Tensor[BelRobinson] - calculate the Bel-Robinson tensor
Calling Sequences
BelRobinson(g, W, indexlist)
Parameters
g - a metric tensor on a 4-dimensional manifold
W - (optional) the Weyl tensor of the metric g
indexlist - (optional) the keyword argument indexlist = ind, where ind is a list of 4 index types "con" or "cov"
Description
Examples
See Also
The Bel-Robinson tensor Bijhk is a covariant rank 4 tensor defined in terms of the Weyl tensor Wijhk on a 4-dimensional manifold by (see, for example, Penrose and Rindler Vol. 1)
Bijhk=14WilhmWj k l m−12gijWlmhn+gilWmjhn+ gimWjlhnW klm n.
The Bel-Robinson tensor is totally symmetric: Bijhk=Bjihk=Bhjik=Bkjhi . The Bel-Robinson tensor is trace-free: gijBijhk=0. If gij is an Einstein metric, that is, Rij=Λgij (where Rij is the Ricci tensor for the metric gij and Λ is a constant), then the covariant divergence of Bel-Robinson vanishes: gil ∇l Bijhk=0. Here ∇l denotes the covariant derivative with respect to the Christoffel connection for gij.
The keyword argument indexlist = ind allows the user to specify the index structure for the Bel-Robinson tensor. For example, with indexlist = ["con", "con", "con", "con"], the contravariant form Bijhk is returned. The default output is the purely covariant form (as above).
This command is part of the DifferentialGeometry:-Tensor package, and so can be used in the form BelRobinson(...) only after executing the commands with(DifferentialGeometry); with(Tensor); in that order. It can always be used in the long form DifferentialGeometry:-Tensor:-BelRobinson.
with⁡DifferentialGeometry:with⁡Tensor:
Example 1.
First create a 4-dimensional manifold M and define a metric gon M. The metric shown below is a homogenous Einstein metric (see (12.34) in Stephani, Kramer et al).
DGsetup⁡x,y,z,u,M
frame name: M
g ≔ evalDG⁡ⅇz⁢dx &t dx+ⅇ−2⁢z⁢dy &t dy+dx &s du−3⁢dz &t dzΛ
g:=_DG⁡tensor,M,cov_bas,cov_bas,,1,1,ⅇz,1,4,ⅇ−2⁢z2,2,2,ⅇ−2⁢z,3,3,−3Λ,4,1,ⅇ−2⁢z2,_DG⁡tensor,M,cov_bas,cov_bas,,1,1,ⅇz,1,4,ⅇ−2⁢z2,2,2,ⅇ−2⁢z,3,3,−3Λ,4,1,ⅇ−2⁢z2,_DG⁡tensor,M,cov_bas,cov_bas,,1,1,ⅇz,1,4,ⅇ−2⁢z2,2,2,ⅇ−2⁢z,3,3,−3Λ,4,1,ⅇ−2⁢z2,_DG⁡tensor,M,cov_bas,cov_bas,,1,1,ⅇz,1,4,ⅇ−2⁢z2,2,2,ⅇ−2⁢z,3,3,−3Λ,4,1,ⅇ−2⁢z2
Calculate the Bel-Robinson tensor for the metric g. The result is clearly a symmetric tensor.
B ≔ BelRobinson⁡g
B:=_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,1,1,1,Λ2⁢ⅇ2⁢z4,_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,1,1,1,Λ2⁢ⅇ2⁢z4,_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,1,1,1,Λ2⁢ⅇ2⁢z4,_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,1,1,1,Λ2⁢ⅇ2⁢z4
Use the optional keyword argument indexlist to calculate the contravariant form of the Bel-Robinson tensor.
B1 ≔ BelRobinson⁡g,indexlist=con,con,con,con
B1:=_DG⁡tensor,M,con_bas,con_bas,con_bas,con_bas,,4,4,4,4,4⁢ⅇ10⁢z⁢Λ2,_DG⁡tensor,M,con_bas,con_bas,con_bas,con_bas,,4,4,4,4,4⁢ⅇ10⁢z⁢Λ2,_DG⁡tensor,M,con_bas,con_bas,con_bas,con_bas,,4,4,4,4,4⁢ⅇ10⁢z⁢Λ2,_DG⁡tensor,M,con_bas,con_bas,con_bas,con_bas,,4,4,4,4,4⁢ⅇ10⁢z⁢Λ2
The tensor B is trace-free.
