Optical scalars

Short description: Set of three scalar functions

In general relativity, optical scalars refer to a set of three scalar functions {θ^ (expansion), σ^ (shear) and ω^ (twist/rotation/vorticity)} describing the propagation of a geodesic null congruence.[1][2][3][4][5]

In fact, these three scalars {θ^,σ^,ω^} can be defined for both timelike and null geodesic congruences in an identical spirit, but they are called "optical scalars" only for the null case. Also, it is their tensorial predecessors {θ^h^ab,σ^ab,ω^ab} that are adopted in tensorial equations, while the scalars {θ^,σ^,ω^} mainly show up in equations written in the language of Newman–Penrose formalism.

## Contents

* 1 Definitions: expansion, shear and twist
* 1.1 For geodesic timelike congruences
* 1.2 For geodesic null congruences
* 2 Definitions: optical scalars for null congruences
* 3 Applications in decomposing the propagation equations
* 3.1 For a geodesic timelike congruence
* 3.2 For a geodesic null congruence
* 3.3 For a restricted geodesic null congruence
* 4 Spin coefficients, Raychaudhuri's equation and optical scalars
* 5 See also
* 6 References

## Definitions: expansion, shear and twist

### For geodesic timelike congruences

Denote the tangent vector field of an observer's worldline (in a timelike congruence) as Za, and then one could construct induced "spatial metrics" that

(1)hab=gab+ZaZb,hab=gab+ZaZb,hba=δba+ZaZb,

where hba works as a spatially projecting operator. Use hba to project the coordinate covariant derivative ∇bZa and one obtains the "spatial" auxiliary tensor Bab,

(2)Bab=hachbd∇dZc=∇bZa+AaZb,

where Aa represents the four-acceleration, and Bab is purely spatial in the sense that BabZa=BabZb=0. Specifically for an observer with a geodesic timelike worldline, we have

(3)Aa=0,⇒Bab=∇bZa.

Now decompose Bab into its symmetric and antisymmetric parts θab and ωab,

(4)θab=B(ab),ωab=B[ab].

ωab=B[ab] is trace-free (gabωab=0) while θab has nonzero trace, gabθab=θ. Thus, the symmetric part θab can be further rewritten into its trace and trace-free part,

(5)θab=13θhab+σab.

Hence, all in all we have

(6)Bab=13θhab+σab+ωab,θ=gabθab=gabB(ab),σab=θab−13θhab,ωab=B[ab].

### For geodesic null congruences

Now, consider a geodesic null congruence with tangent vector field ka. Similar to the timelike situation, we also define

(7)B^ab:=∇bka,

which can be decomposed into

(8)B^ab=θ^ab+ω^ab=12θ^h^ab+σ^ab+ω^ab,

where

(9)θ^ab=B^(ab),θ^=h^abB^ab,σ^ab=B^(ab)−12θ^h^ab,ω^ab=B^[ab].

Here, "hatted" quantities are utilized to stress that these quantities for null congruences are two-dimensional as opposed to the three-dimensional timelike case. However, if we only discuss null congruences in a paper, the hats can be omitted for simplicity.

## Definitions: optical scalars for null congruences

The optical scalars {θ^,σ^,ω^}[1][2][3][4][5] come straightforwardly from "scalarization" of the tensors {θ^,σ^ab,ω^ab} in Eq(9).

The expansion of a geodesic null congruence is defined by (where for clearance we will adopt another standard symbol ";" to denote the covariant derivative ∇a)

(10)θ^=12ka;a.

Comparison with the "expansion rates of a null congruence": As shown in the article "Expansion rate of a null congruence", the outgoing and ingoing expansion rates, denoted by θ(ℓ) and θ(n) respectively, are defined by

(A.1)θ(ℓ):=hab∇alb,

(A.2)θ(n):=hab∇anb,

where hab=gab+lanb+nalb represents the induced metric. Also, θ(ℓ) and θ(n) can be calculated via

(A.3)θ(ℓ)=gab∇alb−κ(ℓ),

(A.4)θ(n)=gab∇anb−κ(n),

where κ(ℓ) and κ(n) are respectively the outgoing and ingoing non-affinity coefficients defined by

(A.5)la∇alb=κ(ℓ)lb,

(A.6)na∇anb=κ(n)nb.

Moreover, in the language of Newman–Penrose formalism with the convention {(−,+,+,+);lana=−1,mam¯a=1}, we have

(A.7)θ(l)=−(ρ+ρ¯)=−2Re(ρ),θ(n)=μ+μ¯=2Re(μ),

As we can see, for a geodesic null congruence, the optical scalar θ plays the same role with the expansion rates θ(ℓ) and θ(n). Hence, for a geodesic null congruence, θ will be equal to either θ(ℓ) or θ(n).

The shear of a geodesic null congruence is defined by

(11)σ^2=σ^abσ¯^ab=12gcagdbk(a;b)kc;d−(12ka;a)2=gcagdb12k(a;b)kc;d−θ^2.

The twist of a geodesic null congruence is defined by

(12)ω^2=12k[a;b]ka;b=gcagdbk[a;b]kc;d.

