Clarifying spatial distance measurement

Abstract

We examine length measurement in curved spacetime, based on the 1+3-splitting of a local observer frame. This situates extended objects within spacetime, in terms of a given coordinate which serves as an external reference. The radar metric is shown to coincide with the spatial projector, but these only give meaningful results on the observer's 3-space, where they reduce to the metric. Examples from Schwarzschild spacetime are given.