I have heard that photons do not have any mass which is why they travel at the speed of light. However consider the following thought experiment. You are box in space accelerating at 1 G. From your perspective, you are in a gravitational field of 1G. If you shine a light from one wall of the box to the other (across the gravitational field), it will hit the wall at a slightly lower point that it would if the box were traveling at constant velocity. This is because the wall will have moved compared to where it would have been had the box been traveling at constant velocity. From the perspective of the observer in the box, the light beam has been bent by the gravitational field. The same will apply with a gravitational field created by the existence of an object with mass. We can therefore say that the photon is attracted by an object with mass. Does it not follow that the photon must its self have mass?