# Photon Acceleration?

*Most recent answer: 01/08/2012*

Q:

You have an an answer to
If photons are massless, how come light are affected to gravity? dosent gravity only effect particles with mass? So for light to be bent or curved by gravity dosent that mean that photons have to have mass?
- Stian Dahl (age 21)
Vestfold Norway
Mike W.
Photons are affected by gravity. Whatever is affected by gravity usually follows Newton's second law F = ma. How do photons accelerate by gravity? In special relativity, photons have an invariant velocity of c is an inertial reference frame. Does the motion of photons in a gravitational field differ than the motion of particles with rest mass in a gravitational field?
Dpug S.

- Doug Snyder (age 61)

Los Angeles, CA, USA

- Doug Snyder (age 61)

Los Angeles, CA, USA

A:

The law F=ma gets farther and farther from being true as the objects involved move at bigger fractions of the speed of light, in the chosen reference frame. For light, or other things traveling at or near c, it's not even close.

That leaves your very interesting question. Say that light is going straight down in a gravitational field. Its speed is fixed at c, so it isn't accelerating. How do we then describe the effects of the gravity on it? It turns out that this requires a change in our basic picture of space and time. We need General Relativity. The geometry of space gets distorted, and clocks that were identically constructed run at different rates depending on whether they're up or down from each other. The frequency of falling light increases. All these effects are measured.

Mike W.

That leaves your very interesting question. Say that light is going straight down in a gravitational field. Its speed is fixed at c, so it isn't accelerating. How do we then describe the effects of the gravity on it? It turns out that this requires a change in our basic picture of space and time. We need General Relativity. The geometry of space gets distorted, and clocks that were identically constructed run at different rates depending on whether they're up or down from each other. The frequency of falling light increases. All these effects are measured.

Mike W.

*(published on 01/08/2012)*