General Relativity: An Introduction for Physicists · Problem 9.16
The Schwarzschild geometry — Hobson, Efstathiou & Lasenby
Physics · photon-round-trip-schwarzschild
Problem
In the Schwarzschild geometry, a photon is emitted from a coordinate radius r=r2 and travels radially inwards until it is reflected by a fixed mirror at r=r1, so that it travels radially outwards back to r=r2. How long does the round trip take according to a stationary observer at infinity?