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.