Please someone could explain me why the curves have no intrinsic geometry? With surfaces I can see that there are two kind of geometries, i.e. the euclidean one (related to euclidean isometries) and the intrinsic (related to isometries on surfaces), but I am not able to make a comparison/analogy with curves. Thanks.
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One possible interpretation for the statement «curves have no intrinsic geometry» in the context of Riemannian geometry is the following:
Most of this is proved implicitly when one shows that a curve can always be parametrized by arc-length, after classifying $1$-dimensional differentiable manifolds.
A cuter statement, although partial, is that the complete $1$-dimensional manifolds up to isometry are in bijective correspondence with $(0,\infty]$.
Likewise, the local version of this is: any two $1$-dimensional Riemannan manifolds are locally isometric.
Curves have the same intrinsic geometry that a straight line has. If $B$ is between $A$ and $C$ then the distance from $A$ to $B$ (measured along the curve, i.e. the arc length) plus the distance from $B$ to $C$, is the distance from $A$ to $C$. If a string winds around through space and you pick it up and lay it down unrolled so that it's a straight line, you don't change any of the intrinsic distances; the length of the curve from $A$ to $B$ is still the same. Hence the curvature doesn't matter.