Why is light subject to gravity if it has no mass? Theory of General Relativity.

Albert Einstein, a German-born physicist, developed the comprehensive physical theory of relativity, which includes general relativity. Einstein had the idea in 1915. It uses the ability of space to "curve" as an explanation for gravity, or more precisely, it connects the gravitational force to the shifting geometry of space-time. (The gravity of Einstein)

Today, replacing those devised by Isaac Newton several centuries earlier, the mathematical equations of Einstein's general theory of relativity are the most reliable technique to predict gravitational interactions.

Numerous tests conducted throughout the past century have established the reliability of both special and general relativity. Astronomers detected the deflection of light from far-off stars as the starlight in 1919, which was the first significant test of general relativity passed in front of our sun, demonstrating that gravity does, in fact, bend or warp space.