The Radius of a Rotating Disk Must Also Contract

v2: general rewrite specifying origin and nature of the identity error

v3: added paragraph to rigidity section pointing out impossibility of rigid radial rod

v4: general rewrite focusing on operationally defining the radius of a rotating disk

v5: changed title

v6: added circularity argument to radial misidentification section

v7: made non-Euclidean radius/radii explicit, showed that material path length contracts to R in the lab frame

v8: added standard textbook citation