Model vacuum state leads to the observed
small nonzero value of the cosmological constant, dominated by gravitational
drag of virtual gluonic strings at the strong interaction scale.
