Does a Black Hole Accelerate Light?

If light, that is of course traveling in the speed of light, go to the gravitational field of a black hole, the black hole will apply a force on the light and it will accelerate, and by accelerating the light will go faster than the speed of light?
- Guilherme Marchetto (age 15)
Americana, S?Paulo, Brazil

The formal definition of acceleration is the change in a velocity vector over time, which includes changes in both direction and magnitude (speed). So acceleration doesn't have to be a change in speed—it can also be a change in direction.

When light passes by a black hole, its speed doesn't change, but its path curves. Astronomers have directly observed this bending of light around massive objects in space (here's a !). Light can even go into orbit around the black hole, just like a spaceship around the Earth—although spaceships can orbit at many different distances from the Earth by varying their speeds, while light can only orbit a black hole at one special distance. 

So the path of light can be bent by gravity, just like the path of a spaceship. However, when a spaceship returns to Earth by falling through the atmosphere, it speeds up as its gravitational potential energy is converted to kinetic energy (energy of motion). When light "falls" towards a massive object like a black hole, it can't speed up. Instead, its color changes, becoming bluer (as seen locally) as it moves into an area farther "downhill" in the gravitational field. Bluer light has a higher frequency, and more energy per photon. When light travels away from a massive object, the opposite happens: the light becomes redder, and loses energy. This effect is called gravitational redshift, and it has been observed in many experiments. GPS satellites even have to in order to work properly.

Rebecca Holmes

(published on 12/18/2014)