= Affine rescaling of a null geodesic tangent
{title2=$k\mapsto k/A,\quad h\mapsto h/A$}
A change $\lambda\mapsto A\lambda+B$ rescales a <geodesic> tangent and all its linear Killing constants by $1/A$. A null norm remains zero and cannot fix $A$, unlike proper-time normalization of a material particle. Ratios of the constants, such as the <photon> <impact parameter> $b=|h|/(ck)$ in <Schwarzschild spacetime>, are independent of the affine scale. A physically normalized <four-momentum> fixes the <photon> <energy> and hence selects a tangent scale.
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