Can light age?

Dominic - Well, light is made up of these particles we call photons which seem to basically be quite timeless. They don't age when they're travelling through free space. They can be absorbed by things they pass through. But it's actually quite useful particularly to people like me who are astronomers that light can travel for billions of years through free space completely unchanged because that means we can look back through the universe and see light that was emitted billions of years ago, and see what objects in the universe look like in the very distant past.

Dave Ansell - The one way it does change if it's come an awful long way is it can get red shifted. So, things which are an awful long way away from us seem to look redder than they should do because the space inside is stretched and they stretch the photons.

Dominic - That's right. When you're looking at these very old objects, you're seeing the fact that the universe is expanding around us, over periods of billions of years. As that universe expands, light has a particular wavelengths associated with it and that wavelengths get stretched. Red light has longer wavelengths than blue light. So, this light becomes redder as it gets older.