Is Light Really Slowing Down or Is It Just More Matter Between Us and the Stars?

[Geekchick]

Alright, so my friend and I were looking at articles on light possibly slowing down and he was saying that the article (http://www.theage.com.au/articles/2002/08/07/1028157961167.html) was assuming that the matter between the Earth and the quasars is constant. So his reasoning was what if instead of light traveling slower there is simply more matter between the Earth and the stars now? and since light slows as it moves through matter that's what is causing light to look like its slowing down? but I was thinking that since motion slows time isn't it possible that the light and time are slowing at the same rate so that would cancel his theory, right? Can someone explain this to me. Note that I know next to nothing about physics.

thanks!

 

[atyy]

It's not actually about light slowing per se, they presented it that way for simplicity. It's about a change in the "fine structure constant", which is a dimensionless number that is a ratio of several constants including the speed of light, Planck's constant and electron charge. So if the fine structure constant changes, we can say it's the speed of light or Planck's constant or the electron charge or some mixture of them that has changed.

What they measured was also not the speed of light per se. They know that a particular chemical can absorb light of several frequencies. The difference between the frequencies is characteristic of that chemical, and related to the fine structure constant. So in this way they can compare the fine structure constant in stuff far away and "long ago" with the fine structure constant of stuff here and "now".

Although there is some evidence suggesting that the fine structure constant is not constant, the evidence is not (yet?) strong enough to be accepted as "fact".

The fundamental constants and their variation: observational status and theoretical motivations
Jean-Philippe Uzan
http://arxiv.org/abs/hep-ph/0205340

Variation of the constants in the late and early universe
Jean-Philippe Uzan
http://arxiv.org/abs/astro-ph/0409424