An observer stationed wherer = ro watches a light signal emitted from a point where r...
An observer stationed wherer = ro watches a light signal emitted from a point where r = r1. It travels radially inwards and is reflected by a fixed mirror at r = r2, so that it travels back to its point of origin at r = r1. How long does the round-trip take according to the observer at r=ro? (Assume that 2m < r2 <ri <ro.)