system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know redshifting is due to the universe expanding? What if it's an effect similar to friction, so the wave loses energy as it travels through space, and it's just more noticeable from the extremely distant galaxies?

I ask party out of curiosity, but also because the idea of space expanding so fast that light can never reach us really kinda freaks me out.
gpt
With respect to photons losing energy as they travel, one has to understand that, although we perceive light as taking billions of years to reach us, in the frame of reference of a photon, the journey is instantaneous. There is no opportunity for a photon to "leak" energy, or change in any other way, as it travels.

In fact, the same argument has been reversed to demonstrate that another type of particle, the neutrino, must have mass: neutrinos come in different varieties, and the ratio of those varieties change as neutrinos move through space. This implies that the neutrinos have time to randomly change from one type to another as they travel, meaning that they travel at less than the speed of light, meaning that, unlike photons, they must have a nonzero rest mass.