- #26

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There is no such thing as "the" tangent vector, because of the normalization issue. But as long as the invariant mass of the object is constant, the 4-momentum is certainlyThat's the length of the 4-momentum which isnotthe tangent vector.

*a*tangent vector; you just use proper time divided by invariant mass as the parameter (instead of proper time by itself).

It is true, though, that this no longer works for the case of changing invariant mass.