Astronomers have long known that light bends when it passes a star. Photons have no mass for gravity to act on, but Einstein’s theory that matter changes space predicts it. He imagined a light shining in a lift that is going up, as shown in Figure 5.10. As the lift rises, the light curves relative to it, so if gravity equates to an acceleration, stars should bend light passing by, and they do.
The light passing a star bends because, from its perspective, it is going in a straight line, just as light in the lift is. Stars then don’t bend light by pulling it, but by changing the space around to make it bend.
This model also predicts that a star will bend light by changing the space around it, as the gravity gradient can cause refraction (3.6.2). Light bends when it enters water because it spreads more slowly in it, as slowing down one side of a spreading wave skews it that way. Water then bends light by being a denser medium not by exerting a force.
Similarly, the gravity gradient around the sun slows down light as it get closer because the network has more to do. In effect, space becomes denser closer to the sun, and light waves refract when they enter a denser medium. Light then bends when it passes a star for the same reason that it bends when it enters a denser medium like water.
A large mass like the sun alters the space closer to it to make it a denser medium for light, so light bends. Gravity then does curve space, as Einstein said, but both arise from the quantum field around the sun. How then does the gravity gradient affect time?