David McMahon is a physicist at Los Alamos interested in exotic propulsion who has written a very useful series of self-study advanced physics books. The one I use here is his "Relativity, DeMystified". His solution 9.3 for gravity-free Minkowski space-time using spherical-polar coordinates shows that for that particular choice, the Penrose-Rindler null tetrads are

l = (cdt +dr)/2^1/2

for the retro-causal "destiny" advanced light ray propagating positive energy backwards in time along the past light cone.

n = (cdt - dr)/2^1/2

for the usual causal "history" retarded light ray propagating positive energy forward in time along the future light cone.

The remaining two complex null tetrads are

m = (rd(theta) + irsin(theta)d(phi)/2^1/2

and its complex conjugate m*

This can be generalized for the static LNIFs of the Schwarzschild black hole outside the event horizon

l' = [(1-rs/r)^1/2cdt + dr/(1 - rs/r)^1/2]/2^1/2 etc

g00 = -1/grr = 1 - rs/r

as well as the dark energy observer-dependent de Sitter accelerating universe solution with us at r = 0.

l" = [(1 - Lambda r^2)^1/2cdt + dr/(1 - Lambda r^2)^1/2]/2^1/2

goo = - 1/grr = 1 - Lambda r^2

Lambda = Einstein's cosmological constant = 1/Area of retro-causal future event horizon

~ dark energy density

It From Qubit: Wheeler-Feynman Null Tetrad Gravity

Jack Sarfatti

Local observers are defined by orthonormal “non-holonomic” (aka “non-coordinate”) tetrad gravity fields (Cartan’s “moving frames”). The tetrads are spin 1 vector fields under the 6-parameter homogeneous Lorentz group SO1,3 of Einstein’s 1905 special relativity. You can think of the tetrad gravity fields as the square roots of Einstein’s 1916 spin 2 metric tensor gravity fields. We will see that we must also allow for spin 0 and spin 1 gravity because the spin 1 tetrads, in turn, are Einstein-Podolsky-Rosen entangled quantum states of pairs of 2-component Penrose-Rindler qubits in the quantum pre-geometry.

to be continued (work in progress)