[thesharkboy]

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.

[Zeneta]

That's my point.

Photons have long been described as massless particles which don't interact with each other

Einstein's Theory of General Relativity says that they do interact with each other.