Tensor[RicciSpinor] - compute the spinor form of the trace-free Ricci tensor
σ - a solder form
R - (optional) the Ricci tensor for the metric determined by the solder form σ
Let g be a metric tensor. The trace-free Ricci tensor for g is defined by Tij=Rij−14gijS , where Rij is the Ricci tensor and S=gijRij the Ricci scalar of g.
The command RicciSpinor(sσ ) first computes the metric tensor g defined by the solder form s. The trace-free Ricci tensor T for g is then computed and converted, using the solder form σ to a rank 4 covariant spinor with index type TABA'B' . (See convert/DGspinor.) Finally, a scalar factor of −12 is introduced according to standard conventions. See Stewart, page 85.
If the Ricci tensor R for the metric g has been previously computed, then the Ricci spinor will be computed more quickly using the second calling sequence RicciSpinor(σ, R).
This command is part of the DifferentialGeometry:-Tensor package, and so can be used in the form RicciSpinor(..) only after executing the commands with(DifferentialGeometry); with(Tensor); in that order. It can always be used in the long form DifferentialGeometry:-Tensor:-RicciSpinor.
First create a vector bundle M with base coordinatest,x,y,z and fiber coordinates z1,z2,w1,w2.
frame name: M
Define a metric g on the base. For this example we use the Godel metric. (See (12.26) in Exact Solutions to Einstein's Field Equations.) Note that we have adjusted the metric to conform to the signature conventions 1,−1,−1,−1 used by the spinor formalism in the DifferentialGeometry package. See SpacetimeConventions.
g ≔ evalDG⁡dt &t dt+ⅇx⁢dt &s dz−dx &t dx−dy &t dy+1⁢ⅇ2⁢x⁢dz &t dz2