Where's the energy going, supposedly? I'd assumed "the light's losing energy" had been firmly ruled out long ago—it was a potential explanation that occurred to

Where's the energy going, supposedly? I'd assumed "the light's losing energy" had been firmly ruled out long ago—it was a potential explanation that occurred to me the very first time I heard about the observed red shift of distant galaxies, so I figured it must be very and obviously wrong if I'd never heard an actual physicist even mention the possibility of that as a notable factor. [EDIT] Wikipedia "Tired Light" article: > The concept was first proposed in 1929 by Fritz Zwicky, who suggested that if photons lost energy over time through collisions with other particles in a regular way, the more distant objects would appear redder than more nearby ones. Oh, so, what I might have guessed, "it hits stuff sometimes". Article goes on to make it seem like there's a lot working against the notion, including that distant images ought to be a lot fuzzier if light's interacting with other stuff along the way.