# Photons not Accelerating

*Most recent answer: 10/22/2007*

Q:

Light can’t accelerate right? Then what happens to the photon when it enters an extreme gravitational well and is lenzed or bent around objects? Surels a change in of a photon’s light involves some magnitude of acceleration. So if accelerated charged particles produce photons, what do accelerated photons produce?

- SJ

JHU, Baltimore, MD

- SJ

JHU, Baltimore, MD

A:

You are right in your first sentence -- light cannot accelerate.

The paradox of the second sentence is resolved by noticing that a large piece of space with a gravitating mass in it does not constitute an "inertial reference frame". Newton’s first law says that an object with no force on it travels in a straight line at constant speed, if it is measured in an inertial reference frame. To observe a deviation from "straight" means either there is some acceleration going on, or the reference frame is not inertial, or both.

Einstein’s general theory of relativity is built on top of the special theory of relativity, and says that everywhere, space is "locally Lorentzian", in that if you look in a small enough patch, light should travel in straight lines where straight is defined by Euclidean geometry. But on larger distance scales, when gravity is present, the definition of "straight" gets changed to "that path a light ray takes", which may not agree with the Euclidean idea of straight.

A fun read is the first couple of chapters of Misner, Thorne, and Wheeler’s otherwise daunting book, "Gravitation".

Tom

The paradox of the second sentence is resolved by noticing that a large piece of space with a gravitating mass in it does not constitute an "inertial reference frame". Newton’s first law says that an object with no force on it travels in a straight line at constant speed, if it is measured in an inertial reference frame. To observe a deviation from "straight" means either there is some acceleration going on, or the reference frame is not inertial, or both.

Einstein’s general theory of relativity is built on top of the special theory of relativity, and says that everywhere, space is "locally Lorentzian", in that if you look in a small enough patch, light should travel in straight lines where straight is defined by Euclidean geometry. But on larger distance scales, when gravity is present, the definition of "straight" gets changed to "that path a light ray takes", which may not agree with the Euclidean idea of straight.

A fun read is the first couple of chapters of Misner, Thorne, and Wheeler’s otherwise daunting book, "Gravitation".

Tom

*(published on 10/22/2007)*