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 thats 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.