description of an idea that goes back at least two hundred years, to a time when there were two theories about light:
one, which Newton favored, was that it was composed of particles; the other was that it was made of waves. We now
know that really both theories are correct. By the wave/particle duality of quantum mechanics, light can be regarded as
both a wave and a particle. Under the theory that light is made up of waves, it was not clear how it would respond to
gravity. But if light is composed of particles, one might expect them to be affected by gravity in the same way that
cannonballs, rockets, and planets are. At first people thought that particles of light traveled infinitely fast, so gravity
would not have been able to slow them down, but the discovery by Roemer that light travels at a finite speed meant that
gravity might have an important effect.
https://apod.nasa.gov/apod/ap170216.html
