# Does light accelerate as it nears a black hole? [duplicate]

As light is affected by gravity ( gravitational lending and black holes), it would seem that gravity causes acceleration.

Acceleration has two parts: direction and magnitude. It is clearly evident that gravity affects the former of the two. It would seem that it does not affect the later component because light is currently at the universal 'speed limit' (not counting taychons).

What is going on here? Is the magnitude component affected by gravity?

## 1 Answer

Light always travels at the same speed. What happens as photons move through changes in gravitational potential is that they gain and lose energy, which manifests itself as red shift or blue shift.

In the specific case of light moving towards a black hole, where an object with a rest mass would gain kinetic energy as it fell, a photon is blue-shifted, carrying more energy (and momentum) for each photon - in classical terms, the light is shifted to a higher frequency i.e. a shorter wavelength.

For the reverse situation, light coming from deep within a potential well to an observer far away, the light is red shifted to a lower frequency (higher wavelength), with lower energy per photon.

• So do you mean that the light would have a higher frequency to 'store' the excess energy? Thxs for the speedy answer :). Commented Mar 2, 2016 at 13:26
• Sort of a spin off... But can we produce energy by somehow red shifting it? Commented Mar 2, 2016 at 14:08
• An observer near the black hole and right beside the light (using his own rod and clock) will measure the standard c. However, an observer far from the black hole (using his own rod and clock) sees that the speed of light is less than c as it passes near the black hole. Commented Mar 2, 2016 at 20:08