Recently it has been affirmed here (again) that the quantity called "interval (also 'spacetime interval' or 'invariant interval')" is referring to two (in general distinct) events as arguments, such as εA and εB, and is denoted (as shorthand) by Δs2.
Consequently, writing out the concrete arguments explicitly, the "interval between" (also called "interval of") events εA and εB is denoted as Δs2[ εA,εB ],
where by definition Δs2[ εA,εB ]≡Δs2[ εB,εA ].
If two events are "timelike separated" (such as events ε(PH) and ε(PJ) where participant P took part in both, having met and passed participant H, and separately met and passed participant J) then the "interval between" these two events is of opposite sign than the "interval between" any two events which are "spacelike separated".
Following the convention set out in Wikipedia (even though it is just inverse from the convention adopted in the answer referred to above) therefore explicitly: Δs2[ ε(PH),ε(PJ) ]<0.
With this convention, considering three events which are (pairwise) "timelike separated" from each other (such as events ε(PH), ε(PJ), and ε(PK), where participant P's meeting/passing of participant J had been between participant P's meetings/passings of participants H and K, respectively) therefore holds
Δs2[ ε(PH),ε(PK) ]<Δs2[ ε(PH),ε(PJ) ],
Δs2[ ε(PH),ε(PK) ]<Δs2[ ε(PJ),ε(PK) ],
and the "inverse triangle inequality":
√−Δs2[ ε(PH),ε(PJ) ]+√−Δs2[ ε(PJ),ε(PK) ]≤√−Δs2[ ε(PH),ε(PK) ].
Note the square roots, "√ ", operating on each term of the latter inequality. Correspondingly, the (any) "interval, Δs2", is considered a "squared quantity" (and appropriately its name involves an exponent: " 2").
My question:
Is there a name for the corresponding linear quantity "Δ¯s" ?,
for which (written in shorthand) sgn[ Δ¯s ]≡sgn[ Δs2 ]
and
Δ¯s≡sgn[ Δs2 ]×√sgn[ Δs2 ]×Δs2 ],
and therefore in turn
Δs2≡sgn[ Δ¯s ]×Δ¯s×Δ¯s,
and such that (again following the sign convention of Wikipedia) the "inverse triangle inequality" relating three pairwise "timelike separated" events is (simply, linearly)
¯s[ ε(PH),ε(PK) ]≤¯s[ ε(PH),ε(PJ) ]+¯s[ ε(PJ),ε(PK) ],
provided
¯s[ ε(PH),ε(PK) ]<¯s[ ε(PH),ε(PJ) ]<0
and
¯s[ ε(PH),ε(PK) ]<¯s[ ε(PJ),ε(PK) ]<0.
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