Full Idea
In Einstein's general theory, gravity acts also on energy and momentum, not simply on mass. For example, massless photons of light feel the gravitational attraction of the Sun and can be deflected.
Gist of Idea
In general relativity the energy and momentum of photons subjects them to gravity
Source
Frank Close (Theories of Everything [2017], 5 'Planck')
Book Reference
Close,Frank: 'Theories of Everything' [Profile Books 2017], p.99
A Reaction
Ah, a puzzle solved. How come massless photons are bent by gravity?