The discovery, Lukin said, runs contrary to decades of accepted wisdom about the nature of light. Photons have long been described as massless particles which don't interact with each other shine two laser beams at each other, he said, and they simply pass through one another.
Massless particles ? really ?
Wouldn't that be a direct challenge to Einstein's theory ?
How could gravity affect something that has no mass ?
That's the whole point of Einstein's Theory of General Relativity. Gravity is essentially a manifestation of the curvature of space-time, which affects everything, including photons.
It does so by distorting space - the old steel ball on a rubber membrane example.
“How could gravity affect something that has no mass ?”
Easy. Gravity warps space, and the path of the photons is curved along with it.