The Einstein radius is a concept from gravitational lensing, which is the bending of light caused by the gravitational field of massive objects, such as galaxies or galaxy clusters. When a light source (like a distant star or galaxy) is perfectly aligned with a massive foreground object (the lens), the light from the source can be bent around the lens, creating multiple images or a ring-like structure known as an "Einstein ring.
Articles by others on the same topic
For a point lens of mass at observer distance and source distance in a nearby Euclidean geometry, the Einstein radius in the lens plane is . Its angular version is . This is the ring radius for perfect alignment and the characteristic transverse scale of a gravitational microlensing event. A crossing timescale is , proportional to the square root of lens mass at fixed geometry and speed.