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. 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 pull light but change space to make it bend.
This model also expects stars to bend light because the gravity gradient around a star will cause refraction (3.6.2). Light refracts when it enters a denser medium like water because it spreads more slowly in it, as slowing down one side of a wave skews it that way. Water then bends light because it is a denser medium, not by exerting a force.
Likewise, the gravity gradient of the sun makes space closer to it a slower medium for light because the quantum network has more to do. In effect, gravity makes space denser closer to the sun, so light is bent when it enters it, just as it is bent when it enters a denser medium like water.
The sun then bends light because its gravity makes the space closer to it a denser medium, so gravity does curve space as Einstein said, but why does it also affect time?