Optical appearance of a collapsing object as seen by a radially infalling observer

Abstract

We investigate the optical appearance of a collapsing object relative to a radially infalling observer. The surface of the collapsing object is modelled as a worldline of a radially infalling geodesic in Schwarzschild spacetime. The relative shift of the frequency of photons emitted from the collapsing object as measured in the frame of the infalling observer was calculated numerically. Our results imply that a black hole formed from a collapsing star would not necessarily appear black to an infalling observer.