Light is, to my knowledge, made up of particles that behave in a wave-like manner (just like all matter in the universe, technically). Going by that, should it not be possible to make these particles speed up or slow down? Movement is energy, and I can think of three energetic forces that might have an impact on light:
1. Heat. Higher temperatures make all other forms of matter speed up, I don't see why light would be an exception. I'm not sure whether this has been tested, but logically there should be some kind of difference in speed between the light being released from an immensely hot light source (let's say a star) and the light lost out in freezing space. Furthermore, what state of matter is light exactly? Plasma?
2. Distance. I know this is wrong, but I don't understand how waves (or wave-like particles) maintain 100% of their energy when crossing the absolute emptiness of space. Movement is energy, including wave-like movement, and therefore some of that energy should be expended. Especially in the case of mass, when it is solid, physical matter that must move back and forth. There is something going on here that I am not aware of, I'm sure. But I felt like I should mention it.
3. Force. As with any object, when light hits something, some of its energy should be transferred into the other object. According to this, light should slow down a bit each time it hits something, for instance an atmospheric molecule.
Sorry to go off topic, and sorry for asking such a childish question, but thinking about point #3, where exactly does light go? When I turn the light off in my room, why don't latent photons keep bouncing around, at least for a little while? Why does light stop? What is the end of the life of a photon? Does it just rest on something, and if so, should this not cause them to build up over lengths of time? Furthermore, what is the structure of a photon? Just some things I'm curious about.