# Light and Relativity

1. Dec 13, 2009

### Dragohunter

2 questions

Does light have a frame of reference?

Although there is no speed greater than c, can it be said that the change in distance between two identifiable objects changed at a rate greater than c?

For example one person I know said this:

Last edited: Dec 13, 2009
2. Dec 13, 2009

### bcrowell

Staff Emeritus
Are you asking whether it's possible to have a frame of reference that's moving along with a beam of light? If so, then the answer is no. For example, the $\gamma$ factor linking this frame to the frame of any material object would be infinite.

Your friend is correct. What's forbidden by relativity is much more specific. For example, suppose you have an observer who takes measurements in a certain reference frame. Relativity forbids a particle from whizzing right past the observer's location at a speed greater than c. It doesn't forbid distant objects from moving at greater than c relative to the observer. For example, you can say that galaxies beyond the edge of the observable universe are moving away from us at greater than c; however, it's impossible for us to observe those galaxies.