* Idea: The spacetime (squared) interval between two points, which conceptually encodes all the information in the metric, but does not mention a differentiable structure in its definition and is therefore appealing for generalizations of spacetimes (e.g., discrete ones), at least at the kinematical level.
$ Def: Given two points x, y ∈ M, the world function is defined as
σ(x, y):= ± \(1\over2\)S 2(x, y) ,
where S(x, y) is the geodesic distance between
x and y if it is defined, and the sign depends on whether
x and y are or are not, respectively, causally related.
* Example: In Minkowski space,
ση(x, y) = \(1\over2\)(xm − ym) ηmn (xn −yn) .
* Properties: It is symmetric, non-negative, and satisfies (in any dimension, with signature (−, +, ..., +), and under the appropriate differentiability assumptions)
(∂σ(x, y)/∂xa) gab(x) (∂σ(x, y)/∂xb) = −2 σ(x,y) , det(∂2σ(x, y) ∂xa∂yb) ≠ 0 ,
limy → x σ(x, y) = 0 , limy → x ∂σ(x, y)/∂xa = 0 ,
limy → x ∂2σ(x, y) / ∂xa∂yb = −gab(x) .
(These limit properties explain why S 2
is used rather than S.)
> Online resources: see Wikipedia page.
And Gravitation > s.a. spacetime structure.
* Idea: All curvature tensors can be written as coincidence limits of derivatives of the world function, and Einstein's equation becomes a set of fourth-order partial differential equations for σ.
@ General: in Synge 60; Rylov AdP(63).
@ Special cases: Roberts ALC(93)gq/99 [in FLRW spacetime].
@ Applications: Bahder AJP(01)gq [spacetime navigation]; Le Poncin-Lafitte et al CQG(04) [and light deflection]; > s.a. tests of general relativity with light.
@ And quantum gravity: Álvarez PLB(88) [quantum spacetime]; Rylov JMP(90) [discrete spacetime]; Kothawala PRD(13)-a1307 [minimal length]; Jia a1909 [quantum causal structure, including matter].
@ Related topics: in Ottewill & Wardell PRD(11)-a0906 [derivatives, transport equation approach].
– journals – comments
– other sites – acknowledgements
send feedback and suggestions to bombelli at olemiss.edu – modified 14 sep 2019