  Quantum gravity has been so elusive because we have tried to approach it by
two paths which can never meet: quantum mechanics and general relativity. These
contradict each other not only in superdense regimes, but also in the vacuum.
  We explore a straight road to quantum gravity here--the one mandated by
Clifford-algebra covariance. This bridges the gap from microscales--where the
massive Dirac propagator is a sum over null zig-zags--to macroscales--where we
see the energy-momentum current, *T and the resulting Einstein curvature, *G.
For massive particles, *T flows in the "cosmic time" direction,
y^0--centrifugally in an expanding universe.
  Neighboring centrifugal currents of *T present opposite spacetime vorticities
*G to the boundaries of each others' worldtubes, so they advect--i.e. attract,
as we show here by integrating a Spin^c-4 Lagrangian by parts in the spinfluid
regime.
  This boundary integral not only explains why stress-energy *T is the source
for gravitational curvature *G, but also gives a value for the gravitational
constant, kappa(x^0) that depends on the current scale factor of our expanding
Friedmann 3-brane. On the microscopic scale, quantum gravity appears naturally
as the statistical mechanics of null zig-zags of massive particles in
"imaginary time," y^0.

General Relativity and Quantum Cosmology