In practice, a geodesic null congruence is usually defined by either its outgoing (ka=la) or ingoing (ka=na) tangent vector field (which are also its null normals). Thus, we obtain two sets of optical scalars {θ^(ℓ),σ^(ℓ),ω^(ℓ)} and {θ^(n),σ^(n),ω^(n)}, which are defined with respect to la and na, respectively.

## Applications in decomposing the propagation equations

### For a geodesic timelike congruence

The propagation (or evolution) of Bab for a geodesic timelike congruence along Zc respects the following equation,

(13)Zc∇cBab=−BbcBac+RcbadZcZd.

Take the trace of Eq(13) by contracting it with gab, and Eq(13) becomes

(14)Zc∇cθ=θ,τ=−13θ2−σabσab+ωabωab−RabZaZb

in terms of the quantities in Eq(6). Moreover, the trace-free, symmetric part of Eq(13) is

(15)Zc∇cσab=−23θσab−σacσbc−ωacωbc+13hab(σcdσcd−ωcdωcd)+CcbadZcZd+12R~ab.

Finally, the antisymmetric component of Eq(13) yields

(16)Zc∇cωab=−23θωab−2σ[bcωa]c.

### For a geodesic null congruence

A (generic) geodesic null congruence obeys the following propagation equation,

(16)kc∇cB^ab=−B^bcB^ac+Rcbadkckd^.

With the definitions summarized in Eq(9), Eq(14) could be rewritten into the following componential equations,

(17)kc∇cθ^=θ^,λ=−12θ^2−σ^abσ^ab+ω^abω^ab−Rcdkckd^,

(18)kc∇cσ^ab=−θ^σ^ab+Ccbadkckd^,

(19)kc∇cω^ab=−θ^ω^ab.

### For a restricted geodesic null congruence

For a geodesic null congruence restricted on a null hypersurface, we have

(20)kc∇cθ=θ^,λ=−12θ^2−σ^abσ^ab−Rcdkckd^+κ(ℓ)θ^,

(21)kc∇cσ^ab=−θ^σ^ab+Ccbadkckd^+κ(ℓ)σ^ab,

(22)kc∇cω^ab=0.

## Spin coefficients, Raychaudhuri's equation and optical scalars

For a better understanding of the previous section, we will briefly review the meanings of relevant NP spin coefficients in depicting null congruences.[1] The tensor form of Raychaudhuri's equation[6] governing null flows reads

(23)ℒℓθ(ℓ)=−12θ(ℓ)2+κ~(ℓ)θ(ℓ)−σabσab+ω~abω~ab−Rablalb,

where κ~(ℓ) is defined such that κ~(ℓ)lb:=la∇alb. The quantities in Raychaudhuri's equation are related with the spin coefficients via

(24)θ(ℓ)=−(ρ+ρ¯)=−2Re(ρ),θ(n)=μ+μ¯=2Re(μ),

(25)σab=−σm¯am¯b−σ¯mamb,

(26)ω~ab=12(ρ−ρ¯)(mam¯b−m¯amb)=Im(ρ)⋅(mam¯b−m¯amb),

where Eq(24) follows directly from h^ab=h^ba=mbm¯a+m¯bma and

(27)θ(ℓ)=h^ba∇alb=mbm¯a∇alb+m¯bma∇alb=mbδ¯lb+m¯bδlb=−(ρ+ρ¯),

(28)θ(n)=h^ba∇anb=m¯bma∇anb+mbm¯a∇anb=m¯bδnb+mbδ¯nb=μ+μ¯.

## See also

* Raychaudhari equation
* Congruence (general relativity)

## References

1. ↑ 1.0 1.1 1.2 Eric Poisson. A Relativist's Toolkit: The Mathematics of Black-Hole Mechanics. Cambridge: Cambridge University Press, 2004. Chapter 2.
2. ↑ 2.0 2.1 Hans Stephani, Dietrich Kramer, Malcolm MacCallum, Cornelius Hoenselaers, Eduard Herlt. Exact Solutions of Einstein's Field Equations. Cambridge: Cambridge University Press, 2003. Chapter 6.
3. ↑ 3.0 3.1 Subrahmanyan Chandrasekhar. The Mathematical Theory of Black Holes. Oxford: Oxford University Press, 1998. Section 9.(a).
4. ↑ 4.0 4.1 Jeremy Bransom Griffiths, Jiri Podolsky. Exact Space-Times in Einstein's General Relativity. Cambridge: Cambridge University Press, 2009. Section 2.1.3.
5. ↑ 5.0 5.1 P Schneider, J Ehlers, E E Falco. Gravitational Lenses. Berlin: Springer, 1999. Section 3.4.2.
6. ↑ Sayan Kar, Soumitra SenGupta. The Raychaudhuri equations: a brief review. Pramana, 2007, 69(1): 49-76. [arxiv.org/abs/gr-qc/0611123v1 gr-qc/0611123]

0.00

(0 votes)

Original source: https://en.wikipedia.org/wiki/Optical scalars. Read more

|

Retrieved from "https://handwiki.org/wiki/index.php?title=Physics:Optical_scalars&oldid=4183918"