Can we distinguish between black holes and wormholes by their Einstein-ring systems?

Abstract

For the past decade, gravitational lensing in the strong gravitational field has been studied eagerly. It is well known that, for the lensing by a black hole, an infinite number of Einstein rings are formed by the light rays which wind around the black hole nearly on the photon sphere, which are called relativistic Einstein rings. This is also the case for the lensing by a wormhole. In this paper, we study the Einstein ring and relativistic Einstein rings for the Schwarzschild black hole and the Ellis wormhole, the latter of which is an example of traversable wormholes of the Morris-Thorne class. Given the configuration of the gravitational lensing and the radii of the Einstein ring and relativistic Einstein rings, we can distinguish between a black hole and a wormhole in principle. We conclude that we can detect the relativistic Einstein rings by wormholes which have the radii of the throat $a\ensuremath{\simeq}0.5\text{ }\text{ }\mathrm{pc}$ at a Galactic center with the distance 10 Mpc and which have $a\ensuremath{\simeq}10\text{ }\text{ }\mathrm{AU}$ in our Galaxy using the most powerful modern instruments which have the resolution of ${10}^{\ensuremath{-}2}\text{ }\text{ }\mathrm{arcsecond}$ such as a 10-meter optical-infrared telescope. The black holes which make the Einstein rings of the same size as the ones by the wormholes are galactic supermassive black holes and the relativistic Einstein rings by the black holes are too small to measure with the current technology. We may test the hypotheses of astrophysical wormholes by using the Einstein ring and relativistic Einstein rings in the future.