h ≔ InverseMetric⁡g
h:=_DG⁡tensor,M,con_bas,con_bas,,1,4,2⁢ⅇ2⁢z,2,2,ⅇ2⁢z,3,3,−Λ3,4,1,2⁢ⅇ2⁢z,4,4,−4⁢ⅇ5⁢z,_DG⁡tensor,M,con_bas,con_bas,,1,4,2⁢ⅇ2⁢z,2,2,ⅇ2⁢z,3,3,−Λ3,4,1,2⁢ⅇ2⁢z,4,4,−4⁢ⅇ5⁢z,_DG⁡tensor,M,con_bas,con_bas,,1,4,2⁢ⅇ2⁢z,2,2,ⅇ2⁢z,3,3,−Λ3,4,1,2⁢ⅇ2⁢z,4,4,−4⁢ⅇ5⁢z,_DG⁡tensor,M,con_bas,con_bas,,1,4,2⁢ⅇ2⁢z,2,2,ⅇ2⁢z,3,3,−Λ3,4,1,2⁢ⅇ2⁢z,4,4,−4⁢ⅇ5⁢z
ContractIndices⁡h,B,1,1,2,2
_DG⁡tensor,M,cov_bas,cov_bas,,1,1,0,_DG⁡tensor,M,cov_bas,cov_bas,,1,1,0,_DG⁡tensor,M,cov_bas,cov_bas,,1,1,0,_DG⁡tensor,M,cov_bas,cov_bas,,1,1,0
The covariant divergence of the tensor B1 vanishes. To check this, first calculate the Christoffel connection C for the metric g and then calculate the covariant derivative of B1.
C ≔ Christoffel⁡g
C:=_DG⁡connection,M,con_bas,cov_bas,cov_bas,,1,1,3,−1,1,3,1,−1,2,2,3,−1,2,3,2,−1,3,1,1,Λ⁢ⅇz6,3,1,4,−Λ⁢ⅇ−2⁢z6,3,2,2,−Λ⁢ⅇ−2⁢z3,3,4,1,−Λ⁢ⅇ−2⁢z6,4,1,3,3⁢ⅇ3⁢z,4,3,1,3⁢ⅇ3⁢z,4,3,4,−1,4,4,3,−1,_DG⁡connection,M,con_bas,cov_bas,cov_bas,,1,1,3,−1,1,3,1,−1,2,2,3,−1,2,3,2,−1,3,1,1,Λ⁢ⅇz6,3,1,4,−Λ⁢ⅇ−2⁢z6,3,2,2,−Λ⁢ⅇ−2⁢z3,3,4,1,−Λ⁢ⅇ−2⁢z6,4,1,3,3⁢ⅇ3⁢z,4,3,1,3⁢ⅇ3⁢z,4,3,4,−1,4,4,3,−1,_DG⁡connection,M,con_bas,cov_bas,cov_bas,,1,1,3,−1,1,3,1,−1,2,2,3,−1,2,3,2,−1,3,1,1,Λ⁢ⅇz6,3,1,4,−Λ⁢ⅇ−2⁢z6,3,2,2,−Λ⁢ⅇ−2⁢z3,3,4,1,−Λ⁢ⅇ−2⁢z6,4,1,3,3⁢ⅇ3⁢z,4,3,1,3⁢ⅇ3⁢z,4,3,4,−1,4,4,3,−1,_DG⁡connection,M,con_bas,cov_bas,cov_bas,,1,1,3,−1,1,3,1,−1,2,2,3,−1,2,3,2,−1,3,1,1,Λ⁢ⅇz6,3,1,4,−Λ⁢ⅇ−2⁢z6,3,2,2,−Λ⁢ⅇ−2⁢z3,3,4,1,−Λ⁢ⅇ−2⁢z6,4,1,3,3⁢ⅇ3⁢z,4,3,1,3⁢ⅇ3⁢z,4,3,4,−1,4,4,3,−1
nablaB1 ≔ CovariantDerivative⁡B1,C
nablaB1:=_DG⁡tensor,M,con_bas,con_bas,con_bas,con_bas,cov_bas,,3,4,4,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,3,4,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,3,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,4,3,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,4,4,3,24⁢ⅇ10⁢z⁢Λ2,_DG⁡tensor,M,con_bas,con_bas,con_bas,con_bas,cov_bas,,3,4,4,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,3,4,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,3,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,4,3,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,4,4,3,24⁢ⅇ10⁢z⁢Λ2,_DG⁡tensor,M,con_bas,con_bas,con_bas,con_bas,cov_bas,,3,4,4,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,3,4,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,3,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,4,3,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,4,4,3,24⁢ⅇ10⁢z⁢Λ2,_DG⁡tensor,M,con_bas,con_bas,con_bas,con_bas,cov_bas,,3,4,4,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,3,4,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,3,4,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,4,3,1,−2⁢Λ3⁢ⅇ8⁢z3,4,4,4,4,3,24⁢ⅇ10⁢z⁢Λ2
Divergence ≔ ContractIndices⁡nablaB1,1,5
Divergence:=_DG⁡tensor,M,con_bas,con_bas,con_bas,,1,1,1,0,_DG⁡tensor,M,con_bas,con_bas,con_bas,,1,1,1,0,_DG⁡tensor,M,con_bas,con_bas,con_bas,,1,1,1,0,_DG⁡tensor,M,con_bas,con_bas,con_bas,,1,1,1,0
The divergence of the Bel-Robinson tensor is not automatically zero; the divergence vanishes when the metric g is an Einstein metric. To check this, compute the Ricci tensor of g.
