# What will be speed of light near black holes?

1. May 28, 2015

### Prashan Shan

black holes pulls light right,
then when a black hole pulls photon will the speed that photon increase?

2. May 28, 2015

### wabbit

No, the speed of light does not change near a black hole. Why do you think the photon's speed should increase ?

3. May 28, 2015

### phinds

As the rabbit said, the speed of light in a vacuum is c.

Far away from a black hole, the speed of light in a vacuum is c.

Near a black hole the speed of light in a vacuum is c.

The speed of light in a vacuum is c.

4. May 28, 2015

### nitsuj

lol likely because gravity "pulls" things, pretty easy to intuit the OP is envisioning that "pull" on a photon.

No it will not increase the speed of the photon. The photon is already traveling at an invariant speed, but that doesn't mean nothing happens. That "pull" "stretches" or "compresses" the wavelength of the photon, changing frequencies. Which one depends on your perspective / which way the light is traveling through the "distorted" spacetime.

5. May 28, 2015

### wabbit

The thing is, it's not really wrong to think that there's a "pull", the issue is, what does a pull do to a photon since it cannot change its speed ? As you mentionned it acts on its frequency, but the intuition as to why it does so is not obvious.

Prashan Shan, perhaps you know than a photon's energy is $E=h\nu$ where $\nu$ is its frequency. You can think of that as kinetic energy since the photon does not have a rest mass - now what does gravitational attraction do to the kinetic energy of a particle as it gets closer to the attracting mass?

Last edited: May 28, 2015