The equivalence principle (EP) - which is the basis of general relativity – states that you cannot distinguish between an object’s behaviour in a uniform gravitational field from that in a uniformly accelerating frame. If light travels vertically in a gravity field it loses or gains energy, and experiments confirm that gravity pulls on light as if it possessed inertial mass m=E/c^2. So light travelling vertically obeys the EP. When Einstein first calculated the bending of light near the sun he got a value of 0.8 arcsec, using the above reasoning. But later corrected this to 1.75 arcsec, because light refracts in a gravitational field. So the light bends doubly. Consider the two cases: 1. An astronaut is in a room inside a rocket that is accelerating. A beam of light passes horizontally across the room and strikes the opposite wall lower down by distance d. 2. An astronaut is in a room inside a rocket that is stationary on the earth. A beam of light passes horizontally across the room and strikes the opposite wall lower down. Does light fall by d (gravity pulling on light’s inertial mass only)? Or does it fall by 2d (due to refraction and gravitational pull)? If it’s 2d, doesn’t that violate the EP?