Protons don't decay (as far as we can tell), they get ripped apart in black holes which evaporate into photons, mostly. All that's left are photons, moving at

Protons don't decay (as far as we can tell), they get ripped apart in black holes which evaporate into photons, mostly. All that's left are photons, moving at the speed of light, experiencing no time, and never meeting each other as the universe expands away faster than they move. "It's hard to bore a photon." -- Roger Penrose