|MadSci Network: Physics|
It is not true that only massive objects can be deflected by a gravitational field. In fact, everything that has energy feels the warping of spacetime. For example, an excited nucleus is `heavier' than the same nucleus in the ground state: the additional energy contributes to its mass and inertia. Photons do not have rest mass, all right. But they do have energy according to E=hbar x omega!
According to General Relativity, objects in free fall subject to a gravitational field follow a path which is called a geodesic. A geodesic is (locally) the shortest connection between two points in spacetime (not space!). A photon is such an object, and thus moves on a geodesic.
According to the conservation of momentum, a massive body must also feel a recoil (or drag) as a photon whizzes by, but that effect is certainly much too small to be detectable.
Hope that helps,
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