Monday, November 12, 2018

general relativity - Given the Wikipedia notion of "arc length", how is its manifestly real "signed variant" to be called and denoted?


I am dissatisfied with the presentation (not to say "definition") of "arc length", in its "Generalization to (pseudo-)Riemannian manifolds", as given in Wikipedia. (Who isn't?. But I'll sketch it here as a starting point anyways.) Namely:




[arc] length of curve $\gamma$ as $$\ell[~\gamma~] := \int_0^1 dt~\sqrt{\pm~g[~\gamma'[~t~],\gamma'[~t~]~]},$$



where [...] the sign ["$\pm$"] in the square root is chosen once for a given curve, to ensure that the square root is a real number.




In contrast, more useful I find the following definition variant (which is broadly similar to the above, but different in some decisive details): $$\int_0^1 dt~(\pm)[~t~]~\lvert\sqrt{(\pm)[~t~]~g[~\gamma'[~t~],\gamma'[~t~]~]}~\rvert,$$ where the sign "$\pm$" is chosen separately for each individual value $t$,
or in other words, the sign "$(\pm)[~t~]$" is chosen as a function of "the variable $t$",
to ensure that the square root is a real number for each individual value $t$
(and with all other symbols the same as in the Wikipedia presentation above).


Is this latter definition variant already known by some particular name and notation in the literature ?


And vice versa: Has the name "signed arc length" and/or the symbol "$s[~\gamma~]$" been used in any other sense (inconsistent with this latter definition variant); at least within the context of discussing pseudo-Riemannian manifolds ?


Note on notation:



The symbol "$(\pm)[~t~]$" for denoting "the appropriate sign as a function of the variable $t$" has been used above in order to mimic the symbol "$\pm$" which appears (presently) in the Wikipedia article. A more explicit and perhaps more established notation for this function would be "$\text{sgn}[~g[~\gamma'[~t~],\gamma'[~t~]~]~]$".


Documentation of prior research (in response to a deleted answer):


As of recently, Google searches for "signed arc length" or "signed arclength" seem to yield fewer than 100 distinct results, several of which even dealing with general relativity (and hence with spacetime, and/or pseudo-Riemannian manifolds as models of spacetime), but none of them (except this PSE question) presenting in this context anything resembling the sought particular expression.*


My attempts at a web search for this particular expression didn't seem to bring up any relevant results either; even considering several different choices of notation.


(*: In order to make this determination I've been trying to match symbols or items of the given notations of the documents I had found to the following notions (here in my specific, but generally of course arbitrary notation):




  • spacetime, as set of events $\mathcal S$,





  • a strictly ordered subset of spacetime, $\Gamma \subset \mathcal S$, and




  • two signed measures $\mu_s$ and $\mu_g$ for which




$$\forall x \in \Gamma : \lim_{ A \rightarrow x }~\left( |~\mu_s[~A~]~| ~ \mu_s[~A~] - \mu_g[~A~] \right) = 0,$$



  • or at least one signed measure $\mu_s$ together with real numbers $g[~x, \Gamma, \mathcal S~]$ which are not necessarily positive, for which



$$\forall x \in \Gamma : \lim_{ A \rightarrow x }~\left( |~\mu_s[~A~]~| ~ \mu_s[~A~] \right) = g$$ ).



Answer



I don't remember having seen the specific expression of the proposed "signed arc length" either (anywhere but related to the OP question), nor anything resembling (1) the more abstract expression for determining the sought resemblance.


For naming this proposed functional from the set of curves (or rather, arcs) into the set of real numbers (incl. $\mathbb R_{-}$) the choice "signed arc length" seems reasonable, but not very specific (2).


However I can think of several names which on first sight may seem reasonable and more evocative, but which refer(3) instead to (largely) inapplicable notions; for instance:




  • not "Lorentzian arc length", not "sub-Lorentzian arc length", not even "signed Lorentzian (arc) length";





  • not "(Synge's) World (arc) function", not "Minkowski arc length";




  • not "pseudo-Riemannian arc length";




even though, on the other hand, the definition of "pseudo-(arc)-length", (eq. 11.59) itself may well allow an interpretation which includes the proposed functional.


But if relying on other's interpretation is not an option then, as far as I know, it remains only to call the (definition, and any accordingly determined value) of the proposed functional "proper arc length".


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