R ≔ RicciTensor⁡g
R:=_DG⁡tensor,M,cov_bas,cov_bas,,1,1,Λ⁢ⅇz,1,4,Λ⁢ⅇ−2⁢z2,2,2,Λ⁢ⅇ−2⁢z,3,3,−3,4,1,Λ⁢ⅇ−2⁢z2,_DG⁡tensor,M,cov_bas,cov_bas,,1,1,Λ⁢ⅇz,1,4,Λ⁢ⅇ−2⁢z2,2,2,Λ⁢ⅇ−2⁢z,3,3,−3,4,1,Λ⁢ⅇ−2⁢z2,_DG⁡tensor,M,cov_bas,cov_bas,,1,1,Λ⁢ⅇz,1,4,Λ⁢ⅇ−2⁢z2,2,2,Λ⁢ⅇ−2⁢z,3,3,−3,4,1,Λ⁢ⅇ−2⁢z2,_DG⁡tensor,M,cov_bas,cov_bas,,1,1,Λ⁢ⅇz,1,4,Λ⁢ⅇ−2⁢z2,2,2,Λ⁢ⅇ−2⁢z,3,3,−3,4,1,Λ⁢ⅇ−2⁢z2
evalDG⁡R−Λ⁢g
The Weyl tensor, if already calculated, can be used to quickly compute the Bel-Robinson tensor.
W ≔ WeylTensor⁡g
W:=_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,2,1,2,−Λ⁢ⅇ−z2,1,2,2,1,Λ⁢ⅇ−z2,1,3,1,3,−3⁢ⅇz2,1,3,3,1,3⁢ⅇz2,2,1,1,2,Λ⁢ⅇ−z2,2,1,2,1,−Λ⁢ⅇ−z2,3,1,1,3,3⁢ⅇz2,3,1,3,1,−3⁢ⅇz2,_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,2,1,2,−Λ⁢ⅇ−z2,1,2,2,1,Λ⁢ⅇ−z2,1,3,1,3,−3⁢ⅇz2,1,3,3,1,3⁢ⅇz2,2,1,1,2,Λ⁢ⅇ−z2,2,1,2,1,−Λ⁢ⅇ−z2,3,1,1,3,3⁢ⅇz2,3,1,3,1,−3⁢ⅇz2,_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,2,1,2,−Λ⁢ⅇ−z2,1,2,2,1,Λ⁢ⅇ−z2,1,3,1,3,−3⁢ⅇz2,1,3,3,1,3⁢ⅇz2,2,1,1,2,Λ⁢ⅇ−z2,2,1,2,1,−Λ⁢ⅇ−z2,3,1,1,3,3⁢ⅇz2,3,1,3,1,−3⁢ⅇz2,_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,2,1,2,−Λ⁢ⅇ−z2,1,2,2,1,Λ⁢ⅇ−z2,1,3,1,3,−3⁢ⅇz2,1,3,3,1,3⁢ⅇz2,2,1,1,2,Λ⁢ⅇ−z2,2,1,2,1,−Λ⁢ⅇ−z2,3,1,1,3,3⁢ⅇz2,3,1,3,1,−3⁢ⅇz2
BelRobinson⁡g,W
_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,1,1,1,Λ2⁢ⅇ2⁢z4,_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,1,1,1,Λ2⁢ⅇ2⁢z4,_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,1,1,1,Λ2⁢ⅇ2⁢z4,_DG⁡tensor,M,cov_bas,cov_bas,cov_bas,cov_bas,,1,1,1,1,Λ2⁢ⅇ2⁢z4
DifferentialGeometry, Tensor, Christoffel, CovariantDerivative, CurvatureTensor, RicciTensor, WeylTensor
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